User manual | FISHERIES RESEARCH REPORT NO. 156, 2006 stocks in the Swan-Canning Estuary

FISHERIES RESEARCH REPORT NO. 156, 2006
Review of fishery resources and status of key fishery
stocks in the Swan-Canning Estuary
K. A. Smith
Fisheries Research Division
Western Australian Fisheries and Marine Research Laboratories
PO Box 20 NORTH BEACH
Western Australia 6920
Fisheries Research Report
Titles in the fisheries research series contain technical and scientific information that
represents an important contribution to existing knowledge, but which may not be suitable
for publication in national or international scientific journals.
Fisheries Research Reports may be cited as full publications. The full citation is:
Smith, K. A. 2006. Review of fishery resources and status of key fishery stocks in the SwanCanning Estuary, Fisheries Research Report No. 156, Department of Fisheries, Western
Australia, 84p.
Numbers 1-80 in this series were issued as Reports. Numbers 81-82 were issued as
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Enquiries
Department of Fisheries
3rd floor The Atrium
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PERTH WA 6000
Telephone (08) 9482 7333
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Website:
http://www.fish.wa.gov.au
Published by Department of Fisheries, Perth, Western Australia. May 2006.
ISSN: 1035 - 4549
ISBN: 1 877098 87 6
An electronic copy of this report will be available at the above website where parts may be
shown in colour where this is thought to improve clarity.
Fisheries Research in Western Australia
The Fisheries Research Division of the Department of Fisheries is based at the Western
Australian Fisheries and Marine Research Laboratories, PO Box 20, North Beach (Perth),
Western Australia, 6920. The Fisheries and Marine Research Laboratories serve as the
centre for fisheries research in the State of Western Australia.
Research programs conducted by the Fisheries Research Division and laboratories investigate
basic fish biology, stock identity and levels, population dynamics, environmental factors, and
other factors related to commercial fisheries, recreational fisheries and aquaculture. The
Fisheries Research Division also maintains the State data base of catch and effort fisheries
statistics.
The primary function of the Fisheries Research Division is to provide scientific advice
to government in the formulation of management policies for developing and sustaining
Western Australian fisheries.
Fisheries Research Report [Western Australia] No. 156, 2006
Contents
1.0 Introduction...........................................................................................................
7
2.0 Environmental factors affecting fish in the Swan-Canning Estuary ............... 8
2.1 Environmental changes since European settlement......................................... 8
2.2 Phytoplankton blooms and fish kills................................................................ 9
2.3 Karlodinium micrum bloom and fish kill, April-June 2003............................. 10
2.4 Seasonal patterns in water quality and phytoplankton..................................... 11
3.0 Swan Estuary commercial fishery........................................................................
3.1 Early years of fishing........................................................................................
3.2 Commercial fishery catch and effort statistics.................................................
3.3 Commercial fishing effort and cpue..............................................................
3.4 Seasonality of commercial landings.................................................................
3.5 Commercial catches from 1921 to 1974..........................................................
3.6 Commercial catches from 1975 to 2004..........................................................
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16
4.0 Swan Estuary recreational fishery.......................................................................
4.1 Recreational fishery catch and effort statistics . ..............................................
4.2 Composition of the recreational fishery catch..................................................
4.2.1 Creel and phone surveys........................................................................
4.2.2 ‘Swanfish’ fishing tournament...............................................................
4.2.3 Melville Amateur Angling Club (maac).............................................
4.3 Maac catch and effort trends, 1986 to 2003..................................................
4.3.1 Total catch and effort..............................................................................
4.3.2 Catches of key species............................................................................
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5.0 Discussion................................................................................................................
5.1 Commercial and recreational catch composition and catch shares..................
5.2 Catch trends and changes in species abundance..............................................
5.3 Future fishery assessments...............................................................................
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6.0 Status of key fishery stocks................................................................................... 25
6.1 Major target species.......................................................................................... 25
6.1.1 Blue swimmer crab................................................................................. 25
6.1.2 Western school prawn............................................................................. 27
6.1.3 Perth herring........................................................................................... 28
6.1.4 Cobbler...................................................................................................31
6.1.5 Yellowtail trumpeter...............................................................................33
6.1.6 Sea mullet...............................................................................................35
6.1.7 Yellow-eye mullet...................................................................................37
6.1.8 Tailor.......................................................................................................39
6.1.9 Black bream............................................................................................ 41
6.1.10 Australian herring.................................................................................. 45
Fisheries Research Report [Western Australia] No. 156, 2006
6.1.11 Bar-tailed flathead..................................................................................
6.1.12 Small-toothed flounder..........................................................................
6.1.13 Yellowfin whiting..................................................................................
6.2 Minor species....................................................................................................
6 2.1 Trevally (or Skipjack).............................................................................
6.2.3 Tarwhine.................................................................................................
6.2.4 Yellowtail scad.......................................................................................
6.2.5 Mulloway................................................................................................
6.2.7 Common blowfish..................................................................................
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58
7.0 References............................................................................................................... 60
8.0 Tables and figures................................................................................................... 66
9.0 Appendices.............................................................................................................. 83
Appendix 1. Pre- July 1989 format of commercial fisher
compulsory monthly catch returns........................................................................... 83
Appendix 2. 2003 format of commercial fisher compulsory monthly
catch returns............................................................................................................. 84
Fisheries Research Report [Western Australia] No. 156, 2006
Review of fishery resources and status of key
fishery stocks in the Swan-Canning Estuary
K. A. Smith
Summary
This report synthesises data from the commercial and recreational fisheries of the Swan-Canning
Estuary (hereafter Swan Estuary), as well as relevant biological and environmental data. This
information is used to provide an overview of the fisheries, assess the current status of important
stocks and highlight changes in the abundances of various fish species in the estuary over recent
decades.
Blue swimmer crab is currently the largest component of both commercial and recreational landings
and the most valuable component of commercial landings in the Swan Estuary. From 1995 to
2004, commercial crab catches averaged 19 t per year. The 2000/01 recreational crab catch was
estimated to be 33 t. This suggests that the recreational sector has recently taken ~63% of the
total annual crab catch (by weight). The recreational crab catch has probably been of a similar or
greater magnitude to the commercial crab catch in the Swan Estuary since the 1970s.
From 1995 to 2004, commercial finfish catches averaged 36 t per year. The 2000/01
recreational catch of finfish was estimated to be 28 t. This suggests that the recreational sector
has recently taken ~45% of the total finfish catch (by weight) in the Swan Estuary. However, over
this 10 year period, commercial landings of finfish declined considerably, and so in latter years
the recreational catch of finfish has been >50%. Black bream is currently the most commonly
retained finfish species in the recreational fishery. The majority (>80% by weight) of black bream
in the Swan Estuary are taken by recreational fishers.
From 2000 to 2004, the commercial fishery in the Swan Estuary had 4 licensees. In these years,
the commercial fishery retained about 15 species per year. The main species in commercial
landings were blue swimmer crabs, sea mullet, Perth herring, black bream, yellow-eye mullet and
tailor. Minor commercial species included yellowfin whiting, Australian herring, mulloway, bar-tail
flathead, small-tooth flounder, yellowtail scad, tarwhine and yellowtail trumpeter.
Recreational fishery landings in the Swan Estuary are more diverse than commercial landings,
although there were also about 15 common species retained in recent catches (not including
blowfish which is the most commonly caught species). The main species in recreational landings
were blue swimmer crabs, black bream, Australian herring, tailor and bar-tail flathead. Minor
recreational species included yellowfin whiting, small-tooth flounder, tarwhine, yellowtail trumpeter,
six-lined trumpeter, garfish, mulloway, cobbler, yellowtail scad, pink snapper and trevally. Many of
the yellow eye-mullet, mulloway and yellowtail scad retained by recreational fishers are juveniles.
Limited data is available on the discarded component of the recreational catch, but it is likely to
include juveniles of various other species including trevally, tailor and black bream.
Although many species are common to both recreational and commercial landings, there appear
to be few sources of conflict over the current allocation of fishery resources in the estuary. The
vast majority of the recent commercial catch comprised crabs, sea mullet, Perth herring and
black bream. Of these species, only black bream and crabs are targeted by recreational fishers.
Since 1990, the catch rate of black bream has increased and the catch rate of crabs has been
stable which suggests that availability of these species to each sector is adequate.
Commercial and recreational fishery catch trends suggest marked changes in the abundance of
numerous fish species in the Swan Estuary since 1990 or earlier. The annual commercial finfish
catch steadily declined between 1975 and 2000. Catch declines are partly due to declines in effort
and declines in targeting of certain species. However, environmental factors and fishing pressure
Fisheries Research Report [Western Australia] No. 156, 2006
are also likely to have impacted on stocks and led to real declines in fish abundance in the Swan
Estuary. After 1990 the total annual finfish commercial CPUE declined, the species composition of
the finfish catch changed considerably and crabs became the main focus of the fishery.
Commercial and recreational fishery catch trends suggest that cobbler, Western school prawn and
Perth herring stocks in the Swan Estuary have declined significantly over a period of decades.
Sea mullet, yellow-eye mullet, Australian herring and yellowtail trumpeter may also have declined
in abundance in the estuary.
Fishery catch trends suggest that blue swimmer crabs, black bream, tarwhine, yellowtail scad
and blowfish have increased in abundance in the Swan Estuary since 1990. Bar-tail flathead,
tailor, flounder and six-lined trumpeter appear to have experienced stable or fluctuating long-term
abundance. The catch rates of these latter three species, and also Australian herring, appear to
be influenced by strong fluctuations in recruitment.
The impact of environmental factors are likely to be at least as significant as the impact of
fishing pressure on stock abundances in the Swan Estuary. The estuary and its catchment have
been highly modified since European settlement. Often the ecological impacts of these changes
are unclear, but it is reasonable to assume that widespread losses of aquatic habitat, the input
of contaminants, reduced freshwater flows and deepening of the estuary mouth have altered
the composition of the fish community and been detrimental to numerous estuarine-dependant
species. A reduction in water quality has caused many recent ‘fish kills’ and probably also had
other less obvious impacts on fish.
In 2003, a persistent bloom of the toxic dinoflagellate, Karlodinium micrum, resulted in widespread
kills of fish and invertebrates in the upper Swan Estuary. This was the largest recorded fish kill
in the estuary. Affected species included black bream, Perth herring, Swan River goby, flathead,
mulloway, flounder and mullet. Anecdotal reports suggested that various juvenile fish, crabs,
prawns and bait worms were also affected. In 2004 and 2005, K. micrum blooms again caused
fish kills in the estuary, suggesting that these may be regular events in the near future.
Fish kills in the Swan Estuary have tended to occur during summer/autumn in the upper estuary.
Therefore, fish that typically occur in this season/location are at highest risk. Fish kills are likely
to impact most severely on species that exist as discrete, self-replenishing populations within the
estuary and spend a significant part of their life cycle (including eggs, larvae, juveniles and/or
adults) in the upper estuary. Species that aggregate to breed in the upper estuary in summer/
autumn are particularly vulnerable, including black bream, Perth herring, bar-tailed flathead and
yellowtail trumpeter. Other species are likely to be indirectly affected through loss of prey and
forced redistribution within the estuary.
Historically, assessments of fish stocks in the Swan Estuary have relied on catch and effort
data from the commercial fishery. A significant reduction in the number of commercial fishers
has reduced the amount of annual data now available, although the fishery still yields valuable
information for stock assessments of some species. Cessation of commercial fishing in the
estuary in the near future would be problematic because an alternative, ongoing data source
is yet to be established. In future, annual data on the relative abundance and size structure of
many stocks could be provided by recreational fishers, from either angling club catch records or
angler logbooks. Such data is complementary to creel/phone survey estimates of total catch and
effort in the estuary.
Future monitoring of fish communities in the Swan Estuary should include a combination of
methods including annual fishery catch and effort data, regular recreational fishery surveys and
occasional fishery-independent surveys. Independent surveys would be a desirable component of
any future monitoring strategy, providing validation of fishery data and also collecting information
about juveniles of target species and the status of non-fishery species, which are important to
the ecological function of the estuary.
Fisheries Research Report [Western Australia] No. 156, 2006
1.0
Introduction
Assessments of fish stocks in the Swan-Canning Estuary (hereafter Swan Estuary) have
historically relied on catch and effort data from the commercial fishery. However, there
have been very significant changes in recent decades that have made commercial data less
useful for this purpose. Since 1975, commercial catch and effort levels in the estuary have
steadily declined, while recreational catches have increased. Recreational catches of many
species now exceed commercial catches of the same species. Although data from the
commercial fishery are still valuable because they provide a long time series of information
and are the only available index of abundance for some species, the commercial catch is no
longer representative of total fishery landings and is no longer the major source of fishing
mortality in the Swan Estuary. Recreational catch and effort data must now be included in
stock assessments in the Swan Estuary.
Recreational catch and effort data in this estuary have mainly become available since the
mid-1990s and a significant amount of recreational data is now available from various
sources (Malseed and Sumner 2001, Henry and Lyle 2003, this report). The inclusion of
recreational data in assessments not only provides a more representative picture of total
catch, but also overcomes some of the historical limitations of commercial data, such as
being restricted to the middle estuary (the commercial fishery area) and being restricted to
commercial target species.
In the interpretation of recreational and commercial fishery catch trends, consideration must
be given to the impact of environmental factors, which are likely to be at least as significant as
the impact of fishing pressure in the Swan Estuary. The estuary and its catchment have been
highly modified since European settlement. Unfortunately, the lack of long-term biological
monitoring in the estuary makes it very difficult to separate the impacts of fishing and nonfishing factors on fish stocks. Many physical changes (e.g. dredging) are well documented in
the historical literature (e.g. Riggert 1978), although the ecological impacts of these changes
are less clear. It is reasonable to assume that widespread losses of aquatic habitat, the input
of contaminants, reduced freshwater flows and deepening of the estuary mouth have altered
the composition of the fish community and been detrimental to numerous estuarine-dependant
species. A reduction in water quality has caused many ‘fish kills’ over a period of decades and
probably also had other less obvious impacts on fish.
Section 2 of this report summarises some of the environmental factors that are likely to have
impacted on fish stocks in the Swan Estuary in recent decades. Eutrophic conditions in the
estuary have caused numerous algal blooms and several major fish kills recently and so
this report focuses particularly on the impacts of these factors on fish stocks. Sections 3-5
summarise catch and effort data from the commercial and recreational fisheries of the estuary,
with an emphasis on data collected since 1975. Some of these data have not previously been
reported. A summary of catch and effort prior to 1975 can be found in Lenanton (1978, 1984).
Section 6 includes an assessment of the status of the key fishery stocks in the Swan Estuary,
using environmental and biological information together with fishery catch and effort trends.
Fisheries Research Report [Western Australia] No. 156, 2006
2.0
Environmental factors affecting fish in the
Swan-Canning Estuary
2.1 Environmental changes since European settlement
The Swan-Avon River drains the largest catchment in Western Australia (~121,000 km2) (Pen
1999). The ‘estuary’ (i.e. tidal sections) occupies an area of approximately 53 km2 (SRT
2001).
The Swan Estuary and its catchment have been highly modified since European settlement in
1829. Upstream of the estuary, much of the catchment and freshwater environments are now in
poor condition. ‘River training’ (straightening and dredging of channels) along 187 km of the
Avon River was conducted from the mid-1950s to early 1970s to increase maximum flow rates
and reduce flooding (Harris 1996). As a result, much sediment was mobilised and fish habitat
was lost (instream and bank vegetation was cleared, woody debris was removed, river pools
filled with sediment). The Avon catchment is ~70% cleared. This has led to rising groundwater,
increased runoff, higher maximum flows, sedimentation, eutrophication and salinisation. Most
sections of the Avon River and its tributaries are now brackish or saline (Pen 1999).
Freshwater input to the Swan Estuary has been dramatically reduced since settlement. In
particular, Mundaring Weir (Helena River) was completed in 1902 and the Canning Dam was
completed in 1940. Climate change has further reduced freshwater input to the estuary, with a
marked reduction in annual rainfall since the 1970s. The average annual inflow to Perth dams
from 1975 to 2003 was equal to only 49% of the average inflow from 1911 to 1974 (Water
Corporation data). In future, the pressure to divert surface and groundwater resources away
from the river is likely to increase with human population growth and continuing low rainfall.
In addition to reduced rainfall and diversion of flows, both of which have reduced freshwater
input, entrance modifications have acted to create a more ‘marine’ (saline) environment within
the estuary. Increased flushing and tidal range resulted after a limestone bar at the entrance was
removed to construct Fremantle Harbour in 1897. The entrance is now dredged to a depth of
13 m, although the original entrance depth was only 2 m. For >100 years, a railway bridge at
North Fremantle inadvertently functioned as a weir, restricting outflow, but this was removed
in 1968 (Riggert 1978). Before removal of the bar, most of the estuary was fresh or brackish
(Thomson et al. 2001).
The Swan Estuary has been affected by various contaminants including pesticides, petroleum
products and heavy metals. Heavy metals tend to remain in sediments for long periods and
also bioaccumulate (Gerritze et al. 1998). Past and present sources of metal contamination
include urban and agricultural runoff, rubbish tip leachate and antifouling paints (Riggert
1978). Recently, the disturbance of acid-sulphate soils along the foreshore of the upper
estuary has emerged as another potential source of heavy metal contamination (Department of
Environment 2004).
Eutrophication is a major environmental problem in the Swan Estuary and has been occurring
since settlement. Past and present anthropogenic sources of nutrients include seepage from
foreshore rubbish tips and septic tanks, urban stormwater runoff, nutrients from agricultural
and urban fertilisers, effluent from tanneries, abattoirs, breweries, laundries and fertiliser
factories (Riggert 1978). Treated sewage effluent was discharged from Burswood Island from
1912 to 1936. The symptoms of eutrophication (poor water quality, macroalgal blooms) were
Fisheries Research Report [Western Australia] No. 156, 2006
most evident during the 1950s and 60s and have since lessened. However, nutrient input
remains relatively high and blooms of phytoplankton, macroalgae and exotic weeds continue
to occur. The Ellen Brook catchment is currently one of the major sources of nutrients to the
estuary (Gerritse et al. 1998, SRT 2000a).
The problem of eutrophication has been exacerbated by the loss of foreshore and aquatic
vegetation, which would otherwise absorb some of the nutrients. Approximately half of
the foreshore of the middle estuary has been altered (Riggert 1978). Extensive areas of
mudflats along the shoreline have been reclaimed; creeks and swamps have been replaced by
channels and pipes; many areas have been dredged to improve navigation or provide spoil for
reclamation; wash from boat traffic has eroded river banks; many retaining walls have been
built (mostly prior to the 1940s) to reclaim land, build roads, bridges, etc. All of these works
have contributed to a loss of natural vegetation and fish habitat.
2.2 Phytoplankton blooms and fish kills
Although eutrophication and algal blooms are not the only environmental threats to fish in the
Swan Estuary, special consideration is given to these issues here because of the potentially
large and immediate impacts on some stocks from associated fish kills. Recent changes in
catchment management have attempted to reduce nutrient input to the catchment but, even if
successful, eutrophic conditions in the estuary are likely to continue for decades. Therefore,
major algal blooms and fish kills, such as occurred in 2003, are likely to reoccur.
Bloom-forming species of phytoplankton, including green algae, diatoms, dinoflagellates and
blue-green algae, are typically present in the river at low densities and only form blooms under
favourable conditions. Some species bloom predictably on a seasonal basis, while others tend
to bloom only occasionally. Not all blooms are harmful to fish or other organisms.
Nuisance blooms (and resultant fish kills) tend to occur shortly after storm events in summer or
autumn, when organic material is transported from the catchment into the estuary. This creates
favourable bloom conditions, i.e. high nutrients, warm water and low flow. The intensity and
frequency of nuisance blooms and fish kills may have increased in recent decades due to the
input of anthropogenically-derived nutrients from the catchment. However, comprehensive
records of the timing and magnitude of fish kills in south-western Australian estuaries were not
kept until quite recently (T. Rose, Department of Environment, pers. comm.). Therefore, the
cause of previous fish kills and trends in frequency are unclear.
Asphyxiation is the typical cause of fish deaths during a bloom, via several mechanisms
including i) anoxic or hypoxic conditions arising from consumption of dissolved oxygen
during bloom development and/or decomposition; ii) physical clogging of gills by algal cells;
iii) damage to gill surfaces by toxins, particularly those produced by dinoflagellates and bluegreen algae.
To date, no comprehensive, quantitative surveys of fish kills have been conducted in the Swan
Estuary. The available data about recent fish kills focuses on larger species of fish, with little
data available to assess impacts on small fish or invertebrates. Lack of data should not be
interpreted as evidence for a lack of impact. Anoxic conditions and algal toxins, which are
common causes of local fish kills, are likely to kill a wide range of gill breathing organisms
including many small fish and invertebrates. For example, a quantitative survey of a fish kill
in the nearby Serpentine River (Peel-Harvey Estuary) in February 2004, found that anoxic
Fisheries Research Report [Western Australia] No. 156, 2006
conditions killed at least 17 species of finfish and at least 1 species of crustacean. Dead finfish
ranged from <10 mm to >400 mm in length (Smith et al. 2004).
A summary of recorded fish kills associated with algal blooms in the upper Swan Estuary, from
1978 to 1994, can be found in Hosja and Deeley (1994). A summary of more recent fish kills
is listed in Table 1. The main causes of fish kills have been algal blooms and chemical spills.
Regardless of cause, major fish kills have tended to occur during summer/autumn (January to
May) in the upper estuary (i.e. upstream of Heirisson Island in the Swan River and in the upper
Canning River) (Fig. 1). Therefore, this season/location has the highest likelihood of future
kills and species that occur here are at the highest risk.
2.3 Karlodinium micrum bloom and fish kill, April-June 2003
From April to June 2003, a persistent bloom of the toxic dinoflagellate, Karlodinium micrum,
resulted in widespread kills of fish and invertebrates in the upper Swan Estuary. To date, this
is the largest recorded fish kill in the estuary.
K. micrum is a marine species well adapted to estuarine conditions. Cells possess a flagella
(tail-like structure) that assists them to migrate vertically through the water column. This
allows them to exploit optimum conditions of light, temperature, salinity and nutrients at
various depths. K. micrum photosynthesises, and so uses sunlight to convert nutrients to
energy. When nutrients (mainly N or P) are in short supply, it can also prey on other algal
species to obtain energy. K. micrum may also produce a toxin that prevents other algal species
from consuming it. The environmental cue that prompts toxin production is unclear.
K. micrum blooms have been recorded at various locations around the world and are characterised
by an oily scum floating on the surface and a fishy smell. Under bloom conditions, K. micrum
is harmful to fish because cells clog the gills and the toxin destroys blood vessels, especially
in the gills. Both effects result in asphyxiation of fish. In smaller fish, other areas of the body
are also damaged by the toxin. K. micrum is likely to also affect other gill-breathing organisms
such as crustaceans and molluscs.
K. micrum was not detected in the Swan Estuary until 2003. However, this species may
previously have been present in the estuary at low levels, not detectable in routine phytoplankton
monitoring. In April 2003, a K. micrum bloom commenced in the Maylands area of the estuary.
It spread upstream and also spread downstream to Perth Water and the Canning River. Unusual
background conditions contributed to the bloom – several rainfall events between February and
May transported pulses of nutrients (particularly soluble oxidised nitrogen) into the river. A
large rainfall event in April triggered the bloom. A period of calm, sunny weather from April to
June provided ideal conditions for algal growth. May was unusually warm. The bloom ended
in mid-June, when several weeks of rain reduced salinity levels in the estuary and caused the
bloom to collapse. K. micrum has a low tolerance to freshwater.
During the 2003 bloom, an estimated 200,000 individuals of black bream (Acanthopagrus
butcheri) were collected by the Swan River Trust Cleanup Program. Public reports about
the fish kill focused on black bream because individuals are relatively large, conspicuous and
of high fishery value, but other species were also affected. Large numbers of Perth herring
(Nematolosa vlaminghi) and Swan River gobies (Pseudogobius olorum), and smaller numbers
of bar-tailed flathead (Platycephalus endrachtensis), mulloway (Argyrosomus japonicus),
small-toothed flounder (Pseudorhombus jenynsii) and mullet (Mugilidae) were reported dead
10
Fisheries Research Report [Western Australia] No. 156, 2006
but not collected. Anecdotal reports suggested that juvenile fish, crabs, prawns and bait worms
were also killed. A total of 7.8 t of dead fish were collected during the clean up operation.
This figure is an underestimate of the total quantity of fish affected, because only those fish
that floated to the surface or to the sides of the river were collected. Significant quantities may
have remained in deeper water, or been consumed by scavenging birds and crabs. Also, small
fish are likely to have decomposed before the opportunity arose to collect them. Therefore,
semi-quantitative data collected during the clean up operation provided an underestimate of the
total species and number of fish affected.
Annual blooms of K. micrum in the Swan Estuary and regular fish kills now appear likely, as
evidenced by the high cell densities of K. micrum that were recorded in summer/autumn of
2004 and 2005 (Swan River Trust data). Cell densities in 2004 and 2005 were higher than in
2003 and, although a toxic bloom did not form in these years, fish kills still occurred due to
de-oxygenation of the water.
Other south-west estuaries are also at risk of regular fish kills due to K. micrum. A bloom of
K. micrum was responsible for fish deaths in the Collie River in early June 2003, at
approximately the same time as the bloom in the upper Swan Estuary.
Many fish species are likely to be directly or indirectly impacted by K. micrum blooms.
Mortality of prey species may subsequently reduce food availability for other species not
directly killed by the bloom. Alternatively, mortality of predators may benefit some prey
species. Also, many fish swim away from, or avoid, areas of high K. micrum cell densities
(F. Valesini, Murdoch University, unpubl. data). Therefore, phytoplankton blooms not resulting
in fish kills may still impact on fish and fisheries. For example, fishery catch rates in bloomaffected areas may be reduced while catch rates in unaffected areas may be elevated.
2.4 Seasonal patterns in water quality and phytoplankton
A major factor affecting the abundance and distribution of phytoplankton in the Swan Estuary,
and also the migration and distribution patterns of many fish, is the movement of a salt wedge,
which underlies fresh surface waters and extends furthest upstream in summer (Kurup et al.
1998). Coinciding with the movement of the salt wedge are seasonal changes in river flow,
temperature, salinity, turbidity and nutrient levels.
Water quality parameters and phytoplankton abundance are measured monthly by the Swan
River Trust (SRT) (SRT 2000b). Seasonal patterns in nutrients, water quality and phytoplankton
in the Swan Estuary are described by Hosja and Deeley (1994), Gerritse (1999) and SRT
(2000c), and are summarised below.
Middle and upper estuary. In winter, high rainfall washes organic material from the
catchment into the estuary, which results in fresh, surface waters containing elevated levels of
nitrogen. Biologically-available phosphorus is low at this time. Phosphorus tends to bind to
suspended particles and sink to the bottom (from where it is released in summer). In winter,
low temperatures, low light levels and high rates of flushing prevent phytoplankton blooms
from establishing, despite high nitrogen levels. Low light levels are due to short day lengths
and high turbidity, mainly due to tannins in the water.
In spring, rainfall and river flushing rates decrease, and temperatures rise. Under these
conditions, the elevated levels of nitrogen that still persist in the system facilitate phytoplankton
blooms. Spring blooms tend to occur as a succession of blooms by different species. The
Fisheries Research Report [Western Australia] No. 156, 2006
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decomposition of one bloom releases nutrients back into the water and facilitates a new bloom
by a different phytoplankton species. Spring blooms of green algae typically occur in the
middle and upper estuary in October-November. Spring blooms are often significant in scale,
but are part of the natural cycle in the system and are typically harmless.
In response to declining river flow, the location of the salt wedge begins to move upstream
in spring and penetrates furthest upstream in summer. This saline water underlies a shallow,
fresh, surface layer in the middle and upper estuary. There is limited vertical mixing between
layers. The saline water is typically low in oxygen and high in nutrients. Biological activity
(from bacteria, etc) rapidly removes oxygen from bottom waters, resulting in hypoxic or anoxic
conditions. These conditions cause the release of ammonium and soluble phosphate from
sediments, further increasing nutrient levels in bottom waters of the middle and upper estuary.
Under such stratified conditions, blooms of dinoflagellates can occur because these cells can
migrate vertically between the oxygen-rich surface layer and the nutrient-rich bottom layer.
‘Summer’ conditions prevail in the Swan Estuary until the onset of autumn rains, the timing and
magnitude of which can vary among years. Nuisance blooms, especially by dinoflagellates, are
most likely to occur under ‘summer’ conditions and are generally restricted to the middle and
upper estuary. During summer and early autumn, warm water and high light levels are ideal for
rapid algal growth, although bloom events will not occur if nutrients are limited. Episodic rainfall
events at this time can add a pulse of nutrients to the system and allow a phytoplankton bloom to
occur. A bloom is more likely to occur if a rainfall event in summer/autumn is followed by a dry
period. If rains fall over an extended period, a bloom is unlikely to develop because increased
flows will flush the river and prevent blooms from establishing. Blooms in summer/autumn are
unpredictable, due to the erratic timing of rainfall and nutrient input during that period.
Lower estuary. Phytoplankton and nutrient cycles in the lower estuary are distinct from the
middle and upper estuary. The lower estuary system is dominated by marine species and
tends to be nitrogen limited. Spring blooms occur earlier in the lower estuary, from August
to September. During summer/autumn, the lower estuary is generally well flushed by low
nutrient, ocean water. Phytoplankton growth occurs at these times, but generally does not
result in a bloom. Occasional minor algal blooms can occur when summer or autumn rain
events wash a pulse of nutrients (especially nitrogen) into the lower estuary.
3.0
Swan Estuary commercial fishery
3.1 Early years of fishing
‘Commercial’ fishing in the Swan Estuary undoubtedly commenced shortly after European
settlement in 1829. Unfortunately, annual records of commercial catch and effort are
incomplete prior to 1939 and so little is known of the catch in these early years. A significant
quantity of locally caught fish was presumably needed to supply a Perth population of almost
30,000 in 1880 and almost 50,000 by 1890. Also, the proclamation of the first Fisheries Act
and the issue of the first fishing licence in 1899 suggest that considerable fishing effort was
occurring by this time (Lenanton 1978, 1984). From 1899 until 1940, any person who caught
fish for sale or used a seine net to catch fish was required to hold a licence. From 19401949, any person catching fish by any type of net was required to hold a licence. However,
the distinction between commercial and recreational fishing was not made until 1949, when
separate ‘professional’ and ‘amateur’ licences were introduced.
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3.2 Commercial fishery catch and effort statistics
Historically, catch and effort statistics supplied by commercial fishers via compulsory monthly
returns have been the main (and often only) source of information for fish stock assessments
in the Swan Estuary. Commercial fishery catch and effort data are valuable for this
purpose because they are long-term, continuous time-series. At the same time, the utility
of commercial fishery catch and effort data to provide an index of fish abundance is limited
by several factors, which are outlined below. Catch trends must always be interpreted with
consideration of these factors.
Changes in market demand for key target species have had a major influence on catch levels
in the fishery. Peaks in catch and effort in the estuary occurred around 1920 and again around
1940 to alleviate local food shortages after World War 1 and during World War 2, respectively
(Fig. 2). At certain times during 1919, up to 130 men (mainly returned soldiers) were engaged
in commercial fishing (Lenanton 1978). Many fish from the Swan Estuary (and adjacent
Peel-Harvey Estuary) were canned or smoked for human consumption, a practice that
occurred from the late 1800s until the 1950s (Lenanton 1978, 1984). The demand for canned
fish (especially Perth herring, but also sea mullet (Mugil cephalus), yellow-eye mullet
(Aldrichetta forsteri), garfish (Hemiramphidae), pilchards (Sardinops sagax) and cobbler
(Cnidoglanis macrocephalus) was greatest in the 1940s. During the 1960s and 1970s, Perth
herring, sea mullet and yellow-eye mullet were targeted to supply the western rock lobster
fishery with local bait, and were the dominant species in the Swan Estuary catch in these
decades. During the 1980s, markets for these species as bait or for human consumption
declined. Perth herring is no longer caught for human consumption and now supplies a minor
recreational bait market only.
After 1969 the Department of Fisheries (DoF) adopted a policy to i) not issue any new
commercial licences and ii) limit transferability of licences. In subsequent years, a ‘Voluntary
Fishery Adjustment Scheme’ (VFAS) significantly reduced fishing effort in the Swan Estuary
commercial fishery. The number of registered vessels declined from approximately 30 vessels
operating annually throughout the 1960s and 1970s to only 4 vessels operating per year from
2000 to 2004. The departure of certain fishers (especially those that fished part-time) from
the fishery and the greatly reduced competition between fishers may have altered the overall
targeting practices of the fishery, leading to a change in the composition of the catch.
Various changes in formatting of compulsory monthly fishing returns are likely to have altered
the quality and quantity of reported catch and/or effort data. Significant changes in the format
of compulsory monthly fishing returns occurred in July 1989 (Appendices 1, 2). New returns
provided additional data about fishing effort and location. Various extra minor changes were
made to the format of returns after 1989.
Compulsory monthly commercial catch and effort statistics from July 1975 onwards are held
in the Department of Fisheries Catch and effort statistics (CAES) database. Catches of each
species in the CAES database are recorded as both “landed weight” and “live weight”. In the
Swan Estuary commercial fishery, there is negligible difference between total annual “landed”
and “live” weights, indicating that most species are landed whole (Fig. 3).
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3.3 Commercial fishing effort and cpue
Currently, the following 4 measures of monthly commercial fishing effort can be calculated
from compulsory monthly catch and effort returns:
1) Number of boats = the total number of boats registered in the fishery. This is the only type
of effort data consistently available between 1912 and 1976. However, it may overestimate
actual effort expended because it includes inactive boats.
2)Mean monthly number of active boats = the average number of boats that fished (i.e.
recorded some catch) per month.
3) Fishing day = a day spent fishing, regardless of what gear type or how many gear types were
used on that day. This may underestimate actual effort expended, particularly when multiple
gear types are fishing simultaneously.
4) Gear day = a day spent using a particular gear type. The number of gear days may exceed
the number of fishing days per month because a fisher may use >1 gear type per day.
In general, catch-per-unit-effort (CPUE) derived from any of the above units of effort does
not provide a precise index of abundance for any species because actual (real) effort cannot be
determined. Also, monthly catch returns cannot be used to determine the actual effort used to
target each species, due to the multi-species nature of the fishery. This can be illustrated with
a hypothetical example – in January 2003, a fisher reported 20 days of gill netting and a total
catch of 500 kg of black bream and 200 kg of sea mullet. From this limited data, the number
of days spent specifically targeting each species cannot be determined. In the absence of any
other information, 20 days of effort could be (and usually is) allocated to each species, and
the estimated monthly catch rates would be (500 ÷20 =) 25 kg/day and (200 ÷20 =) 10 kg/day,
respectively. However, if 19 days was actually spent targeting bream and only 1 day spent
targeting mullet, then the actual catch rates would be (500 ÷19 =) 26 kg/day and (200 ÷1 =)
200 kg/day, respectively. This problem with the measurement of actual effort may be largely
overcome in future by implementation of a daily or trip logbook.
Not withstanding these problems, commercial CPUE is the best available index of abundance
for many species and is still widely used as an indicator in stock assessments.
All of the above measures of effort follow similar annual trends (Fig. 4a). However, there are
minor differences and the choice of effort used to calculate CPUE has an effect on the trend in
CPUE, especially after 1989 (Fig. 4b). For example, CPUE calculated from “kg/gear day” and
“kg/fishing day” both suggest a decline in catch rate since 1989. By contrast, CPUE calculated
from “kg/registered boat” or “kg/active boat” suggests that catch rate has been stable since
1990. In summary, an assessment of the status of the fishery based on CPUE trends in recent
years is most optimistic if CPUE is estimated by “kg/ registered boat” and least optimistic if
CPUE is estimated by “kg/ gear day”. This report uses “kg/ gear day” to estimate CPUE after
1975 because ‘gear day’ is considered to be the best available approximation of actual effort.
3.4 Seasonality of commercial landings
The commercial catch of each key target species in the estuary is highly seasonal (Fig. 5).
Environmental factors (e.g. fluctuations in salinity) and behavioural factors (e.g. spawning
migrations) affect the distribution of fish in the estuary and determine their abundance (and
hence availability) within the commercial fishery zone. Preferential targeting can also result in
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seasonal catches - lower value species may only be targeted during months when higher value
species are not available. These factors mean that monthly trends in commercial catch rates of
particular species do not always reflect trends in overall estuarine abundance.
Commercial landings of Perth herring, Australian herring (Arripis georgianus), tailor
(Pomatomus saltatrix), bar-tailed flathead, whiting (Sillago schombergkii), small-toothed
flounder and sea mullet typically peak in spring (Fig. 5). Cobbler and black bream are mostly
caught in winter. Yellow-eye mullet is mostly caught in autumn/winter. Yellowtail scad and
blue swimmer crab are mostly caught in summer/autumn.
3.5 Commercial catches from 1921 to 1974
Incomplete records of annual catch in the Swan Estuary commercial fishery are available for
the period 1912 to 1939. Annual landings appear to have been relatively high in these years, as
suggested by a total catch of 252 t (including 227 t of finfish) that was recorded in 1921 (Fig.
2). Notably, historic peaks in annual landings of yellow-eye mullet and tailor of 44 t and 16 t,
respectively, were recorded in 1921 (Fig. 6).
From 1939 to 1960, landings were dominated by finfish, but also contained significant quantities
of blue swimmer crabs (Portunus pelagicus) and prawns (Metapeneas dalli and Penaeus
latisulcatus). Over this period, annual catches of finfish, crabs and prawns varied markedly but
averaged 58 t, 11 t, 3 t per year, respectively. The total fishery catch was dominated by Perth
herring from 1942 to 1946 and by sea mullet from 1947 to 1959 (Fig. 6).
After 1959, the catches of finfish increased dramatically and peaked at 322 t in 1973 (Fig.
2). The increase was mainly due to landings of Perth herring, although sea mullet, cobbler
and yellow-eye mullet were also abundant in the catch (Fig. 6). Peak catches of Perth herring
occurred in 1968 (178 t), 1972 (147 t), 1973 (159 t) and 1975 (138 t). Sea mullet landings
averaged 61 t per year from 1960 to 1974, including an historic peak of 116 t in 1961. Cobbler
landings averaged 33 t per year from 1960 to 1974, including peaks of 57 t in 1960 and 50 t in
1970. Yellow-eye mullet landings averaged 15 t per year from 1960 to 1974, including peaks
of 29 t in both 1960 and 1970. These values were similar to a later peak in yellow-eye mullet
landings of 30 t, which occurred in 1988 (Fig. 6).
From 1939 to 1974, tailor, bar-tailed flathead, yellowtail trumpeter (Amniataba caudavitatta)
and mulloway also made small but significant contributions to the total commercial catch in
some years. Annual landings of tailor were <5 t, except for landings of 5-9 t per year in 194351 and in 1961. Annual landings of flathead averaged 2 t, with a peak of 5 t in 1945. Annual
landings of yellowtail trumpeter averaged 1 t, with a peak of 7 t in 1964. Mulloway incurred a
period of relatively high annual commercial catches (averaging 2.7 t) from 1963 to 1986 (Fig.
6). However, after peaking at 8 t in 1965, the mulloway catch and CPUE trend was downward,
with the exception a relatively high catch in 1975.
From 1939 to 1974, annual crab catches fluctuated greatly (ranging from <1 t to 24 t), although
the overall trend was stable (Fig. 2). In contrast, prawn catches occurred within two main
periods, with peaks of 11 t in 1948 and 14 t in 1959, respectively. After 1959, prawn landings
declined. The last significant commercial catch (3 t) of prawns was recorded in 1975.
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3.6 Commercial catches from 1975 to 2004
From 1975 to 2004, six species comprised the majority of annual commercial landings, namely
Perth herring, sea mullet, yellow-eye mullet, cobbler, black bream and blue swimmer crabs.
In this period, annual landings of tailor, bar-tailed flathead, yellowtail trumpeter and mulloway
were relatively low (mostly <5 t each) compared with landings of these species prior to 1975
(Fig. 6). Minor catches of small-toothed flounder, white bait (Hyperlophus vittatus) and
Australian herring were also taken between 1975 and 1999.
From 1975 to 2004, the total annual catch of the fishery declined significantly due to a steadily
decline in finfish landings over this period (Figs. 7, 8). For example, finfish landings from
1995-99 equalled only 27% (by weight) of the landings from 1975-79.
Between 1975 and 2004, annual landings of each major finfish target species declined
markedly. The annual catch of Perth herring declined from 137 t to <10 t, sea mullet declined
from 54 t to <5 t, yellow-eye mullet declined from 24 t to <5 t, and cobbler declined from 31
t to <100 kg.
Between 1975 and 1999, annual effort also declined due to a reduction in the number of
licensees (via a voluntary fishery adjustment scheme) (Fig. 7). As a result, the total annual
CPUE increased from 1975 to 1990 but then declined gradually after 1990 (Fig. 7). This
downward trend was driven by the catch of finfish. The annual finfish CPUE steadily declined
from ~70 kg/gear day in 1990 to ~40 kg/gear day in 1999, despite the reduction in fishing effort
(Fig. 8). In particular, the annual CPUEs of Perth herring, yellow-eye mullet, small-toothed
flounder and cobbler each declined in this period.
The only major finfish species to exhibit an increase in annual catch and CPUE after 1990 was
black bream (Fig. 6). Annual catches of black bream increased markedly between 1980 and
1999, despite sharply declining effort levels over this period.
Between 1975 and 2004, blue swimmer crabs became an increasingly important component
of the total commercial fishery catch. In contrast to finfish trends, annual landings of crabs
were relatively stable from 1975 to 2004 and the crab CPUE increased markedly after 1990
(Fig. 8). Crab CPUE showed a strong inverse relationship with effort, i.e. CPUE increased as
effort decreased. Crab CPUE rose from approximately 5kg/day in the 1980s to approximately
25 kg/day in the late 1990s.
From 1975 to 1990, crabs contributed approximately 10% (by weight) to total annual landings,
but this proportion increased significantly during the 1990s and was approximately 40% in
1999. Since 1975, annual crab catches have ranged from 4 to 35 t. Catches averaged 19 t from
1995 to 2004 (Table 2).
The period from 2000 to 2004 represented a period of stable commercial effort levels in the
Swan Estuary, following many years of declining numbers of licensees (Fig. 7). From 2000 to
2004, four licensees operated in the fishery.
In 2004, crabs contributed 41% to the weight and 58% to the value of the total annual catch.
The finfish catch contained about 15 taxa although ~90% of the weight and value of the total
finfish catch comprised only 7 species (sea mullet, Perth herring, black bream, yellowfin
whiting, yellowtail trumpeter, yellow-eye mullet and tarwhine (Rhabdosargus sarba).
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4.0 Swan Estuary recreational fishery
4.1 Recreational fishery catch and effort statistics
Recreational anglers and netters were reported to have been very active in the Swan Estuary
as early as 1912 (Lenanton 1978). Even in the early years of the fishery, the total recreational
catch was substantial and possibly exceeded the commercial catch in some years. Lenanton
(1978) suggested that the recreational crab catch was equal to the commercial catch in the
1970s.
Estimates of annual recreational catch in the Swan Estuary are not available prior to the late
1990s, although the number of recreational netting licences issued by the Department of
Fisheries gives some indication of earlier effort levels. From 1971 to 1976, the total number
of recreational net licences issued at Perth and Fremantle increased from 5,624 to 11,948
(Lenanton 1978). At this time, the primary target species of net fishers were prawns (50-60%
of fishers), finfish (25%) and rock lobster. This licenced component of the recreational catch
was probably much less than the unlicenced component, which comprised line-caught finfish
and crabs (Lenanton 1978).
Recreational catch and effort levels are likely to have increased significantly since the 1970s, in
proportion to the growth of Perth’s human population from 832,760 in 1976 to >1.4 million in
2004. Available data suggests that recreational catches of many species exceeded commercial
catches of the same species in recent years.
The following four major sources of data on the recreational fishery are currently available:
1)A creel survey was conducted in the lower Swan Estuary in 1998-99 (Malseed and Sumner
2001).
2)A national phone survey collected data from the entire Swan Estuary in 2000-01 (Henry and
Lyle 2003).
3)‘Swanfish’ is a recreational angling tournament hosted by the Melville Amateur Angling
Club (MAAC) and is held annually on the last weekend in February. It is open to the general
public and fishing effort is spread throughout the Swan Estuary. Since 2000, competitors
have been asked to return a ‘catch card’ with numbers of each species retained and discarded
during the event and details of their fishing location.
4)Monthly catch records have been kept by the MAAC since 1986. They are the only
available time-series of recreational catch for the Swan Estuary, including trends in CPUE
and average fish size. Records kept by the club include the number of participating anglers
and the numbers and total weights of each species presented at the ‘weigh-in’ after each
estuary fishing day held by the club.
4.2 Composition of the recreational fishery catch
4.2.1 Creel and phone surveys
The 1998-99 creel survey and the 2000-01 phone survey both indicated that blue swimmer
crab was the species most frequently retained by recreational fishers in the estuary (Table 3).
However, there was a considerable difference in the total annual crab catch estimated by the
two surveys. In 1998-99, an estimated 20,875 crabs (~ 7.3 t) were retained. In 2000-01, an
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estimated 148,341 crabs (~ 33.1 t) were retained. The 2000-01 survey may have over-estimated
boat-based catches of all species (N. Sumner, DoF, pers. comm.). However, the shore-based
component of the catch suggested that at least 17,357 crabs (~3.9 t) were caught in 2000-01
and the 1998-99 survey indicated that the shore-based catch is much smaller than the boatbased catch in the Swan Estuary. Also, recent annual landings of crabs by recreational fishers
are believed to be considerably larger than recent commercial landings (~ 16.5 t, in Table 2)
(L. Bellchambers, DoF, pers. comm.). Therefore, a total estimated crab catch of 33.1 t in 200001 is not unrealistic.
The 1998-99 survey was focused on crabs (hence its location in the lower estuary) and provided
incomplete data about finfish catches (especially catches in the upper estuary). The limited
data from 1998-99 indicated that tailor, whiting, black bream, Australian herring, flathead
and yellowtail trumpeter were the most frequently retained target finfish species in the lower
estuary. Minor quantities of small-toothed flounder, butterfish (Pentapodus vita), tarwhine,
trevally (Pseudocaranx spp.), mulloway, yellow-eye mullet and yellowtail scad (Trachurus
novaezelandiae) were also retained. The 2000-01 survey indicated that black bream was the
main target species throughout the entire estuary. Other major species in the 2000-01 catch
were Australian herring, tailor, whiting and flathead. Minor species in the catch were flounder,
tarwhine, ‘baitfish’, garfish, yellowtail trumpeter, cobbler, mulloway, pink snapper (Pagrus
auratus) and yellowtail scad.
Although not targeted, blowfish (Torquineger pleurogramma) was the most frequently caught
finfish in both surveys, indicating that this species was extremely abundant in the estuary in
1998-99 and 2000-01. A total of 116,079 blowfish were reported in 2000-01.
Catch weight was not measured in either survey but can be estimated from known average
weights of each species. The estimated catch weight of black bream in 2000-01 was 16,273 kg
(= 35,334 fish), which was considerably greater than the reported catch of any other individual
finfish species (Table 3). Some uncertainty is associated with the estimation of boat-based
catches in 2000-01. However, since black bream are mainly caught from the shore and sample
sizes were high, the estimated catch for this species is reasonably reliable.
The weight of the total recreational finfish catch in 2000-01 was estimated to be ~28.2 t, but
there is considerable uncertainty associated with this value due to the likely over-estimation of
boat-based catches.
4.2.2 ‘Swanfish’ fishing tournament
The MAAC have been collecting catch cards from competitors in the annual ‘Swanfish’
tournament since 2000. A high proportion of competitors return their catch cards, partly
because it entitles them to entry into a random prize draw. In 2004, the format of the catch
cards was altered to include slightly more detail about fishing location and more details of
discarded fish, especially blowfish. About 50% of competitors recorded blowfish catches in
2004. Blowfish catches were usually not recorded in previous years.
In 2004, over half of the total reported catch (retained + discarded fish) at the ‘Swanfish’
tournament comprised only 2 species - blowfish (37%) and black bream (21%) (Table 4).
Excluding blowfish, black bream represented 29-60% of the total reported catch each year. On
average, 9 species (black bream, yellowtail trumpeter, flathead, tarwhine, yellowfin whiting,
six-lined trumpeter (Pelates sexlineatus), tailor, flounder, cobbler) comprised 90% of the total
catch each year (excluding blowfish).
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4.2.3 Melville Amateur Angling Club (maac)
From 1986 to 2003, the MAAC catch included approximately 40 taxa of finfish. Twelve species
comprised 90% of the total catch over this period (yellowfin whiting, cobbler, Australian
herring, black bream, yellowtail trumpeter, flathead, yellow-eye mullet, trevally, tailor, sixlined trumpeter, flounder, tarwhine) (Table 5). However, the catch composition differed
considerably among years. In particular, the catches of cobbler, yellow-eye mullet and flounder
progressively declined over this period. From 2000 to 2003, 11 species comprised 90% of the
total catch (Australian herring, black bream, flathead, yellowfin whiting, tarwhine, six-lined
trumpeter, trevally, yellowtail trumpeter, tailor, wrasse/groper, sea garfish (Hyporhamphus
melanochir). Recent MAAC annual catches contained more trumpeter and tarwhine and less
tailor than the overall Swan Estuary recreational catch estimated by the 1998-99 creel survey or
the 2000-01 phone survey. However, despite differences in the proportions of each species in
the total catch, the MAAC data are in reasonable agreement with the creel and phone surveys
as to the main species retained by recreational fishers in the Swan Estuary.
4.3
Maac catch and effort trends, 1986 to 2003
Catches recorded by the MAAC at ‘weigh-in’ events following each Estuary Field Day may
underestimate the total catch per day because some retained fish may not have been included.
Also, discarded fish are not recorded by the Club. In all years, some ‘high-grading’ of catch
may have occurred to maximise competition points. The Club allocates competition points to
members on the basis of the number and weight of fish caught, and the number of species in
the catch. For these reasons, the data from ‘weigh-ins’ may slightly under-estimate the actual
daily catch and slightly over-estimate the average weight of all captured fish. Also, due to
the competitive nature of the club, the MAAC daily catch rates and average fish sizes may be
higher than those for all recreational fishers in the Swan Estuary.
From 1986 to June 1991, the MAAC had a self-imposed bag limit of 2 fish per species per
fisher. From July 1991 onwards the bag limit was increased to 3. After mid-1991, the Club
catch rates of several key species increased and the average size of several key species
decreased, probably as a result of the increase in Club bag limits.
4.3.1 Total catch and effort
From 1986 to 2003, total annual MAAC effort ranged from 414 to 3,560 angler days, while total
annual catch ranged from 112 fish (64 kg) to 1,013 fish (257 kg) (Fig. 9a). After peaking in 1994,
there was a slight downward trend in the average annual CPUE (Fig. 9b). The average size of
retained fish (regardless of species) was anomalously high in 1987 due to two large mulloway in
the catch. The reported number of species in the total catch ranged from 9 to 25 per year, and was
relatively stable after 1991, with approximately 20 species retained per year.
From 1986 to 2003, MAAC Estuary Field Days typically occurred monthly. Monthly catch
and effort tended to peak during warmer months and so the main fishing season each year was
generally summer/autumn.
4.3.2 Catches of key species
Black bream. The MAAC annual CPUE of black bream peaked in 1994 (0.7 fish/angler day)
and again in 2003 (0.5 fish/angler day) (Fig. 10a). The higher catch rates after mid-1991 were
Fisheries Research Report [Western Australia] No. 156, 2006
19
greater than might be expected from the effect of the change in bag limit alone, suggesting an
increase in the abundance of black bream in the estuary since 1992. From 1992 to 2003, the
average size of black bream was stable at approximately 0.5 kg (Fig. 11a).
Tarwhine. From 1992 to 2003, the annual CPUE of tarwhine steadily increased, suggesting a
steady increase in abundance of this species in the estuary over this period. A peak (0.3 kg) in
the average size of retained fish occurred in 2002, followed by a peak in CPUE in 2003 (~0.5
fish/angler day).
Cobbler. The annual CPUE of cobbler steadily declined from 1990 to 2000, despite the
increase in bag limit in mid-1991. The trend almost certainly reflects a severe decline in
abundance in the estuary over this period. Since 2000, MAAC members have chosen to release
any cobbler caught, although the number caught since this time has been minimal. Despite
the change in bag limit, the average size of cobbler increased after 1992 (peaking at ~0.7 kg),
suggesting recruitment failure.
Bar-tailed flathead. From 1992 to 2003, the annual CPUE of flathead fluctuated considerably
although the overall trend was stable. The CPUE tended to be higher (0.6-0.7 fish/angler day)
between 1994 and 2000, suggesting higher abundances during the late 1990s. The average size
of flathead peaked (0.4 kg) in 1999, consistent with higher CPUE and the possibility of a strong
recruitment event around this time.
Small-toothed flounder. The annual CPUE of flounder declined from 1988 to 2003, despite
the increase in bag limit in mid-1991. There was a minor increase in CPUE during the late
1990s. The long-term trend suggests a decline in abundance in the estuary since 1987. CPUE
was particularly low after 2000. Anecdotal reports from MAAC members also suggest that
flounder has been relatively rare in the estuary in recent years. The average size of flounder
was relatively stable across all years (~0.26 kg), except for a slight decrease (minimum ~0.18
kg) in the late 1990s, consistent with higher CPUE and the possibility of a weak recruitment
event at this time.
Australian herring. The annual CPUE of Australian herring fluctuated considerably between
1986 and 2003, with peaks in 1993 (0.5 fish/angler day), 1999 (0.9 fish/angler day) and
2003 (0.5 fish/angler day). The average size of Australian herring retained by the MAAC
also fluctuated over this period, peaking at 0.19 kg in 1999. There appears to be an inverse
relationship between CPUE and average fish size, suggesting recruitment-driven variations in
the catch of this species.
Yellow-eye mullet. The annual CPUE and average fish size of yellow-eye mullet each peaked
during the early 1990s. From 1999 to 2003, CPUE and fish size have been relatively low,
suggesting a lower abundance of this species and smaller average size in the estuary in recent
years.
Yellowtail scad. This species was not retained by MAAC members until 1992. Since then
annual CPUE has fluctuated between zero and 0.08 fish/angler day and the average fish size
has been stable (~0.09 kg).
Tailor. The annual CPUE of tailor ranged from 0.1 to 0.6 fish/angler day from 1986 to 2003.
CPUE did not increase after mid-1991, despite the increase in bag limit. CPUE was relatively
low from 2001 to 2003. Overall, trends suggest a slight decline in tailor abundance in the
estuary since 1986. The average size of tailor was stable from 1986 to 1997. The average
size then increased sharply to a peak of ~0.4 kg in 2000 before declining to ~0.25 kg in 2003.
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The 2000 peak in average fish size corresponded to a minimum in CPUE, suggesting poor
recruitment (i.e. an absence of small fish) at this time.
Trevally. (Psuedocaranx wrighti and P. dentex). The annual CPUE of trevally ranged from 0.1
to 0.6 fish/angler day and the average fish size ranged from 0.1 to 0.3 kg. The annual CPUE
peaked at 0.6 fish/angler day in 1997, followed three years later by a peak in average fish size
(0.3 kg) in 2000. Variations in the catch rate and average size of trevally retained by MAAC
since 1987 appear to have been mainly influenced by changes in bag and size limits. CPUE
was relatively high from 1991 to 1997, following an increase in the self-imposed MAAC bag
limit in 1991. The CPUE was slightly lower from 1997 to 2003, following an increase in the
legal minimum size in 2003. The average fish size increased after 1997.
Six-lined trumpeter. The annual CPUE of six-lined trumpeter was relatively low (~0.15 fish/
angler day) until 1993. CPUE then increased and peaked at 0.9 fish/angler day in 1998, before
steadily declining until 2003. The average fish size peaked at 0.17 kg in 1991, and a lesser
peak of 0.14 kg occurred in 1999 and 2000. A peak in CPUE during the mid-1990s followed
by a peak in fish size in 1999-2000 is suggestive of strong recruitment in the mid-1990s.
Yellowtail trumpeter. The annual CPUE of yellowtail trumpeter was relatively low (~0.2 fish/
angler day) until 1993. The CPUE then increased dramatically and peaked at 0.7 fish/angler
day) in 1994. From 1994 to 2003, annual CPUE trended downwards. The average fish size
decreased in 1991 (coinciding with an increase in the bag limit) but was then stable until 1999.
From 1999, the average fish size declined and reached a minimum of 100 g in 2003 (Fig. 11).
Overall, data suggest that yellowtail trumpeter became progressively smaller and less abundant
in the estuary after 1994.
Whiting. Three species of whiting are caught by the MAAC, the most common being yellowfin
whiting. A period of relatively high annual CPUE occurred from 1991 to 1998, including a
peak of 1.3 fish/angler day in 1998. CPUE from 1999 to 2003 were considerably lower (0.20.4 fish/angler day). Trends in CPUE suggest that yellowfin whiting were more abundant in
the estuary from 1991 to 1998 than in subsequent years. From 1986 to 2003, the average fish
size was relatively stable, ranging from 0.1 to 0.3 kg.
Between 1986 and 2004, King George whiting (Sillaginoides punctata) and trumpeter whiting
(Sillago maculata) were caught intermittently in the estuary by the MAAC, mainly during the
early 1990s.
5.0
Discussion
5.1 Commercial and recreational catch composition and
catch shares
The period 1975-2004 encompassed a marked shift in the composition of the commercial
catch in the Swan Estuary. The proportion of the total catch represented by finfish declined
and crabs became the main focus of the fishery. Also, the species composition of the finfish
catch changed considerably. Most notably, the proportion of cobbler declined to almost
zero while the proportion of black bream increased. After 1999, the proportion of species
previously considered as minor or non-target species increased significantly. These species
included yellowtail scad, tarwhine, roach (Gerres subfasciatus) and whiting (mainly Sillago
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21
schombergkii). Elasmobranch (sharks and rays) catches also increased slightly in recent years,
although they remained a very minor (<500 kg) component of the annual catch (Fig. 8).
Long-term records of total annual recreational catch in the Swan Estuary are not available but
records of MAAC catch since 1986 suggest that some of the changes in the composition of
the commercial catch also occurred in the composition of the recreational catch. In particular,
the proportion of cobbler in the MAAC catch declined while the proportion of black bream
increased.
In recent years, commercial fishers in the Swan Estuary retained ~15 species per year. The
primary species in recent commercial catches were blue swimmer crabs, sea mullet, Perth
herring, black bream, yellow-eye mullet and tailor. Secondary commercial species included
yellowfin whiting, Australian herring, mulloway, bar-tailed flathead, small-toothed flounder,
yellowtail scad, tarwhine and yellowtail trumpeter.
The recreational catch in the Swan Estuary is more diverse than the commercial catch, although
there are also ~15 common species retained (not including blowfish which is the most commonly
caught species). The primary species in recent recreational catches were blue swimmer crabs,
black bream, Australian herring, tailor and bar-tailed flathead. Secondary recreational species
included yellowfin whiting, small-toothed flounder, tarwhine, yellowtail trumpeter, six-lined
trumpeter, garfish, mulloway, cobbler, yellowtail scad, pink snapper and trevally.
Blue swimmer crabs is currently the largest component of both commercial and recreational
landings and the most valuable component of commercial landings in the Swan Estuary. From
1995 to 2004, commercial crab catches averaged 19 t per year. The 2000/01 recreational crab
catch was estimated to be 33 t. This suggests that the recreational sector has recently taken
~63% of the total crab catch (by weight). The recreational crab catch has probably been of a
similar or greater magnitude to the commercial crab catch in the Swan Estuary since the 1970s
(Lenanton 1978).
The 2000/01, the recreational catch of finfish was estimated to be 28 t. From 1995 to 2004,
commercial finfish catches averaged 36 t per year. This suggests that the recreational sector has
recently taken ~45% of the total finfish catch (by weight) in the Swan Estuary. However, over
this 10 year period, the commercial landings of finfish declined considerably, and so in latter
years the recreational catch of finfish has been >50%.
Although many species are common to both recreational and commercial catches, there appear
to be few sources of conflict over the current allocation of fishery resources in the estuary. The
vast majority of the recent commercial catch comprised crabs, sea mullet, Perth herring and
black bream. Of these species, only black bream and crabs are targeted by recreational fishers.
Since 1990, the CPUE of black bream has increased and the CPUE of crabs has been stable
which suggests that availability these species to each sector is adequate.
5.2 Catch trends and changes in species abundance
Annual commercial catch rates were not consistently available until ~1950 and annual
recreational catch rates were not consistently available until ~1990. Therefore, fishery records
provide very limited information about changes in fish abundance that occurred in the Swan
Estuary prior to these dates. Importantly, the absence of catch and effort data from the earliest
years of the fishery means that quantitative information about the ‘virgin state’ of the fish stocks
and the initial impacts of fishing in the estuary is not available. Early reports by Department of
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Fisheries and others suggest that early fishing had a significant impact on local fish abundance
and that at least some estuarine stocks were already depleted by the time commercial fishery
catch records were introduced (Colebatch 1929).
Trends in commercial and recreational catches suggest the abundances of many fishery species
in the estuary have changed over the last 2-3 decades (Table 6). While some species exhibited
stable or increasing abundance, of most concern are the various species that appear to have
declined in abundance over this period.
In the commercial fishery, there was a continuous decline in the total annual finfish catch after
1975 and a decline in the annual finfish CPUE after 1990. The decline in annual commercial
catch and CPUE after 1990 was the combined effect of declines in the catches of Perth herring,
sea mullet, yellow-eye mullet and cobbler. Historically, these were the main target species in
the fishery and were known to be relatively abundant in the estuary at least until the 1980s
(Loneragan et al. 1989). Declines in commercial catches of these species were likely to be due
to a combination of factors. Firstly, total effort in the fishery was significantly reduced (Fig. 7).
Secondly, targeting of some species was reduced. Thirdly, the abundance of the major target
species may have declined.
There is a body of evidence to suggest that the abundances of Perth herring, sea mullet,
yellow-eye mullet and cobbler in the estuary have declined. The evidence includes trends
in commercial and recreational catches, trends in catches from fishery-independent sampling
(I. Potter, Murdoch University, unpubl. data) and anecdotal reports from commercial and
recreational fishers.
In addition to declines in the primary species mentioned above, commercial fishery catch trends
since ~1940 suggest declines in the abundances of school prawns, tailor, bar-tailed flathead
and mulloway, and recreational catch trends since 1990 suggest declines in the abundances of
yellowtail trumpeter and small-toothed flounder.
Commercial and recreational catch trends indicate that five fishery species may have increased
in abundance in the Swan Estuary since 1990. Recreational catch rates suggest an increase
in black bream, tarwhine and Australian herring, while commercial catch rates suggest an
increase in black bream, blue swimmer crabs, tarwhine and yellowtail scad. Black bream
and blue swimmer crabs each fetch a relatively high market price and are likely to be targeted
by commercial fishers whenever available. Hence, marketability is not likely to have been a
major factor influencing catch trends of these species. However, the higher commercial catch
of yellowtail scad may reflect the increased marketability of this species rather than higher
abundance (R.J. Bales, commercial fisher, pers. comm.). Commercial and recreational catches
of tarwhine have increased in many south-west estuaries recently, suggesting a widespread
regional increase in abundance of this species (DoF, unpubl. data).
The increasing trend in the recreational CPUE of Australian herring was probably an artefact
of a localised peak in recruitment in 1999-2000, rather than a long-term increasing trend in
stock abundance. A localised peak also occurred in the commercial CPUE in 2000. However,
the longer-term commercial catch trend since 1975 in the Swan Estuary was decreasing.
Commercial and recreational catch rates of Australian herring elsewhere along the west coast
of Western Australia (where the same stock is targeted) have declined over recent decades.
Marked fluctuations in the catch rates and/or average fish weight of several species suggest
strong variations in annual recruitment. In 2001, a relatively low recreational CPUE and a peak
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23
in average fish size suggest poor juvenile recruitment by tailor to the estuary in this year. In
1991, the recreational CPUE of King George whiting peaked, followed by a peak in average
fish size in 1993, suggesting strong juvenile recruitment and subsequent growth in the estuary.
A period of elevated CPUE followed by a peak in average body size of six-lined trumpeter
suggest strong recruitment in the mid-1990s.
Annual catches of bar-tail flathead and small-toothed flounder have been highly variable,
probably as a result of highly variable recruitment. Since 1939, there have been several discrete
periods of high commercial landings for both species (Fig. 6). Peaks in flounder catches were
followed by peaks flathead in catches a few years later, suggesting that pulses of recruitment
in these two species were driven by the same environmental factors. Commercial catch
trends suggested strong recruitment by flounder and flathead in the late 1950s. Commercial
and recreational catch trends suggested strong recruitment by flounder in the late 1980s and
strong recruitment by flathead in the early 1990s. Catch rates of flounder and flathead have
declined since the early 1990s and late 1990s, respectively. These species are of high value
to each sector, and so low catches are probably not market driven and are likely to reflect low
abundance in the estuary in recent years.
5.3 Future fishery assessments
Historically, stock assessments in the Swan Estuary have relied on catch and effort data
from the commercial fishery. A significant reduction in the number of commercial fishers
has reduced the amount of annual data and has eroded the usefulness of commercial data
currently being collected. However, any continuation of this long-term data series will still
yield valuable information for stock assessments of primary species. Cessation of commercial
fishing in the estuary in the near future would be problematic because an alternative, ongoing
data source is yet to be established. This situation has already arisen in another west coast
estuary – commercial fishing in the Leschenault Inlet ceased in June 2001 and no annual data
for assessments have since been available.
In future, annual data on the relative abundance and size structure of many stocks could be
provided by recreational fishers, from either club catch records or angler logbooks. Agreement
between MAAC CPUE trends and commercial and/or fishery-independent survey catch rate
trends for various species, suggests that recreational catch rates can provide useful indices of
fish abundance. Such data is highly complementary to creel/phone survey estimates of total
catch and effort in the estuary.
At present, commercial and recreational fishery data from the Swan Estuary work in concert.
The commercial fishery provides information about species that are not often caught
recreationally (e.g. sea mullet, yellow-eye mullet and Perth herring), while the recreational
fishery provides information about species that are not often caught commercially (e.g. six-lined
trumpeter, blowfish). For species that are caught by both sectors, comparisons of catch trends
provides some validation of the data. In the absence of commercial data, fishery-independent
surveys would be required to provide information about non-recreational species. In any
case, independent surveys would be a desirable component of any future monitoring strategy,
providing validation of fishery data in general. Independent surveys also collect information
about the status of non-fishery species, which are important to the ecological function of the
estuary. It has been demonstrated in similar estuarine systems that baseline fish community
data provide an index of general estuary health, and so have broad applications in estuary and
fishery management (e.g. Harrison and Whitfield 2004).
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Future monitoring of fish communities in the Swan Estuary should include a combination of
methods including annual fishery catch and effort data, regular recreational fishery surveys and
occasional fishery-independent surveys.
6.0 Status of key fishery stocks
This section assesses the status of the key fishery stocks in the Swan Estuary, including comments
on various environmental factors that are likely to have impacted on stocks in the estuary in
recent decades. Environmental factors (including habitat modification, input of pollutants
and climate change) are likely to be at least as significant as the impact of fishing pressure on
stocks in the Swan Estuary. Unfortunately, the lack of long-term biological monitoring in the
estuary makes it difficult to precisely determine the impacts of most environmental factors on
fish stocks.
In discussing environmental factors, this section focuses on the risk to stocks from algal
blooms because the frequency of harmful blooms and associated fish kills in the Swan Estuary
has been relatively high in recent years. In summer/autumn of 2003, a bloom of Karlodinium
micrum resulted in the largest recorded fish kill in the estuary and smaller blooms of K. micrum
occurred again in 2004 and 2005. Such events pose a high risk to certain stocks, especially
estuarine-dependent species. Major fish kills have tended to occur during summer/autumn
(January to May) in the upper estuary (i.e. upstream of Heirisson Island in the Swan River and
in the upper Canning River) (Fig. 1). Therefore, this season/location has the highest likelihood
of future kills and species that occur here are at the highest risk.
6.1 Major target species
6.1.1 Blue swimmer crab
Biology: Blue swimmer crabs (Portunus pelagicus) are distributed throughout the Indo-west
Pacific. They occur across northern Australia from Cape Naturalist (WA) to Eden (NSW),
and also in some areas of South Australia (Kailola et al. 1993). A number of discrete regional
stocks occur along the Western Australian coast. Individuals in the Swan Estuary are believed
to belong to a stock which also occurs in Cockburn Sound, but which is separate to populations
in more southern estuaries (Chaplin et al. 2001, S. de Lestang, pers. comm.).
The main source of crab recruitment to the Swan Estuary is probably from spawning activity in
nearby Cockburn Sound (S. de Lestang, pers. comm.). The majority of spawners in Cockburn
Sound are believed to be residents of local coastal waters, with relatively few individuals from
estuaries. Some crabs from the Swan Estuary may migrate to Cockburn Sound to spawn, but
the size of the blue swimmer crab population in the Swan Estuary is relatively small compared
to those of other estuaries in the region, and the contribution to regional recruitment by Swan
Estuary females is likely to be minor. Hence, recruitment to the Swan Estuary fishery is
independent of population size within the estuary.
The blue swimmer crab is essentially a marine species and can be found in coastal waters and
the lower reaches of estuaries throughout the year. Adults will move into the middle and upper
reaches of estuaries if waters in these areas become sufficiently saline, with a preference for
salinities of 30-40‰ (Potter et al. 1983). In the Swan Estuary, this typically occurs during
summer when freshwater input is low. Therefore, crabs can be found as far upstream as the
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25
middle reaches of the Swan River and lower reaches of the Canning River during warmer
periods of the year (S. de Lestang, DoF, pers. comm).
Mating by blue swimmer crabs occurs during summer in estuarine waters (Potter et al. 1983).
After mating, females migrate to ocean waters (or possibly the lower estuary) to spawn while
males remain in estuaries. Females may remain in ocean waters for extended periods, where
they can spawn several batches of eggs from a single mating event. The low abundance of
females in estuaries during summer results in estuarine fishery landings that are dominated by
male crabs. In the Swan Estuary, males are estimated to comprise 94% of the recreational catch
(Malseed and Sumner 2001).
Blue swimmer crabs are relatively short-lived, with a maximum age of approximately 3
years and maximum carapace width of 200 mm (Kailola et al. 1993). Maturity is reached
at approximately 1 year, and recruitment to the Swan Estuary fishery occurs at an age of
approximately 18 months. In general, blue swimmer crab populations have relatively fast rates
of replacement and can recover quickly from depletion, which allows each stock to sustain
moderately heavy levels of fishing.
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills appear to be a low risk to blue swimmer crabs in the Swan Estuary. Major
fish kills are most likely to occur in summer/autumn in the upper estuary and the majority
of the population would probably be downstream or in ocean waters at this time and not be
directly affected. Any loss of stock would be replenished by juvenile recruitment to the estuary
in the following spring.
Algal blooms pose a lower risk to crabs than to most finfish. Crabs have a lower oxygen demand
than most finfish species and so may be less impacted by hypoxic (low oxygen) conditions than
finfish, although they could be impacted by anoxic (zero oxygen) conditions (S. de Lestang,
DoF, pers. comm.). Also, crabs are scavengers and consume dead fish. In the event of a fish
kill, the surviving crab population may benefit from enhanced food availability.
The main source of crab recruitment to the Swan Estuary is from spawning in nearby Cockburn
Sound. Therefore, environmental impacts in Cockburn Sound may affect the breeding stock
and affect recruitment to the Swan Estuary.
Trophic links: Blue swimmer crabs are bottom-dwelling carnivores and scavengers (Kailola
et al. 1993). They mainly consume various slow-moving or sessile invertebrates including
worms, bivalve molluscs and crustaceans, and may also consume small amounts of seagrass
and algae. They will scavenge on dead fish and larger invertebrates.
Fishery: Blue swimmer crabs are currently the largest and most valuable component of
commercial and recreational fishery landings in the Swan Estuary. In recent years, crabs
contributed 34% by weight and 57% by value to total commercial landings and 52% by weight
to total recreational landings (Tables 2, 3, 4). Catches in both fisheries are seasonal, reaching
a maximum in summer and a minimum in winter (Fig. 5). This pattern reflects the seasonal
movement of adult crabs into estuaries during warmer months.
A minimum legal size of 127 mm carapace width and a daily bag limit of 20 crabs applies to
recreational fishers in the Swan Estuary.
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Stock assessment: Commercial fishery trends suggest that the abundance of crabs in the
estuary has been stable over many decades and possibly increased since 1990. Since the
1970s, the annual commercial catch of blue swimmer crabs in the Swan Estuary has been
highly variable but exhibited an overall stable trend. In the 1990s, the CPUE of crabs increased
dramatically and then stabilised at a relatively high level after 1996. Interpretation of this
trend is confounded by a major reduction in total commercial effort over the same period
and changes in the marketability of other target species, both of which may have increased
the targeting of crabs by the fishery. A record of annual crab catches is not available from the
recreational fishery for comparison and so it is not known whether recreational CPUE also
increased after 1990.
The main source of crab recruitment to the Swan Estuary is a breeding stock in Cockburn
Sound. Therefore, recruitment to the Swan Estuary fishery is not dependent on population size
within the estuary and the fishery is likely to be sustainable at relatively high levels of catch
and effort within the estuary. Fishing mortality in the Swan Estuary probably has little impact
on total stock size or recruitment. The fact that a) the fishery does not target breeding females
and b) males are mainly caught after mating each year, further limits the impact of the Swan
Estuary fishery on the regional crab stock.
6.1.2 Western school prawn
Biology: Western school (or river) prawns (Metapenaeus dalli) occur in the Indo-west Pacific,
in Indonesia and along the west Australian coast, from Mandurah (WA) to Darwin (NT)
(Grey et al. 1983). School prawns (Metapenaeus spp.) generally occur over sand and sandmud bottoms in rivers, estuaries and inshore waters to 50 m (Kailola et al. 1993). They bury
themselves in the sand during day (and on bright moonlit nights) and emerge at night to feed.
In the Swan Estuary, school prawns are restricted to the middle and lower estuary in winter but
spread to the upper estuary in summer, corresponding to intrusion of the salt wedge and more
saline conditions in upper estuary. They are associated with approximately oceanic salinities
in the Swan Estuary, but occur at much lower salinities in the adjacent Peel-Harvey estuary
(Potter et al. 1986, 1989).
The entire life cycle of school prawns, including spawning, is completed in estuaries. Each
estuarine population is a discrete breeding stock. Spawning in the Swan Estuary tends to occur
where salinity is >30 ppt (i.e. in the middle and lower estuary). Fecundity is ~300,000 eggs
(Potter et al. 1986). The larval stages are pelagic and feed near surface for ~2 weeks before
settling into shallow waters. Most small prawns are found in shallow (<1.2 m) water and
tend to move into deeper water as they approach maturity (Potter et al. 1986). School prawns
mature at 1 year of age and spawn soon after, during summer (November-April, but mainly
January-March). They reach a maximum age of ~2 years and length of 190 mm TL.
Fish kills and environmental impacts on stock: Past fluctuations in the abundance of school
prawns in the Swan Estuary have been attributed to variations in rainfall. It has been suggested
that several, consecutive dry winters may increase the survival of young prawns (Potter et al.
1986). Hence, seasonal or annual rainfall patterns and the rate of freshwater runoff from the
catchment may impact on this stock.
Given that juvenile and adult prawns are consumed by a wide range of predators, increases
in the abundance of various predatory species could impact on prawn abundance. It has been
speculated that relatively high numbers of blowfish in the last 1-2 decades years has impacted
on school prawn abundance in the Swan Estuary (R. Lenanton, pers. comm.).
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27
Harmful algal blooms and other factors causing fish kills appear to be a medium risk to school
prawns in the Swan Estuary. Major fish kills are most likely to occur in summer/autumn in
the upper estuary. School prawns are most common in the middle and lower estuary (from
Fremantle to the Narrows Bridge) but do occur in the upper estuary (up to Maylands and in
the Canning River) in summer. Also, spawning occurs during summer and algal blooms at this
time could affect pelagic larval and benthic juvenile stages. Therefore, an algal bloom or other
event resulting in poor water quality during summer in the upper estuary may affect a small
proportion of the total population in the estuary, but this proportion could include spawning
adults, larvae and juveniles.
Although only a minority of the stock is likely to be directly affected by a harmful algal bloom,
any threat to this stock should be considered serious, given the very low abundance in recent
years.
Trophic links: School prawns are bottom-dwelling omnivores and consume small invertebrates
and detritus. Prawns are eaten by many fish, including key fishery species.
Fishery: School prawns are targeted by recreational fishers in the shallow waters of the middle
estuary (i.e. Melville water) during summer. King prawns (Penaeus latisulcatus) are also
targeted in this area. Recreational fishers are restricted to catching prawns by hand-trawl nets
(length < 4m, mesh >16mm) or hand-scoop nets only. A personal daily bag limit of 9 litres and
a minimum legal size of 50 mm applies to recreational fishers in the Swan Estuary.
School prawns become vulnerable to capture at ~50 mm length, which is reached at 9-10
months of age (i.e. in spring). Hence, they are vulnerable to capture prior to reaching maturity.
A closed fishing season is enforced in the Swan Estuary from July 31 to November 1 to allow
prawns to reach maturity prior to capture. To provide habitat protection, there are several
areas permanently closed to the use of hand trawl nets (dragnets). This includes the waters of
the Swan River within 100 metres of the Pelican Point Nature Reserve and the Milyu Nature
Reserve (Como).
Stock assessment: Previous catches of school prawns in the estuary have been highly variable,
reflecting annual fluctuations in spawning and recruitment success. Relatively high catches
occurred in 1977-80 and 1984-85, possibly as a result of two or more consecutive dry winters,
which may increase the survival of young prawns. Very low catches were recorded in 1982
(Potter et al. 1986).
The current abundance of school prawns in the Swan Estuary appears to be very low relative
to historic levels. The last significant commercial catches of school prawn occurred in 1975.
Significant recreational catches were last taken in the late 1990s, suggesting that abundance in
the estuary has been very low since then (Maher 2002). Recreational fishers reported a slight
increase in abundance of school prawns in 2003 and 2004.
This population inherently has limited capacity to recover because it is an isolated breeding
unit and reliant on self-replenishment.
6.1.3 Perth herring
Biology: Perth herring (Nematolosa vlaminghi) are endemic to WA, occurring from Broome to
Bunbury (Kailola et al. 1993). Available data suggest that each estuarine population may be a
discrete stock (Chubb et al. 1984). This notion is supported by the fact that, in some related
species, adults return to spawn each year in their natal river. In the Swan Estuary, adult Perth
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herring migrate from ocean waters to the upper reaches of the estuary to spawn in summer. The
movement of adults while in marine waters has not been examined. However, commercial catches
of Perth herring in coastal waters near the Swan Estuary have been mainly restricted to within a
few kilometres of the estuary mouth, suggesting limited movement away from the estuary.
Mature fish enter the Swan Estuary during spring and move rapidly through the lower
and middle estuary towards spawning areas in the upper estuary (Chubb and Potter 1984).
Spawning locations are upstream of the commercial fishery area in the Swan Estuary. Hence,
the primary peak in commercial catches occurs prior to spawning (September/October) when
adults are migrating upstream through the fishery area to spawning areas. A secondary peak
in catches occurs after spawning (February), when adults are moving downstream through the
fishery area on their return to the ocean. Recreational anglers report that Perth herring form
large aggregations at night under the Narrows Bridge (apparently attracted to the bridge lights)
in September-November, roughly corresponding to the timing of upstream migration. At these
times, mulloway also aggregate under the Narrows Bridge to feed on the Perth herring.
Spawning by Perth herring occurs in the upper estuary (sites 5-8 in Chubb et al. 1984) in
December, and to a lesser extent in January. After spawning, eggs and larvae are likely to
experience limited dispersal downstream due to low flow rates at this time of year (Neira et al.
1992). Small juveniles start to appear in the upper estuary in January and gradually disperse
throughout the upper, middle and lower estuary over the next 1-2 months.
The overall distribution of Perth herring in the Swan Estuary is related to variations in salinity.
The upstream migration of adults coincides with the easing of freshwater flows and the intrusion
of the salt wedge into the upper estuary. At the onset of autumn rains, Perth herring abundance
declines rapidly in the upper estuary. Both juveniles and adults are absent from the upper estuary
from approximately July to September, corresponding to periods of low salinity. While adults
return to ocean waters in winter, juveniles reside in the estuary all year. Juveniles are most
common in the middle estuary, where they occur in all months (Chubb and Potter 1984).
The Swan Estuary population is dominated by individuals aged 0-4 y (Chubb and Potter 1986).
Maturity and recruitment to the commercial fishery occurs at age 2-3 y, and length >160 mm.
The maximum age and length of Perth herring is reported to be 8 y and 360 mm TL (Chubb
and Potter 1986). However, this maximum age was determined by an examination of scale
increments. A preliminary examination of otolith increments suggests a maximum age of at
least 18 years (K. Smith, DoF, unpubl. data).
Fish kills and environmental impacts on stock: A harmful algal bloom or other factor
resulting in a fish kill in the upper estuary during the December/January, or shortly after, could
have a major impact on this stock. Such an event could affect a large proportion of eggs,
larvae, juveniles and breeding adults. In the Swan Estuary, fish kills tend to occur in summer/
autumn in the upper estuary, coinciding with the timing and location of spawning by Perth
herring. Therefore, there is a high likelihood of a major impact on this stock in the near future.
Indeed, during fish kills in April-June 2003, hundreds of dead Perth herring were collected and
many more were observed in the Swan and Canning Rivers (Swan River Trust, unpubl. data).
Locations of dead fish included the Swan River between Ascot and Perth Water and the lower
Canning River near Coffee Point.
In summary, harmful algal blooms resulting in fish kills represent a high risk to this stock.
Major fish kills in summer/autumn in successive years could significantly lower spawning
stock size and reduce annual recruitment by Perth herring.
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29
Trophic links: Perth herring are a small (<300 mm), schooling species. As such, they are
likely to be a component of the diet of some larger predatory species including birds, sea lions,
dolphins and important fishery species such as mulloway and tailor. Recreational fishers
report that small Perth herring are also consumed by large black bream. Depletion of the Perth
herring stock is likely to reduce prey available to various species in the estuary.
Fishery: Perth herring contributed approximately 24% by weight and 4% by value to recent
annual commercial finfish catches in the Swan Estuary (Table 2, 3). Catches occur during all
months of the year, but are generally lowest in May-July when adults migrate downstream of
the fishing area and become unavailable to commercial fishers (Fig. 5). Recreational fishers do
not catch this species in the estuary (Tables 4-6).
Recruitment by Perth herring to the commercial fishery occurs at approximately the same
age/length as maturity (2-3 y / 160 mm). Hence, many individuals are vulnerable to capture in
Swan Estuary before they have an opportunity to breed.
From 1963 to 1988, the annual commercial catch of Perth herring in the Swan Estuary was
>40 t. This relatively lengthy period of stable catches suggests that annual catches of 40 t in the
estuary may have been sustainable under the environmental conditions that existed at that time.
In the 1960s and early 1970s, some annual catches exceeded 100 t, including a peak of 150 t in
1968-69. However, the lower catches around 1970 and during the late 1970s, following these
peaks, suggest that catches of >100 t were unsustainable.
Annual catches of Perth herring from Cockburn Sound were >40 t during the 1970s and
averaged ~15 t per year during the 1990s (DoF unpubl. data). No catches were reported from
Cockburn Sound after 2000.
Stock assessment: The magnitude of past catches and data from previous fishery-independent
surveys (e.g. Loneragan et al. 1989) indicate that Perth herring was formerly one of the most
abundant fish in the estuary. However, commercial catch trends suggest that the abundance of
this species has been declining since the 1980s. All available evidence, including simultaneous
catch declines in adjacent coastal waters where the same stock was commercially targeted until
recently (D. Gaughan, DoF, pers. comm.), anecdotal reports from fishers and low catches in
recent fishery-independent surveys (I. Potter, Murdoch University, unpubl. data), suggests that
the current stock size of Perth herring is very low relative to historic levels and possibly still
declining. Since this species has been subject to relatively low fishing pressure in recent years,
the decline in abundance of this species is probably now being driven by environmental factors
rather than fishing pressure.
Environmental changes over the last 20 years or so may have reduced the ‘carrying capacity’
of the estuary in relation to Perth herring. In particular, poor water quality in the middle/upper
estuary may have affected growth and mortality at all life history stages of this species. If so,
then the relatively low recent (<10 t) annual catches may actually be unsustainable, despite
(apparently) sustainable annual catch levels of 40 t in the Swan Estuary (plus landings in
Cockburn Sound) in earlier years of the fishery.
This population inherently has limited capacity to recover because it is an isolated breeding
unit and reliant on self-replenishment. All available evidence suggests that the current stock
size of Perth herring is already very low relative to historic levels and possibly still declining.
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6.1.4 Cobbler
Biology: Cobbler (Cnidoglanis macrocephalus) are endemic to temperate Australia, occurring
in southern Queensland, New South Wales, South Australia and Western Australia, but not
occurring in Tasmania or Victoria. In WA, they occur southwards from the Houtman-Abrolhos
Islands. Adults and juveniles inhabit estuarine and coastal waters to approximately 30 m depth
(Kailola et al. 1993). The Swan Estuary population is a discrete stock and is self-replenishing.
Genetic and morphological differences indicate that there are low levels of mixing between
marine and estuarine populations and among estuarine populations of cobbler (Kowarsky 1975,
Ayvazian et al. 1994). Therefore, movement of fish between the Swan Estuary and adjacent
marine waters, or adjacent estuaries, is probably negligible.
In the Swan Estuary, cobbler movement follows a seasonal pattern that corresponds to variations
in salinity. Fish are concentrated in the middle estuary in winter, spread upstream to the upper
estuary during spring/early summer (following the upstream intrusion of the salt wedge) and
are most dispersed throughout the estuary in late summer (Kowarsky 1975). Abundance tends
to be highest in the middle estuary throughout the year and larger fish are most often found in
the middle estuary (Nel et al. 1985).
In winter/spring, cobbler form tight aggregations in the middle estuary. For example, Point
Walter is well known to recreational and commercial fishers as a site where aggregations
occur. Winter aggregations at this location have historically been a major component of the
commercial cobbler catch. It is not clear whether the purpose of these aggregations is feeding
or reproduction.
Cobbler attain a maximum size of 760 mm and age of 13 y. In the Swan Estuary, 50% maturity
is reached at an age of 2-3 y and a total length of 385/405 mm (male/female) (Nel et al. 1985).
In the Swan Estuary, spawning occurs from October to December. Eggs are laid in burrows
constructed and guarded by males. Cobbler produce relatively few eggs (500-3500 eggs per
batch), but this low level of fecundity is offset by high parental investment in offspring (i.e.
large egg size and relatively late developmental stage at which larvae emerge from burrows).
This reproductive strategy promotes low dispersal of larvae.
Fish kills and environmental impacts on stock: Observations of cobbler in several south-west
estuaries, including the Swan, indicate that burrows are located under structures such as rocks or
seagrass root mats, which form the roof of the burrow. Hence, reproduction is dependent on the
availability of suitable habitat. Burrow entrances are typically 15-20 cm wide and burrows may
be up to 1.5 m deep, with an expanded brood chamber at the far end (Cliff and Lenanton 1993).
In the Swan Estuary, nesting is known to occur in Alfred Cove in the middle estuary. In previous
years, rock retaining walls adjacent to Riverside Drive (middle estuary) were used as nesting
sites, but reconstruction of these walls to create smoother surfaces with less large crevices has
apparently reduced nesting opportunities in this area (R. Lenanton, DoF, pers. comm.).
In the surf zones of coastal beaches, juvenile cobbler undergo diurnal movement from detached
macrophytes (day) to open sand (night). This movement presumably allows juveniles to avoid
visual predators during the day. Also, there is a positive relationship between the abundance
of juveniles and the amount of drift weed in such habitats. Although these observations are
from marine rather than estuarine habitats, they suggest that the availability of suitable habitat
is generally very important to juveniles in predator avoidance and feeding success (Lenanton
and Caputi 1989, Robertson and Lenanton 1984).
Fisheries Research Report [Western Australia] No. 156, 200631
Harmful algal blooms and other factors causing fish kills are a moderate risk to this stock. In
the Swan Estuary, fish kills tend to occur in summer/autumn in the upper estuary. At this time,
cobbler are dispersed throughout the middle and upper estuary and so it is likely that some
individuals will be affected by an event at this time/location. However, the affected proportion
of the population is likely to be small because the majority of the population appears to reside
in the middle estuary at all times of year.
Overall, the likelihood of a major fish kill directly affecting a large proportion of the cobbler
stock in the Swan Estuary is low. However, any additional threat to this population must
be considered serious, given the very depleted size of the stock. Future fish kills are likely
to include small numbers of cobbler and may have indirect effects on this species, such as
reducing the abundance of prey.
Trophic links: The diet of cobbler consists of benthic invertebrates (polychaetes, molluscs,
crustaceans).
Fishery: Cobbler was previously an abundant and valuable component of commercial and
recreational fishery landings in the Swan Estuary. However, it now represents only a small
proportion of landings by both sectors (Tables 2-6). Cobbler is still an important component
of landings in some other south-western estuaries
There is strong seasonality in the commercial catch of cobbler within the Swan Estuary.
Catches peak in winter, which reflects a concentration of individuals in the fishery area at this
time (Fig. 5). Catches are very low from Nov-March, which may reflect the combined effects
of dispersal throughout the estuary, including movement to areas upstream of the fishery area,
and the low catchability of burrowing males during spring (Nel et al. 1985, Laurenson 1992).
The seasonal pattern of landings has become more pronounced since the 1970s which may be
due to a decline in abundance in the estuary, making catch rates difficult to sustain at times
when breeding fish are unavailable and the remaining fish are widely dispersed.
The legal minimum length of cobbler is 430 mm (whole body weight ~420 g) (Nel et al. 1985),
which is greater than the length at maturity in the Swan Estuary. Hence, individuals may have
an opportunity to breed at least once before caught and retained. The legal minimum length
increased from 230 to 430 mm in 1995. Cobbler retained by the MAAC from 1986 to 1993
included fish of weights 200-400 g, which probably included some older juveniles. Therefore,
immature fish are vulnerable to capture by recreational fishers. Rates of post-release mortality
for cobbler are unknown.
A daily bag limit of 4 cobbler currently applies to recreational fishers in the Swan Estuary.
Stock assessment: Historically, this species was an important target species for commercial
and recreational fishers and was known to be moderately abundant in the Swan Estuary until
the 1980s (Loneragan et al. 1989). However, the abundance of cobbler in the estuary has
since been declining. There is a body of evidence to suggest that the current abundance of this
stock is very low relative to historic levels, including declines in commercial and recreational
catches, declines in catches from fishery-independent sampling (I. Potter, Murdoch University,
unpubl. data) and anecdotal reports from commercial and recreational fishers.
No data about the age or size structure of the cobbler stock in the Swan Estuary has been
collected since the 1980s.
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A decrease in recreational CPUE, coinciding with an increase in the average size of fish
retained by recreational fishers, suggests recruitment failure during the mid-1990s. The
commercial CPUE was maintained (i.e. exhibited ‘hyperstability’) until the stock collapsed in
the mid-1990s, after which time recreational and commercial catches largely ceased.
The decline in abundance of cobbler in the Swan Estuary is likely to be due to the combined
effects of fishing pressure and loss of key habitats. The impacts of fishing pressure on the
population structure and abundance of cobbler have been demonstrated in other south-west
estuaries. In Wilson Inlet, adult cobbler were found to be more abundant in an area near the
estuary entrance that was closed to commercial fishing, than in open areas (Laurenson 1992).
Although this distribution is likely to partly reflect a preference by adults for higher salinities,
it is also likely that fishing contributes to localised depletions, given the moderately sedentary
nature of this species. Large fish, especially females, appear to be most vulnerable to capture.
In Wilson Inlet, catches in closed areas contained higher proportions of older fish and females
than open areas. The sex ratio was balanced in catches from open areas. Males are probably
less vulnerable to capture because they spend part of the year in burrows and also attain a
smaller size-at-age than females.
This population inherently has limited capacity to recover because it is a discrete, selfreplenishing stock. Low breeding stock size, low individual fecundity, a limited availability of
key breeding habitats and fishing pressure would also limited recovery.
6.1.5 Yellowtail trumpeter
Biology: Yellowtail trumpeter (Amniataba caudavittatus) occurs in western and northern
estuaries of Australia, from Cape Leeuwin (WA) to Bowen (Qld) including the Northern
Territory (Hutchins and Swainston 1986). It also occurs in New Guinea. Individuals commonly
form schools over sand and weed bottoms. Yellowtail trumpeter is primarily a marine species
throughout its range, although it is largely confined to estuaries in south-western Western
Australia (Potter et al. 1990).
Individuals can tolerate a very wide range of salinities, from fresh to hypersaline (e.g. Lenanton
1977). The distribution of these fish in the Swan Estuary tends to correspond to salinities of
>30‰, suggesting a preference for oceanic salinities (Wise et al. 1994). In winter, yellowtail
trumpeter is absent from the upper estuary, which experiences low salinity at this time. In
winter, fish also tend to move into deeper waters of the middle estuary and reside below the
halocline, again avoiding low salinities (Wise et al. 1994).
In spring, mature fish migrate to the upper estuary to spawn and only juvenile fish (age class
0+ y) remain in the middle estuary (Wise et al. 1994). Spawning occurs in the lower section
of the upper estuary (between Heirisson Island and the Helena River) from mid-November to
early February, but predominantly in January (Potter et al. 1994). Spawning is presumed to
occur in shallow water (Potter et al. 1994). Fecundity increases with female body size, and
ranges from 50,000 to 705,000 eggs at lengths of 150 to 254 mm (Potter et al. 1994). Eggs and
larvae are pelagic and are potentially dispersed downstream throughout the upper and middle
estuary, although currents are low at the time of spawning. The recruitment of small juveniles
to shallow habitats in the upper estuary commences in January (Wise et al. 1994).
Adults and juveniles, including age 0+ fish, are relatively abundant in the upper estuary during
summer and autumn, and then move downstream to the middle or lower estuary at the onset
of winter rains.
Fisheries Research Report [Western Australia] No. 156, 200633
Maturity is generally attained at the end of the 2nd year, although some individuals mature
at the end of their 1st year. The minimum length at maturity is 130/150 mm (male/female),
corresponding to an age of ~2 y and a body weight of ~55 g (Potter et al. 1994). Yellowtail
trumpeter reach an age of at least 3 years and length of at least 260 mm in the Swan Estuary
(Wise et al. 1994). Females attain a larger maximum size than males. The maximum recorded
length of yellowtail trumpeter is 28 cm (Hutchins and Swainston 1986).
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills are a moderate risk to this stock. In the Swan Estuary, major fish kills
typically occur in summer/autumn in the upper estuary. Adult yellowtail trumpeter aggregate
to spawn in the upper estuary from November to February, with a peak in activity in January.
Therefore, a fish kill in the upper estuary during summer could affect a high proportion of the
breeding stock, many larvae and some juveniles. A significant proportion of juveniles occur
in the middle estuary in summer and so would not be directly impacted by a fish kill in the
upper estuary. Surviving juveniles would mature and spawn the following summer, along
with surviving adults. Hence, following a severe fish kill in the upper estuary, some spawning
would still occur in the following year. Major fish kills in the upper estuary during summer in
2 or more consecutive years would have a more serious impact on this stock.
During fish kills in the upper estuary in April-June 2003, no yellowtail trumpeter were reported
dead. However, because a comprehensive survey of affected fish was not undertaken during
this event, it is possible that some yellowtail trumpeter were killed, including small juveniles.
Relatively low mortality of adults may have been due to a higher tolerance by yellowtail
trumpeter of conditions created by the algal bloom (toxins, gill clogging, low oxygen), or
greater ability to escape, than other fish species. Alternatively, only a small proportion of the
yellowtail trumpeter stock may have been present in the affected area. From March onwards,
adults and juveniles are dispersed throughout the middle and upper estuary. A fish kill in
November-February, when adults and small juveniles are aggregated in the upper estuary
during/after spawning, may have had a greater impact on this stock.
Fish kills may also have indirect effects on yellowtail trumpeter, such as reducing the abundance
of prey. Similarly, mortality of yellowtail trumpeter during a fish kill could indirectly impact
on stocks of larger predatory species.
Trophic links: Yellowtail trumpeter are omnivorous. Their diet includes benthic crustaceans,
polychaetes, molluscs, algae and small fish such as gobies and small clupeids (Wise et al.
1994). Yellowtail trumpeter are probably consumed by numerous larger predatory fish.
Fishery: Yellowtail trumpeter is of low commercial value in the Swan Estuary, representing
approximately <1% of the weight and <1% of the value of recent annual commercial landings in the
estuary (Table 2, 3). Since 1989, commercial landings of yellowtail trumpeter have been negligible
(<1 t), although in earlier years annual catches of up to 7 t were taken. The low recent commercial
catch levels probably reflect low targeting because markets for this species are limited.
In contrast, yellowtail trumpeter is one of the main fish species retained by recreational fishers
in the estuary. Yellowtail trumpeter is among the 5 most commonly caught species and among
the 10 most commonly retained species in the recreational fishery (Tables 4-6). It is mainly
caught by shore-based recreational fishers (Table 4).
There is currently no legal minimum length for yellowtail trumpeter in the Swan Estuary, but
fish <200 mm tend to be released by recreational fishers (K. Smith, DoF, pers. obs.). Since
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Fisheries Research Report [Western Australia] No. 156, 2006
1986, the average size of yellowtail trumpeter retained by the MAAC has typically been >100
g. These sizes are above the size at maturity, suggesting that individuals have a chance to breed
at least once before they are retained (assuming they survive release). A daily bag limit of 16
fish applies to recreational fishers.
Stock assessment: The magnitude of recreational catches and data from various fisheryindependent surveys in the Swan Estuary indicate that this species was, and still is, one of the
most abundant fish in the estuary (e.g. Loneragan et al. 1989, I. Potter, Murdoch University
unpubl. data). However, recreational catch trends suggest that the abundance and average size
of fish has been declining since 1993. Environmental degradation in the upper estuary and
fishing-related mortality represent risks to this species. A steadily decline in average fish size
between 1987 and 2003 is consistent with high rates of total mortality.
The Swan Estuary stock inherently has limited capacity to recover from depletion because
it is an isolated breeding unit, with moderate fecundity and reliant on self-replenishment. A
relatively early age at maturity (1-2 y) would assist the stock to reproduce relatively quickly
and recover from depletion within a few years. However, persistently high annual mortality
(e.g. due to fish kills in consecutive years) could limit recovery. In particular, mortality of
eggs, larvae and small juveniles of yellowtail trumpeter during algal blooms or other harmful
events in the upper estuary is unknown but could be significant.
A considerable number of yellowtail trumpeter are caught and released by recreational fishers.
As an abundant species, it may also be caught and discarded by commercial fishers. However,
the quantities of discards are unknown because no surveys of bycatch have been undertaken
in the commercial fishery. The mortality rate of fish that have been discarded is unknown but,
since discard rates are probably relatively high, post-release mortality could be significant.
6.1.6 Sea mullet
Biology: Sea mullet (Mugil cephalus) have a world-wide distribution, in temperate and
tropical waters, from approximately 42°N to 42°S (Thomson 1963). Juveniles and adults of
sea mullet are tolerant of a wide range of temperatures and salinities, including hypersaline and
fresh waters (Smith and Deguara 2002). Sea mullet occur at all latitudes along the Western
Australian coast. Swan Estuary sea mullet are probably part of a widespread south-western
regional stock. Mixing between estuaries occurs via the migration of adults and the dispersal
of marine eggs and larvae.
Small juvenile sea mullet recruit to the Swan Estuary in autumn/winter (May-Nov) at
approximately 2-3 months of age and 20-30 mm length. They move rapidly through the lower
estuary to settle in the middle and upper estuary and also eventually disperse to the tributaries
of the upper estuary (Chubb et al. 1981). Juveniles occur in the middle and upper reaches of
the Swan Estuary, including freshwater tributaries, at most times of year but tend to occur in
the lower estuary during spring only (Chubb et al. 1981).
Sea mullet reach maturity at an age of 2-4 y. Adults typically reside in estuaries, except during
spawning. In spring/summer, a large proportion of adults, and some 1+ and 2+ juveniles,
migrate downstream and aggregate in the middle or lower estuary (Chubb et al. 1981). In late
summer/autumn, adults migrate to ocean waters where they spawn in autumn/winter. They
may be accompanied by some non-spawning juveniles although most fish probably remain in
the estuary for the duration of their juvenile phase (Smith and Deguara 2002). Adults may not
Fisheries Research Report [Western Australia] No. 156, 200635
spawn every year, and some mature individuals may remain in upper, middle or lower reaches
of the estuary during the spawning season.
Whilst in ocean waters, adults appear to migrate northwards along the coast prior to spawning.
There is no evidence of a significant return (southward) migration along the Western Australian
coast and so adults probably do not return to the same estuary after spawning.
Eggs and larvae may be dispersed north or south of the spawning site, depending on the
direction of coastal currents, before recruiting as juveniles to estuaries. Eggs and larvae are
potentially dispersed large distances during a larval phase lasting 2-3 months. Hence, juveniles
recruiting to the Swan Estuary may have been spawned some distance away from the estuary.
Adults entering the estuary probably originated from another estuary to the south.
Fish kills and environmental impacts on stock: Although sea mullet are known to reside
permanently in sheltered marine waters elsewhere, in south-western Australia the majority of
sea mullet spend their juvenile phase and most of their adult phase in estuaries or rivers and
so can be considered ‘estuarine-dependent’. A general decline in estuarine and riverine habitat
quality throughout south-western Australia may have impacted on growth and mortality of this
stock.
Harmful algal blooms and other factors causing fish kills appear to be a low risk to sea mullet
in the Swan Estuary. Major fish kills are most likely to occur in summer/autumn in the upper
estuary and the majority of the population would probably be elsewhere in the estuary at this
time. In summer/autumn, sea mullet affected by a fish kill in the upper estuary could include
some 0+ and 1+ juveniles, although fish in these age classes would also be widely dispersed
in other parts of the estuary/river. Most adults would be in the lower estuary or ocean at this
time.
Sea mullet consume algae and detritus and so mortality of other fauna during a fish kill would
probably have a low impact on the feeding of this species.
Trophic links: Sea mullet consume algae and detritus and are consumed by numerous larger
predatory fish such as sharks, mulloway, flathead, and tailor (Thomson 1957, Kailola et al.
1993).
Fishery: Sea mullet contributed approximately 22% by weight and 18% by value to recent
annual commercial finfish catches in the Swan Estuary (Tables 2, 3). The commercial catch is
seasonal, peaking in October/November when adults tend to aggregate in the fishery area (Fig.
5). Commercial catches are lowest in winter, when adults migrate to ocean waters to spawn.
Compared to historic levels, recent annual landings of sea mullet in the Swan Estuary and
elsewhere in south-western Australia have been low.
Since 2000, total annual landings of sea mullet in the south-western region of Western Australia
have been declining. Most landings are made by gill nets and beach seine nets in estuaries. In
2004, total commercial landings in the south-western region were approximately 130 t.
There is currently no legal minimum length for sea mullet in the Swan Estuary, but a daily
bag limit of 40 fish applies to recreational fishers. However, recreational catches of this
species in the Swan Estuary are rare because netting is banned in this estuary. Nets are used
by recreational fishers elsewhere in south-western Western Australia to target sea mullet
belonging to the same stock as occurs in the Swan Estuary.
Stock assessment: Commercial catches of sea mullet in the Swan Estuary and other south-west
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estuaries (where the same stock occurs) have steadily declined since the 1950s (DoF, unpubl.
data). The commercial CPUE has been declining since the early 1990s in the Swan Estuary
and has been declining since 1980 in the adjacent Peel-Harvey Estuary. These declines at least
partly reflect a decrease in market demand since the 1980s but probably also reflect a decline
in stock abundance over recent decades. Historically, this species was a major target species in
the Swan Estuary fishery and was known to be relatively abundant in the estuary until the 1980s
(Loneragan et al. 1989). Catches taken during fishery independent surveys in the Swan Estuary
have been declining since the 1980s (I. Potter Murdoch University unpubl. data).
Some fluctuations in CPUE may be due to recruitment variability, which is characteristic
of sea mullet populations (e.g. Smith and Deguara 2002). For example, the average annual
commercial catch and CPUE of sea mullet in the Swan Estuary increased from 1990 to 1997
but decreased from 1997 to 2003. However, it is unclear whether this was due to recruitment
variability because the same fluctuation was not observed in the Peel-Harvey Estuary (DoF
unpubl. data), which presumably experiences similar trends in recruitment by marine larvae of
this species.
Sea mullet within the Swan Estuary are part of a larger stock that is distributed among the
estuaries of south-western Australia. Adults spawn in ocean waters and the annual rate of
juvenile recruitment to the Swan Estuary is not likely to be dependent on adult population
size within the estuary. Therefore, an isolated fish kill in the Swan Estuary is likely to have a
low impact on the stock and a low impact on commercial catches of sea mullet in the estuary.
However, most of the south-western estuaries and rivers where this species occurs are degraded
and experience occasional or annual fish kills. It is likely that the cumulative effects of various
anthropogenic impacts throughout its range are impacting on this stock.
The extent to which the abundance of the south-west sea mullet stock has declined over recent
years is difficult to assess from commercial fishery data due to the confounding effect of
significant variations in effort and market-driven targeting. Unfortunately, fishery-independent
surveys have been infrequent and so provide limited additional information.
6.1.7 Yellow-eye mullet
Biology: Yellow-eye mullet (Aldrichetta forsteri) occur in temperate waters of southern
Australia, including Tasmania, from Shark Bay, WA, to the Hunter River, NSW (Kailola et al.
1993). Yellow-eye mullet within the Swan Estuary are part of a larger stock that is distributed
throughout estuaries and marine waters of south-western Australia. Mixing of the stock
between estuaries occurs via the dispersal of marine eggs and larvae and the movement of
adults.
Yellow-eye mullet is essentially a marine species that utilises coastal waters and the lower,
more saline reaches of estuaries. Older adults mainly occur in coastal waters. Mature fish
migrate from the Swan Estuary to spawn in coastal waters near to the mouth of the estuary
(Chubb et al. 1981). Spawning occurs from April to August. Post-larvae recruit to the Swan
Estuary from marine waters between May and September.
Age 0+ fish occur in shallow waters in the lower, middle and downstream section of the upper
estuary (i.e. mainly below Heirisson Island) throughout the year, and also occur in adjacent
coastal waters (Chubb et al. 1981). Age 1+ and 2+ fish also occur in the same areas but are less
abundant in warmer months, suggesting that many older juveniles migrate to sea in summer.
This probably reflects a preference for cooler water.
Fisheries Research Report [Western Australia] No. 156, 200637
Maturity of yellow-eye mullet is reached at 2-3 y, corresponding to a total length of 300 mm
and a body weight of 297 g (Chubb et al. 1981).
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills appear to be a low risk to yellow-eye mullet in the Swan Estuary. Major fish
kills are most likely to occur in summer/autumn in the upper estuary and the majority of the
population would be downstream or in ocean waters at this time and not be directly affected.
However, fish kills may have indirect effects on yellow-eye mullet, such as reducing the
abundance of prey.
Yellow-eye mullet within the Swan Estuary are part of a larger stock that is distributed among
the estuaries and coastal waters of south-western Australia. Adults spawn in ocean waters and
the annual rate of juvenile recruitment to the Swan Estuary is not likely to be dependent on
adult population size within the estuary. Therefore, high mortality in the Swan Estuary during
one year is likely to have a low impact on the total stock and a low impact on subsequent
fishery catches of yellow-eye mullet in the estuary.
On the other hand, most of the south-western estuaries where this species occurs are degraded
and experience occasional or annual fish kills. While juveniles and adults of this species are
not totally dependent on estuaries (i.e. they also utilise marine habitats), nevertheless many
individuals of this species do occur in estuaries. It is possible that the cumulative effects of
various anthropogenic impacts throughout its range are impacting on the stock.
Trophic links: Yellow-eye mullet is omnivorous, consuming small invertebrates and algae
(Thomson 1957). The population of yellow-eye mullet within the Swan Estuary consists
mainly of small fish (length <30 cm) that probably contribute to the diet of numerous larger
predators such as sharks, mulloway, flathead and tailor (Thomson 1957, Kailola et al. 1993).
Fishery: Yellow-eye mullet contributed approximately 5% by weight and 2% by value to
recent annual commercial finfish catches in the Swan Estuary (Tables 2, 3). It was caught
in minor quantities by recreational fishers (Tables 4-6). Compared to historic levels, recent
annual commercial landings of yellow-eye mullet in the Swan Estuary and elsewhere in southwestern Western Australia have been low.
Since 2000, total commercial landings of yellow-eye mullet in the south-west region of
Western Australia have varied between 40 and 75 t per year. Most landings are made by gill
nets and haul nets in estuaries.
Fishery landings in the Swan Estuary are dominated by older juveniles and young adults, and
these fish tend to migrate to ocean waters during warmer months (Chubb et al. 1981). Hence
commercial catches of yellow-eye mullet Swan Estuary are seasonal, reaching a maximum in
May-July and a minimum in December-January (Fig. 5).
There is currently no legal minimum length for yellow-eye mullet in the Swan Estuary. A
daily bag limit of 40 fish applies to recreational fishers. Since 1986, the body weight of most
yellow-eye mullet retained by the MAAC was between 100 and 200 g, suggesting that most
fish retained by the Club were immature (Fig. 11). The mortality rate of fish that have been
discarded by recreational or commercial fishers is unknown.
Stock assessment: Commercial catches of yellow-eye mullet in south-western estuaries have
steadily declined over recent decades (DoF, unpubl. data). The commercial CPUE has been
declining since the early 1990s in the Swan Estuary and has been declining since 1980 in
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the adjacent Peel-Harvey Estuary. These declines at least partly reflect a decrease in market
demand since the 1980s but could also reflect a decline in stock abundance. This species was
known to be relatively abundant in the Swan Estuary at least until the 1980s (Loneragan et
al. 1989). A decline in catches taken during fishery-independent sampling since the 1980s
(I. Potter, Murdoch University, unpubl. data) and an apparent decline in recreational CPUE
since the mid-1990s suggest a decline in abundance in the Swan Estuary.
The extent to which yellow-eye mullet has declined (if at all) over recent years is difficult to
assess from commercial fishery data due to the confounding effects of significant variations
in effort and market-driven targeting. Unfortunately, fishery-independent surveys have been
infrequent and so provide limited additional information.
6.1.8 Tailor
Biology: Tailor (Pomatomus saltatrix) have a world-wide distribution and occur in temperate
waters of North and South America, east and west coasts of Africa, the Mediterranean Sea and
the Black Sea (Juanes et al. 1996). In Australia, they occur in southern waters from Onslow
(WA) to Fraser Island (Qld). Tailor typically form schools, which occur in continental shelf
waters, coastal waters and estuaries. Stocks on the east and west Australian coasts are separate
(Kailola et al. 1993). Tailor in the Swan Estuary belong to a stock that ranges throughout
southern Western Australia (Edmonds et al. 1999, Young et al. 1999). Genetic mixing over
this region occurs via adult migration and the dispersal of eggs and larvae. Tailor on the lower
west coast are probably a separate stock to those occurring in Shark Bay.
In eastern Australia, adult tailor typically migrate northwards along the coast prior to spawning.
They return southwards after spawning, although it is not clear whether they return to precisely
the same pre-spawning location. Spawning behaviour on the west coast is less well known than
on the east coast because western fish do not form large, conspicuous spawning aggregations.
Water temperature and salinity that is conducive to tailor spawning occurs along much of the
west coast, and so tailor may not need to migrate alongshore to find favourable spawning areas
(unlike on the east coast) (Lenanton et al. 1996). On the west coast, northward movement of
adults does occur and some spawning may occur around Geraldton in spring (Lenanton et al.
1996). However, limited evidence suggests that some spawning also occurs during summer at
offshore reefs in the Perth area and during autumn along the lower west coast, south of Perth.
After a marine larval phase, tailor recruit as small (~40 mm) juveniles to sheltered coastal
areas and estuaries, where they tend to remain until maturity. Juveniles that recruit to the
Swan Estuary are probably spawned either locally or south of the estuary. Ocean circulation
modelling suggests larvae in Perth coastal waters are probably derived from local spawning
sites (Chisholm 2004). However, larvae spawned on the lower west coast in summer/autumn
could potentially be transported northwards to the Swan Estuary by the Capes Current. In
contrast, during the northern spawning season (spring/summer), coastal surface currents are
predominantly northward and so larvae spawned in marine waters off Geraldton are unlikely
to be transported southwards to the Swan Estuary.
Lenanton et al. (1996) estimated that 69% of the lower west coast stock resides in inshore
marine and estuarine habitats and that this component of the stock largely consists of sub-adult
fish <350 mm in length. In the west coast region, tailor reach maturity at 300-350 mm TL,
which corresponds to a body weight of 270-415 g and age 2-3 y (Kailola et al. 1993, Juanes
et al. 1996, DoF unpubl. data).
Fisheries Research Report [Western Australia] No. 156, 200639
Tailor caught in fishery-independent surveys in the Swan Estuary range from 47 to 415 mm in
length, but the majority are juveniles, i.e. <350 mm (Loneragan et al. 1989). Small juveniles
occur in the lower, middle and upper estuary, whereas larger juveniles mostly occur in the
lower and middle estuary (Loneragan et al. 1989).
Tailor reach approximately 150 mm by the end of their first year, and exceed 600 mm TL by
age 5 y (Kailola et al. 1993). Maximum reported size is 1200 mm TL and 14 kg. Maximum
reported age is 9 years (www.fishbase.com). However, age structure is unclear in most stocks,
including south-western Australia, due to difficulties in determining age in tailor.
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills appear to be a low risk to tailor in the Swan Estuary. Major fish kills are most
likely to occur in summer/autumn in the upper estuary and the majority of the population would
probably be downstream (lower or middle estuary) or in ocean waters at this time and not be
directly affected. However, fish kills may have indirect effects on tailor, such as reducing the
abundance of prey.
Tailor within the Swan Estuary are mainly immature fish that comprise a small fraction of a
widespread stock that is distributed among the estuaries and coastal waters of the west coast.
Adults spawn in ocean waters and annual recruitment to the estuary occurs via the influx of
marine post-larvae in summer. The rate of recruitment is independent of the population size
within the estuary. Therefore, high mortality in the Swan Estuary during one year is likely to
have a low impact on the total stock and a low impact on subsequent fishery catches of tailor
in the estuary.
While juveniles and adults of this species are not totally dependent on estuaries (i.e. they also
utilise marine habitats), nevertheless many individuals of this species do occur in estuaries.
Most of the south-western estuaries where juveniles of this species occur are degraded and
experience occasional or annual fish kills. Some coastal waters are also degraded and have
experienced recent fish kills (e.g. pilchard deaths in 1998/99). It is possible that the cumulative
effects of various anthropogenic impacts throughout its range are impacting on the tailor stock,
either directly or through a reduction in prey.
Trophic links: Juvenile tailor consume small fish and invertebrates, while adults mainly
consume fish (Kailola et al. 1993).
Fishery: Tailor in the Swan Estuary are part of a widespread south-western regional stock.
In this region, the majority (94%) of tailor landings are taken on the lower west coast by
recreational fishers in coastal and estuarine waters (Penn 2005). In 2000/01, annual recreational
landings along the lower west coast were estimated to be 187 t (Henry and Lyle 2003). Minor
catches of tailor are taken by commercial fishers in south-west coastal waters, and in the Swan
and Peel-Harvey Estuaries, Hardy Inlet and (to a lesser degree) in south coast estuaries.
Tailor contributed approximately 2% by weight and 2% by value to recent annual commercial
finfish catches in the Swan Estuary (Tables 2, 3). Commercial landings of tailor in the estuary
are seasonal. Highest catches occur from August to December, moderate catches occur
from January to April, and low catches occur from May to July (Fig. 5). Tailor is
highly prized by recreational anglers and was one of the most commonly retained species
in the Swan Estuary recreational fishery in recent years (Tables 4-6). Future recreational
catches of tailor in the estuary may be lower as a result of an increase in the legal minimum
length in 2003 (see below).
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Tailor are vulnerable to capture by recreational fishers from their first year onwards. Prior
to 1999, the average size of tailor retained in the Swan Estuary by the MAAC was <250 g,
indicating that catches were mainly comprised of juveniles. Subsequent catches, from 1999 to
2003, had a slightly larger average size of 250-370 g, but these were still mainly juveniles.
Stock assessment: The total commercial catch and CPUE of tailor in the south-west region
has been stable over an extended period. However, commercial landings may not be a reliable
index of abundance because they represent only 6% of the total west coast catch (Penn 2005).
In contrast, recreational CPUE trends in the Swan Estuary suggest a gradual decline in tailor
abundance in the estuary since 1986/87. Both recreational and commercial CPUE trends suggest
a slight increase in tailor abundance during the late 1990s followed by a marked decline in
abundance, due to poor recruitment, after 2000. The average body size of fish retained by the
MAAC suggest poor recruitment to the estuary from 1998 to 2003, and particularly in 2000/01.
Annual surveys of juvenile recruitment, undertaken since 1995, also indicate that west coast
tailor recruitment was relatively low from 1998 to 2004 (Penn 2005).
Concerns over the status of this stock have led to the daily bag limit for recreational fishers in
the west coast region (including the Swan Estuary) being changed twice since the early 1990s
– from unlimited to 20, then from 20 to 8 per person. In 2003, a provision was added to the
daily bag limit such that only 2 fish >600 mm TL could be retained. However, recent anecdotal
reports from recreational fishers suggest that fish >600 mm TL are very rare, even in offshore
waters where larger tailor are typically found (C. Bibra, pers. comm.). A new upper limit of
500 mm TL has recently been proposed.
At the end of 2003, the legal minimum length (LML) was increased from 250 to 300 mm TL
(i.e. from approximately 165 g to 270 g body weight). The new LML is likely to reduced
recreational tailor landings in the Swan Estuary because the majority of tailor caught in
estuaries are <300 mm. To comply with the LML, most fish caught in estuaries and some
caught in ocean waters must now be released. Post-release mortality of locally caught fish
appears to be relatively low (~3%) within the first few hours of release (Ayvazian et al. 2002)
but longer-term mortality is unknown. A LML of 300 mm TL is still less than the length at
100% maturity, and so some juveniles and young adults are still likely to be caught and retained
in estuaries and ocean waters prior to their first spawning.
Anecdotal reports suggest a recent increase by recreational fishers in targeting larger tailor
around offshore reefs on lower west coast. Targeting of these fish, which appear to be
spawning aggregations, is of concern. Despite bag and size limits, the recreational catch of
tailor in ocean waters is essentially unconstrained.
6.1.9 Black bream
Biology: Black bream (Acanthopagrus butcheri) are endemic to Australia. They inhabit
estuarine waters from Myall Lakes (NSW) to the Murchison River (WA), including Tasmania
but excluding the Great Australian Bight (Kailola et al. 1993). Populations within each estuary
are essentially discrete stocks with very limited movement of adults or eggs/larvae between
estuaries (Norriss et al. 2002, Burridge et al. 2004).
In the Swan Estuary, adults are typically associated with salinities of 10-20 ppt (Sarre 1999).
During winter, when freshwater discharge is high and salinities are very low in the upper
estuary, many adults occur in the middle and lower estuary. In spring/early summer (Oct-Dec),
a large proportion of adults appear to migrate upstream, following the upstream migration of
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41
the salt wedge. Adults remain in the upper estuary until the onset of autumn rains, when they
are either flushed or migrate from the upper estuary.
The movements of tagged and recaptured fish and monthly fishery catch trends are consistent
with these seasonal movement patterns. In March 1995, 767 artificially reared and tagged
juvenile fish were released at Ron Courtney Island (middle estuary) (Dibden et al. 2000).
Recaptures (n = 97) of tagged fish occurred at or upstream of this site, except in winter/early
spring when some fish were recaptured downstream in the middle estuary. The autumn
migration of fish from the upper estuary is followed by a winter peak in commercial catches
in the middle estuary.
Contrary to the general pattern of movement, there is evidence that some adults do not migrate
with the salt wedge and are perhaps resident in each section of the estuary throughout the
year. Firstly, adults are caught in summer/autumn by commercial fishers in the middle estuary
and by recreational fishers in the lower estuary, indicating that not all fish migrate upstream
following the salt wedge. Secondly, large fish are caught by recreational fishers in the upper
estuary in winter (Jun-Sep), even during periods of heavy freshwater discharge. Black bream
are believed to prefer saline or brackish water, but some individuals may persist throughout
the year in the upper estuary by inhabiting deep pockets of brackish water during freshwater
flows (Sarre 1999).
In WA, spawning by black bream may occur in winter, spring or summer depending on the
estuary. In all estuaries, spawning typically occurs at the interface between fresh and brackish
waters (i.e. the boundary of the salt wedge). In the Swan Estuary, spawning mainly occurs
from October to January, with a peak in activity in November (Sarre 1999). At this time, the
boundary of the salt wedge is located in the upper estuary. Spawning is believed to occur in
the upper estuary, from approximately Heirrison Island to Ron Courtney Island (R. Lenanton,
pers. comm). In this area, aggregations of sexually mature fish occur in salinities of 6-25 ppt
but are mainly associated with salinities of 10-19 ppt (Sarre 1999).
During the spawning season, several batches of eggs are released by individual females.
Fecundity increases with female body size. Total fecundity ranges from approximately 350,000
(at 250 mm TL) to 6,600,000 eggs (at 500 mm TL) (Sarre 1999). Egg buoyancy depends on
water density. At salinities and temperatures that are typical of the upper Swan Estuary during
the spawning season, eggs are probably neutrally buoyant (Butcher 1945, Newton 1996,
Partridge et al. 2003). Therefore, eggs could be dispersed downstream by currents although
flow rates at this time are low. Aquarium studies and limited field observations suggest that
larvae are demersal and so are unlikely to disperse far from the spawning site. Small juvenile
black bream are initially distributed in the upper or middle estuary.
Juveniles are typically associated with salinities of 20-30 ppt (Sarre 1999). A low tolerance
to freshwater may partly explain why juveniles never occur in the upstream section of the
upper estuary and are rare throughout the upper estuary in winter/early spring, corresponding
to situations of low salinity (Sarre 1999). After a residence of almost 12 months in the upper
estuary, juveniles may either actively migrate or be flushed downstream at the onset of autumn
rains (Sarre 1999).
Black bream move into deeper water with increasing age. In the Swan Estuary, small juveniles
(16-60 mm) are common during the warmer months in shallow (< 1.5 m depth) beds of
macroalgae (Gracilaria verrucosa). Larger juveniles (> 60 mm) tend to occur over shallow
sand and large adults tend to occur in deeper areas (Sarre 1999).
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Juveniles grow rapidly in their first summer, reaching 100 mm TL after approximately 6
months (Sarre 1999). Length is approximately 125 mm at 1 year and approximately 200 mm at
2 years. Males and females both attain 50% maturity at approximately 2.2 y and 220 mm TL.
Sarre and Potter (2000) observed a maximum age of 21 y among black bream collected from
1993 to 1995 but found that the proportion of fish aged > 5 y in the Swan Estuary population
was small, relative to populations in other estuaries. Black bream exceeding 30 y of age have
been captured in the Blackwood River (S. de Lestang, pers. comm.).
The growth rate of black bream appears to be highly plastic, varying in response to diet, water
temperature and other environmental conditions. For example, in the mid-1990s, average
growth rates in the Swan Estuary were higher than in some other south-western estuaries (Sarre
and Potter 2000). At this time, black bream in the Swan Estuary mainly consumed benthic
invertebrates (mussels, polychaetes, amphipods) and larger fish (>350 mm) also consumed
crabs and fish. In other estuaries, where growth rates were slower, black bream consumed a
higher proportion of algae.
Fish kills and environmental impacts on stock: Harmful algal blooms or other factors
resulting in fish kills in the upper estuary during summer/autumn represent a high risk to this
stock. Black bream occur in this area throughout the year and are likely to be a component
of a fish kill at any time in this location. During summer or autumn, a large proportion of
the population (including eggs, larvae, juveniles and adults) is expected to occur in the upper
estuary. Indeed, black bream were a dominant component of autumn fish kills in the upper
estuary in 2003 and 2004.
In spring/summer (October-January), adults are aggregated to spawn in the upper estuary. In
late summer/autumn, a smaller proportion of spawning adults may be aggregated in the upper
estuary but a large proportion of 0+ juveniles would be present. Harmful algal blooms are less
likely during spring/early summer than in late summer/autumn, although other events such
as chemical spills (e.g. November 1997) have occurred in spring and have resulted in mass
mortality of fish.
During summer/autumn, some juveniles and adults occur upstream and downstream of the
upper estuary and would not be directly affected by a fish kill at this time. This is supported
by the fact that black bream were caught upstream and downstream of the affected area during
and after the 2003 fish kills.
Trophic links: Black bream are omnivorous, consuming a wide range of benthic invertebrates,
algae and small fish. The diet varies between estuaries depending on the availability of food
items. In the Swan River, the dominant dietary component of all black bream of lengths >100
mm are Swan River mussels (Sarre and Potter 2000).
Fishery: The majority (>80% by weight) of recent black bream landings in the Swan Estuary
have been taken by recreational fishers. It is highly prized by anglers and is the most
commonly retained finfish species in the recreational fishery (Tables 4-6). The most recent
survey of the recreational fishery in 2000/01 estimated a retained bream catch of approximately
16 t, and a similar weight of discarded fish (Henry and Lyle 2003). Black bream contributed
approximately 7% by weight and 12% by value to recent annual commercial finfish catches
in the Swan Estuary (Tables 2, 3). Commercial landings of black bream in the Swan Estuary
are highly seasonal (Fig. 5). Most commercial catches occur from July to October, when the
maximum number of adult fish occur in the fishery area (i.e. the middle estuary).
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43
In the Swan Estuary, recreational fishers are subject to a bag limit of 4 black bream, with only 2
fish >400 mm TL allowed to be retained. The legal minimum length is 250 mm (corresponding
to age ~2.7 y and whole body weight ~288 g), which is greater than the size at 50% maturity
(~220 mm, ~193 g). Therefore, most fish have an opportunity to breed before being retained,
if they survive release.
From 1986 to 2003, the vast majority of black bream retained by the MAAC were mature (body
weights 200-800 g). However, black bream are vulnerable to capture by recreational fishers
at approximately 200 mm TL, and many are probably caught and released before reaching
maturity (K. Smith, DoF unpubl. data).
Stock assessment: Fishery catch trends indicate a stable or increasing abundance of black
bream in the Swan Estuary since about 1990. Recent catch levels suggest that this species is
currently one of the more abundant target species in the estuary. However, the abundance of
larger/older fish in 1993-95 (Sarre and Potter 2000) and in 2003/04 (K. Smith, unpubl. data)
was low. In 2003/04, the average size (260-300 mm TL) and age (~3 y) of retained bream in
recreational catches was relatively small (K. Smith, DoF, unpubl. data), compared with the
maximum size (>400 mm) and age (>20 y) for this species.
Relatively high catch rates of small black bream since 1990 suggest that recruitment levels have
been adequate to maintain the stock despite high fishing pressure and the impacts of various
environmental factors in the estuary. However, the relatively small size and age of retained
fish indicates that the stock is growth overfished, which increases the risk of recruitment
overfishing. The relationship between spawning stock size and recruitment of black bream is
not known.
The apparent increase in abundance of black bream suggested by recent catch rates may be
an artefact of two factors. Firstly, an increased proportion of younger fish could influence
catch rates since the catchability of smaller fish is probably higher than that of larger fish.
Secondly, it is possible that changes in the commercial fishery that have occurred since 1990
(reduced effort, changes in catch reporting, entry/exit of fishers from the fishery, etc.) may
have increased the targeting of bream. However, bream has always fetched a good price and
so targeting has probably remained fairly constant.
Black bream population size in the Swan Estuary is likely to vary among years due to variations
in recruitment. Strong year classes suggest high spawning success in 1990/91, 1991/92,
1995/96, 1998/99 and 2003/4, and low spawning success in 1999/00 (Sarre and Potter 2000,
K. Smith, DoF, unpubl. data). In Victoria, populations of black bream also show extremely
variable recruitment (e.g. Morison et al. 1998). Salinity levels are known to affect egg production
and growth, and so variations in rainfall patterns among years could be partly responsible for
recruitment variation by influencing annual spawning output and juvenile survival.
Intense fishing pressure probably results in many black bream being caught and released before
reaching maturity. The mortality of released fish in the Swan estuary is unknown but a study
in the Gippsland Lakes, Victoria, found that post-release mortality of black bream is relatively
high (~23%) when fish were deep-hooked (i.e. throat, gill or gut) and relatively low (~3%)
when shallow-hooked (lip or mouth) (Conron et al. 2004). Generally, larger fish (with larger
mouth gape) are more likely to be deep-hooked.
If fishing techniques can be adjusted to ensure minimal mortality of released fish, then
consideration could be given to alternative management strategies that improve the quality of
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recreational fishing by increasing average fish size, and at the same time decrease the risk of
recruitment overfishing.
Harmful algal blooms and fish kills during summer/autumn in the upper estuary represent a
serious risk to this stock. To fully assess this risk, there is a need for more information about
the movement patterns of adult black bream in the Swan Estuary, including the proportion of
fish that do not migrate and are resident either upstream or downstream of the upper estuary.
These fish are least likely to be directly impacted by a fish kill. The extent to which these
fish contribute to spawning and recruitment is unknown, but may be an important factor
determining the rate of population recovery after a fish kill.
Black bream are inherently vulnerable to depletion because they spend their entire life cycle
in estuaries and there are discrete, self-replenishing stocks in the each estuary. In the Swan
Estuary, high fishing pressure could further limit recovery.
6.1.10 Australian herring
Biology: Australian herring (Arripis georgianus) occur from Shark Bay (WA) southward along
the south coast to the Gippsland Lakes (Vic) (Kailola et al.1993). They constitute a single
stock across this range (Tregonning et al. 1994, How 1997). Adults and juveniles form pelagic
schools in coastal and estuarine waters.
Juveniles occur in Victoria, South Australia and along the south-west coast of WA. Adults
occur only in WA. Maturing juveniles and adults migrate to spawning grounds by travelling
westwards along the southern coast and potentially also northward along the west coast.
Spawning occurs between April and late June in coastal waters of WA, mostly between
Cervantes and Bremer Bay (Fairclough 1998). Pelagic eggs and larvae are transported
southward/eastward by the Leeuwin Current to nursery areas. The westward/northward
distribution of spawning, and the southwards/eastward transport of larvae, depends largely on
the strength of the Leeuwin Current each year.
After spawning, adults disperse and become resident in coastal and estuarine waters of the
south-west region, forming the basis for fishery landings that are taken there throughout
the year. Post-spawning adults enter the Swan Estuary during spring and may remain until
summer/autumn when they return to coastal waters to spawn during winter. Australian herring
is essentially a marine species, and mainly occurs in the lower Swan Estuary.
Juveniles of Australian herring recruit to sheltered coastal waters and estuaries at ~30 mm TL
on the lower west coast, 40-50 mm on the south coast of Western Australia and ~60 mm in
South Australia (Fairclough 1998). In the south-west region, juveniles attain lengths of 140-160
mm TL by the end of their first year (Fairclough 1998). One year old juveniles are smaller on
the south coast and in South Australia, possibly due to slower growth at lower temperatures.
Maturity by both sexes is attained at age 2-3 y. The size at 50% maturity is 196/215 mm TL
(male/female), equivalent to a whole body weight of 83/110 g (Fairclough 1998).
The maximum recorded size for Australian herring is 411 mm TL and 843 g. Most fish in
commercial and recreational landings in Western Australia are 200-269 mm and fish >300 mm
are rare (Fairclough 1998). Maximum age is approximately 14 y, but fish aged >8 y are rare.
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills are a low risk to Australian herring in the Swan Estuary. Major fish kills are
Fisheries Research Report [Western Australia] No. 156, 2006
45
most likely to occur in summer/autumn in the upper estuary and the majority of the population
would be downstream or in ocean waters at this time and not be directly affected. However, fish
kills may have indirect effects on Australian herring, such as reducing the abundance of prey.
Any environmental factor causing high mortality of Australian herring in the Swan Estuary
during one year is likely to have a negligible impact on the total stock and a low impact on
subsequent fishery catches of Australian herring in the estuary. Australian herring within the
estuary are mainly adults and are part of a larger stock that is distributed widely throughout the
coastal waters and estuaries of southern Australia. Spawning occurs in ocean waters and the
annual rate of recruitment by post-spawning adults to the Swan Estuary is not dependent on
the previous population size within the estuary.
The juveniles and adults of this species utilise coastal and estuarine habitats, many of which are
degraded and experience occasional fish kills. While an isolated fish kill in the Swan Estuary
would have a low impact on the stock, it is possible that the cumulative effects of various
anthropogenic impacts throughout its range are impacting on the stock. For example, juvenile
herring grow slightly faster in estuaries than in marine waters and so degradation of estuarine
habitats could impact on stock productivity.
Trophic links: Adult Australian herring consume small fish (e.g. whitebait, anchovies,
garfish, pilchards) and invertebrates. Juveniles feed mainly on small invertebrates, especially
crustaceans (Lenanton et al. 1982). Juvenile and adult Australian herring are consumed by
various large predators such as dolphins, seabirds, sharks, Australian salmon, tailor, Australian
salmon, yellowtail kingfish and mulloway (Kailola et al. 1993).
Fishery: Australian herring in the Swan Estuary are part of a widespread southern Australian
stock. This stock is targeted by recreational fishers and commercial fishers in coastal waters
and estuaries of Western Australia and South Australia. Juveniles of this stock are targeted
by commercial and recreational fishers in South Australia and to a lesser extent in Victoria
(Kailola et al. 1993). Adults are caught in WA, where they are the most common finfish species
in the south-west coastal recreational catch (Penn 2005).
Since 2000, total annual landings of Australian herring in Western Australia have been declining
as a result of declines in commercial catches on the southern coast of WA. In 2004, total
landings in Western Australia were approximately 600-700 t per year, including recreational
and commercial catches (Penn 2005).
Australian herring was among the top 3 finfish species in recent recreational landings in the
Swan Estuary (Tables 4-6). In 2000/01, the retained recreational catch of herring was estimated
to be approximately 2 t (Henry and Lyle 2003). Annual commercial landings of Australian
herring in the Swan Estuary are highly variable but are typically low relative to landings of
other target species. Australian herring contributed approximately 1% by weight and <1% by
value to recent annual commercial finfish catches in the estuary (Tables 2, 3).
Herring migration patterns are reflected in monthly fishery landings in the Swan Estuary.
Commercial catches of herring peak during spring (Sept-Dec), coinciding with an influx
of post-spawning adults to the estuary. Negligible catches occur in winter when adults are
spawning in marine waters (Fig. 5).
From 1986 to 2003, herring catch rates by the MAAC in the Swan Estuary were also seasonal,
typically peaking in summer. The average weight of retained herring was greatest in summer
(peaking at ~200 g in March) and lowest in winter (~130 g in June-July) (K. Smith, DoF,
unpubl. data), reflecting the migration from the estuary by spawning adults in winter.
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A daily bag limit of 40 Australian herring applies to recreational fishers. There is currently
no legal minimum length for this species in the Swan Estuary. From 1987 to 2003, herring
retained by the MAAC in the Swan Estuary were generally adults, with individual fish typically
weighing 100-250 g (Fig. 11a). However, throughout the south-west region, recreational
landings of Australian herring are known to include a small but significant proportion of
immature fish (Fairclough 1998). Approximately 75% of fish in the total recreational catch
are females (Fairclough 1998).
Stock assessment: Trends in Australian herring landings in the Swan Estuary appear to be
driven by highly variable recruitment of adults to the estuary. Peaks in recreational catches in
1993-94, and peaks in both recreational and commercial catches during 1998-2000, suggest
pulses of recruitment to the estuary at these times. There appears to be an inverse relationship
between recreational CPUE and average fish size, consistent with recruitment-driven variations
in the catch of this species (Figs. 11, 12).
The abundance of Australian herring in the estuary, and elsewhere on the west coast, is
determined largely by the strength of the Leeuwin Current each year. A weak current is likely
to result in more fish migrating northwards along the west coast to spawn. Post-spawning fish
then disperse into coastal waters and estuaries, including the Swan Estuary. This annual influx
of post-spawners during spring constitutes the majority of estuary fishery landings. The rate
of recruitment to the Swan Estuary by post-spawners is not dependent on previous population
size within the estuary.
Australian herring within the Swan Estuary are part of a widespread stock that is distributed
around southern Australia. Although degradation of coastal and estuarine habitats could have
a negative impact on Australian herring, the major threat to the stock is fishing pressure. This
stock is subject to strong recreational and commercial fishing pressure across it’s entire range.
Recreational and commercial landings in South Australia and Victoria are comprised entirely
of juveniles and, while most of the fish caught in Western Australia are adults, not all have an
opportunity to spawn before capture. Fishing pressure in all states has the potential to affect
recruitment and stock abundance. Apart from a bag limit, which is probably only occasionally
attained (Roennfeldt 1997), the recreational catch of this species in Western Australia is
unconstrained.
6.1.11 Bar-tailed flathead
Biology: Bar-tail flathead (Platycephalus endrachtensis) is distributed across northern Australia
from Fremantle, WA, to Port Hacking, NSW (Prokop 2002). They also occur in New Guinea
(Allen 1997). They occur on sand, silt and mud habitats in estuaries and are relatively common
in the Swan Estuary compared to their abundance elsewhere.
Stock structure of bar-tail flathead in Western Australia is unknown but, given that spawning
occurs in estuaries, there may be limited mixing between estuarine populations. Therefore,
bar-tailed flathead in the Swan Estuary are probably a discrete breeding stock, reliant on selfreplenishment.
Spawning occurs in the estuary, mainly from December to February (P. Coulson, Murdoch
University, pers. comm.). Larvae have been recorded from the lower, middle and upper estuary
(Neira et al. 1992), suggesting that spawning activity could be widespread in the system. Low
river flows during summer would limit downstream dispersal of pelagic eggs and larvae and
assist in retaining them within the estuary.
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Juveniles and adults occur in the lower, middle and upper estuary throughout the year
(Loneragan et al. 1989, Kanandjembo et al. 2001). However, they mainly occur in the lower
estuary during winter and then disperse as far as the upper estuary during summer/autumn,
following the intrusion of saline water.
In the Swan Estuary, the size at maturity of bar-tail flathead is unclear, but probably occurs
below lengths of 293 mm TL (female) and 222 mm TL (male) (P. Coulson, Murdoch
University, pers comm.). These lengths are equivalent to whole body weights of ~155 g and
~63 g, respectively. Maximum reported size of bar-tail flathead is 100 cm, but in the Swan
Estuary they rare above 55 cm and most fish caught by anglers are 30-45 cm (Prokop 2002).
Maximum age is unknown.
Fish kills and environmental impacts on stock: The abundance of bar-tailed flathead in
the estuary appears to be is highly variable, reflecting trends in annual recruitment. Since
1940, when fishery catch records became available, three periods of high catch rates are
evident (presumably reflecting pulses of recruitment in the estuary). The factors determining
recruitment success are unknown. Interestingly, peaks in flathead catches have been preceded
by peaks in flounder catches a few years earlier, suggesting that recruitment success in these
two species may be influenced by the same environmental factor(s).
Harmful algal blooms and other factors causing fish kills appear to be a moderate risk to
bar-tailed flathead in the Swan Estuary. Major fish kills are most likely to occur in the upper
estuary in summer/autumn. Flathead occur in this area throughout the year. During summer/
autumn, a significant proportion of the population (including eggs, larvae, juveniles and adults)
is expected to occur in the upper estuary. Indeed, flathead were a component of autumn fish
kills in the upper estuary in 2003 and 2004.
However, flathead also occur upstream and downstream of the upper estuary in summer/autumn
and so part of the stock would not be directly affected by a fish kill at this time. Fish kills may
have indirect effects on fish bar-tailed flathead, such as reducing the abundance of prey.
Trophic links: Flathead consume small fish and invertebrates.
Fishery: The majority (~75% by weight) of recent bar-tailed flathead landings in the Swan
Estuary have been taken by recreational fishers. It is among the most commonly retained finfish
species in the recreational fishery (Tables 4-6). The most recent survey of the recreational
fishery in 2000/01 estimated a retained flathead catch of approximately 1.6 t (Henry and Lyle
2003). Flathead contributed approximately 1% by weight and 1% by value to recent annual
commercial finfish catches in the Swan Estuary (Tables 2, 3). Commercial landings of flathead
occurred in all months but were highest from August to November (Fig. 5).
In the Swan Estuary, recreational fishers are subject to a combined flathead/flounder bag limit
of 8 fish. The minimum legal length is 300 mm TL (corresponding to whole body weight
~160 g), which is greater than the size at maturity. Therefore, most fish have an opportunity
to breed before being retained, if they survive release. From 1986 to 2003, the majority of
flathead retained by the MAAC in the Swan Estuary were mature, with individual fish typically
weighing 200-400 g (Fig. 11a).
Stock assessment: Trends in commercial and recreational CPUEs suggest that the abundance
of flathead in the Swan Estuary is highly variable, probably as a result of variable recruitment.
Peaks in CPUE suggest 3 periods of strong recruitment to the estuary: i) in the mid-1940s,
ii) over an extended period in the late 1950s to late 1960s, and iii) in the mid-1990s. A peak
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in 1999 in the average size of flathead retained by the MAAC was also consistent with the
possibility of strong recruitment in the mid-1990s.
Trends in commercial and recreational CPUEs suggest that the abundance of flathead in the
estuary has been declining since the mid-1990s. Harmful algal blooms and other impacts in the
estuary may be contributing to this decline, either directly via mortality of larvae, juveniles and
adults, or indirectly via mortality of prey.
6.1.12 Small-toothed flounder
Biology: Small-toothed flounder (Pseudorhombus jenynsii) are distributed widely across
southern Australia from central Queensland to Exmouth Gulf in WA, but not Tasmania (Prokop
2002). Adults are most often found over sand, mud or gravel in estuaries and bays but can also
occur in shelf waters to depths of ~50 m (Kuiter 1993). Juveniles inhabit coastal waters and
the lower reaches of estuaries (Lenanton 1982). In the Swan Estuary, small-toothed flounder
mainly occur in the lower estuary but may disperse as far as the middle and upper estuary
during summer/autumn (Loneragan et al. 1989).
Stock structure is unknown but small-toothed flounder on the lower west coast, including the
Swan Estuary, are probably a single stock as a result of mixing due to adult movement and
dispersal of planktonic eggs and larvae in marine waters. Spawning occurs in marine waters.
Larvae are transported during flood tides into the lower reaches of estuaries (Young and Potter
2003). Small juveniles <100 mm occur in estuaries in summer/autumn, suggesting spawning
in spring/summer (Lenanton 1977).
Growth and reproduction of small-toothed flounder in south western Australia is poorly
understood. Size and age at maturity is unknown. The maximum age is unknown. The
maximum recorded size is 381 mm TL (Loneragan et al. 1989).
Fish kills and environmental impacts on stock: The abundance of small-tooth flounder
in the estuary is highly variable, reflecting trends in annual recruitment. Since 1940, when
fishery catch records became available, three periods of high CPUE are evident (presumably
reflecting pulses of recruitment to the estuary). The factors determining recruitment success
are unknown. Interestingly, peaks in flounder catches have been followed by peaks bar-tailed
flathead in catches a few years later, suggesting that recruitment in these two species may be
influenced by the same environmental factor(s).
Harmful algal blooms and other factors causing fish kills are a low risk to small-tooth flounder
in the Swan Estuary. Major fish kills are most likely to occur in summer/autumn in the upper
estuary and the majority of the stock would be downstream or in ocean waters at this time
and not be directly affected. However, fish kills may have indirect effects on flounder, such as
reducing the abundance of prey.
Trophic links: Small-toothed flounder consume fish and benthic invertebrates (Schafer et al.
2002).
Fishery: The majority (~70% by weight) of recent flounder landings in the Swan Estuary
have been taken by recreational fishers. The most recent survey of the recreational fishery
in 2000/01 estimated a retained flounder catch of approximately 0.4 t (Henry and Lyle 2003).
Flounder contributed <1% by weight and 1% by value to recent annual commercial finfish
catches in the Swan Estuary (Tables 2, 3). Commercial landings of flounder peaked in spring
(Nov-Dec) and were lowest in winter (May-Sep) (Fig. 5).
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49
In the Swan Estuary, recreational fishers are subject to a combined flathead/flounder bag limit
of 8 fish. The minimum legal length is 250 mm TL (~150 g). From 1986 to 2003, the majority
of flounder retained by MAAC in the Swan Estuary weighed 150-400 g (Fig. 11a).
Stock assessment: Trends in commercial and recreational catch rates suggest that the
abundance of flounder in the Swan Estuary is highly variable, probably as a result of infrequent
pulses of strong recruitment. Peaks in catch rates suggest strong recruitment to the estuary in
approximately 1958 and 1988, and possibly also a minor pulse in the late 1990s.
Trends in catch rates suggest that the abundance of flounder in the estuary has been declining
since 1988 and has been particularly low since 2000. MAAC members agree that flounder has
been relatively rare in the estuary in recent years (D. Cox, MAAC, pers. comm.). The fishery
CPUE of flounder is likely to be a reasonable indicator of abundance because this species is
of relatively high value to each sector and will probably be retained whenever available. Catch
records of the MAAC indicate that flounder was among the more common finfish species in the
club’s catch in the late 1980s and early 1990s, confirming that is highly targeted by recreational
fishers when available.
6.1.13 Yellowfin whiting
Biology: Yellowfin whiting (Sillago schomburgkii), also known as western sand whiting, occur
from Albany to Dampier in Western Australia and in Gulf St Vincent and Spencer Gulf in South
Australia (Kailola et al. 1993). Adults typically occur in open, sandy areas, while juveniles
are typically associated with mangrove, mud or seagrass habitats (Kailola et al. 1993). It is
essentially a marine species. Adults and juveniles are most abundant in shallow (<5 m depth)
inshore marine areas but also occur in estuaries (Lenanton 1982, Hyndes et al. 1996). In the
Swan Estuary, adults and juveniles occur as far upstream as the upper estuary but are most
common in the lower estuary (Loneragan et al. 1989, Kanandjembo et al. 2001).
Stock structure is unknown but yellowfin whiting on the lower west coast, including the Swan
Estuary, are probably a single stock as a result of mixing due to adult movement and dispersal
of planktonic eggs and larvae in marine waters.
Spawning occurs in marine waters. Larvae recruit to inshore and estuarine nursery sites at 1213 mm length (Bruce 1994). Larvae are transported into estuaries during flood tides (Young
and Potter 2003). Spawning by yellowfin whiting occurs later and over a shorter period on the
lower west coast and in South Australia (December to February) than in Shark Bay in (August
to December) (Kailola et al. 1993, Hyndes et al. 1997).
Juveniles grow relatively rapidly and reach 120-130 mm TL (whole body weight of 15-19 g)
by age 1 y and 190-200 mm TL (60-70 g) by age 2 y. On lower west coast, maturity occurs at
the end of the 2nd year in both sexes (Hyndes et al. 1997, Coulson et al. 2005). The maximum
recorded age and length of yellowfin whiting is 14 y and 420 mm, respectively, but fish >6 y
and >350 mm TL are rare on the lower west coast (Kailola et al. 1993, Hyndes et al. 1997).
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills are a low risk to yellowfin whiting in the Swan Estuary. Major fish kills are
most likely to occur in summer/autumn in the upper estuary and the majority of the stock would
be downstream or in ocean waters at this time and not be directly affected. However, fish kills
may have indirect effects on whiting in the estuary, such as reducing the abundance of prey.
Trophic links: Yellowfin whiting consume a wide range of benthic invertebrates (Hyndes
et al. 1997).
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Fishery: The majority of recent yellowfin whiting landings in the Swan Estuary have probably
been taken by recreational fishers. The most recent survey of the recreational fishery in
2000/01 estimated a retained whiting catch of approximately 2 t (Henry and Lyle 2003). This
catch may have included several whiting species, but was probably mainly yellowfin whiting.
Recent annual commercial landings have been <2 t. Whiting (all species) contributed 1%
by weight and 1% by value to recent annual commercial finfish catches in the Swan Estuary
(Tables 2, 3). Commercial landings of whiting were highest in spring (Sep-Dec), moderate in
summer/autumn (Jan-May) and low in winter (Jun-Aug) (Fig. 5).
During the 1990s, total annual commercial landings of yellowfin whiting in the south-west
region (i.e. south of 26º S latitude) increased and peaked at ~65 t (DoF unpubl. data). From
2001 to 2004, total annual landings in this region declined, although they were still within the
typical historical catch range. In 2004, total commercial landings in the south-west region
were approximately 30 t. Most yellowfin whiting landings are made by beach seine, haul and
gill nets in coastal and estuarine waters.
In the Swan Estuary, recreational fishers are subject to a daily bag limit of 16 yellowfin
whiting. There is currently no legal minimum length for this species. From 1986 to 2003,
yellowfin whiting retained by the MAAC typically weighed 100-300 g, suggesting that most
whiting retained by recreational fishers in the Swan Estuary are mature.
Stock assessment: The commercial catch trend for yellowfin whiting suggests an increase
in abundance of this species in the Swan Estuary since 2000. However, annual commercial
catches are generally very low (~400 kg). Recreational catch trends are probably more
indicative of abundance in the estuary. Recreational catch trends suggest that yellowfin
whiting were slightly less abundant, and of smaller size, in the Swan Estuary from 2000 to
2004 than during the 1990s.
6.2 Minor species
6 2.1 Trevally (or Skipjack)
Sand trevally (Pseudocaranx wrighti) and silver trevally (Pseudocaranx dentex) both occur
in the Swan Estuary. Silver trevally attains a larger size but, when small, is very similar in
appearance to sand trevally. Catches of these two species are often grouped together by anglers
and in the scientific literature. Sand trevally may be more common in fishery landings. In
scientific surveys in the Swan Estuary, sand trevally are moderately abundant and have been
caught in shallow water (<2 m) by seine nets, whereas silver trevally are rare and have only
been caught in deeper water using otter trawls (Loneragan et al. 1989).
Biology: Sand trevally are endemic to south-western temperate waters of Australia from
Exmouth Gulf, WA, to Bass Strait (Neira et al. 1998). They form schools that inhabit estuaries,
bays and coastal waters to 35 m. In coastal waters off Perth, sand trevally mainly occur over
sand substrate and are most abundant in deeper (20-35 m) waters (Carter 2005).
Silver trevally occur in temperate and sub-tropical waters of Australia from Rockhampton, Qld,
to North West Cape, WA. They also occur in New Zealand and in subtropical and temperate
waters of the Indian and Atlantic Oceans (Kailola et al. 1993). Silver trevally is mainly a
marine species. Juveniles inhabit estuaries, bays and inner continental shelf waters. Adults
often form demersal schools in deeper waters to 120 m but can also occur at the surface and in
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51
inner shelf waters, bays and large estuaries (Kailola et al. 1993). In the Swan Estuary, trevally
(Pseudocaranx spp.) are found mainly in the lower estuary and occasionally in the middle
estuary (Loneragan et al. 1989).
The stock structure of trevallys on the lower west coast, including the Swan Estuary, is
unknown but both species are probably single stocks as a result of mixing due to adult
movement and dispersal of planktonic eggs and larvae in marine waters. Trevally eggs and
larvae are pelagic.
Sand trevally larvae have been found in Cockburn Sound and in the lower Swan Estuary. In
the estuary, larval abundance declines rapidly with distance from the entrance, which suggests
spawning in nearshore marine waters or at the estuary entrance (Neira et al. 1992). Sand
trevally larvae have been caught in the lower estuary in all seasons except winter, with a peak
in abundance in spring. Spawning by sand trevally occurs from September to March on the
lower west coast, with females spawning more than once during this period (Carter 2005).
Silver trevally can spawn in ocean waters or larger estuaries (Kailola et al. 1993). However,
the absence of larvae suggests that they do not spawn in the Swan Estuary. On the east
Australian coast spawning extends from spring to autumn (Rowling and Raines 2000). The
pelagic larvae can disperse widely across the continental shelf prior to coastal recruitment,
depending on currents (Smith 2003).
The maximum size of sand trevally is probably about 220 mm TL (Hutchins and Swainston
1986). Maximum sizes up to 70 cm TL have been reported (e.g. Gommon et al. 1994), but it
is likely that identification was confused with silver trevally in such cases. In a recent survey
of trawl fishery bycatch on the lower west coast, the maximum size of sand trevally was 213
mm TL (or 120 g) for females and 220mm TL (or 135 g) for males (Carter 2005). Maximum
ages during the survey were 12 y and 11 y, respectively.
The maximum recorded size and age of silver trevally is 94 cm (or 10 kg) and 47 y (Kailola
et al. 1993, Gommon et al. 1994). A recent study over the NSW continental shelf observed
a maximum age of only 24 y, but this population was considered to be growth overfished
(Rowling and Raines 2000).
On the lower west coast, sand trevally attain maturity at approximately 118 mm TL (female)
and 108 mm TL (male), which occurs at 3 y and 1 y of age, respectively (Carter 2005). In this
region, silver trevally mature at approximately 257 mm TL (female) or 265 mm TL (male)
(French 2003). On the east coast, maturity in silver trevally occurs over a size range of 20-28
cm LCF for both sexes (Rowling and Raines 2000). This corresponds to a weight range of
approximately 187-477 g. Age at maturity is probably quite variable for silver trevally but may
typically occur between ages 2-5 y.
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills are a low risk to trevally in the Swan Estuary. Major fish kills are most
likely to occur in summer/autumn in the upper estuary and the majority of the stock would be
downstream or in ocean waters at this time and not be directly affected. However, fish kills may
have indirect effects on trevally in the estuary, such as reducing the abundance of prey.
Trophic links: Trevally are carnivores. Small fish feed mainly on copepods. Larger fish
consume a variety of crustaceans, molluscs, polychaetes and echinoderms (Kailola et al. 1993,
Platell et al. 1997).
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Fishery: Trevally are rarely targeted by commercial fishers in the Swan Estuary (Tables 2, 3).
Surveys in 1998/99 and 2000/01 estimated a minor recreational catch of trevally, which was
taken in the lower estuary (Table 4). In the Swan Estuary, recreational fishers are subject to a
bag limit of 8 trevally.
In 1997, the minimum legal length for trevally was increased from 200 mm to 250 mm TL.
A length of 250 mm TL is equivalent to a whole body weight of approximately 187 g for
silver trevally (Rowling and Raines 2000). At this size, some silver trevally are likely to be
immature. The change in legal length effectively prevented the retention of any sand trevally
by recreational fishers because the maximum length attained by this species is only 220 mm.
From 1986 to 1996, prior to the change in minimum legal length, most trevally retained by
the MAAC weighed 100-160 g and were probably mostly sand trevally. From 1997 to 2003,
most fish retained by the MAAC weighed 180-280 g and were probably silver trevally. These
data indicate that small trevally are vulnerable to capture by recreational fishers and suggest
that both adult sand trevally and juvenile silver trevally are currently caught and released by
recreational fishers in the Swan Estuary.
From 1987 to 2003, trevally CPUE by the MAAC in the Swan Estuary were generally highest
in summer and lowest in winter. The average weight of retained fish was similar between
months (K. Smith, DoF unpubl. data).
The total commercial catch of sand trevally in Western Australia is unknown because
commercial catches of this species are reported as ‘other trevally’, which includes numerous
species. The total Western Australian commercial catch of ‘other trevally’ was 220 t in 2003/04
(Penn 2005), but sand trevally probably comprise a very small proportion of this total. Sand
trevally is taken in relatively large quantities as bycatch by prawn and scallop fishers on the
lower west coast (Platell et al. 1997, Hyndes et al. 1999).
The total Western Australian commercial catch of silver trevally was 6 t in 2003/04, although
some silver trevally may also be reported as ‘other’.
Assessment: Commercial and recreational catches of trevally in the Swan Estuary are
relatively low, and so annual catch rates by each sector may not be a reliable index of stock
abundance. The minimum legal length for trevally increased in 1997, further confounding the
interpretation of trends in recreational CPUE.
The legal minimum length for trevally prevents retention of all sand trevally by recreational
fishers in Western Australia. Such a low level of targeting probably affords reasonable
protection to this stock. The main source of fishing mortality on sand trevally in the south-west
region is probably nearshore trawling, when sand trevally are taken as bycatch.
Silver trevally is relatively slow-growing and long-lived and so is inherently vulnerable to
overfishing. This species has been growth overfished on the east coast. However, silver
trevally are taken in low quantities by commercial and recreational fishers in Western Australia,
compared with catch levels on the east coast (albeit the western stock size may also be smaller).
Most silver trevally retained by recreational fishers on the west coast are probably mature,
although some fish may be caught as young adults prior to their first spawning.
The majority of all trevally caught by recreational fishers in the Swan Estuary are probably
below the legal minimum length and are released. Both adult sand trevally and juvenile silver
trevally are probably caught and released. Rates of post-release mortality of small silver trevally
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53
appear to be low for fish that experience minimal handling and are released immediately after
capture (Broadhurst et al. 2005).
6.2.3 Tarwhine
Biology: Tarwhine (Rhabdosargus sarba) reportedly occur in tropical and subtropical waters
of the Indo-West Pacific, including the Red Sea, east Africa, Australia, China and Japan (Smith
and Heemstra 1984). However, there is some evidence to suggest that these populations
actually consist of several species (Hesp et al. 2004). In Australia, tarwhine are distributed
from Queensland to the Gippsland Lakes, Victoria, and from Albany to Coral Bay, in Western
Australia (Kailola et al. 1993). Adults commonly form schools in coastal waters around reefs
and in estuaries.
Stock structure is unknown, but tarwhine in Western Australia probably comprise a single
genetic stock due to some mixing via movement of adults in ocean waters and dispersal of
eggs/ larvae. There may be sub-populations of adults in each region that largely remain
separate after recruitment, e.g. in Shark Bay.
Throughout Australia, spawning by tarwhine typically occurs in coastal waters, often adjacent
to reefs. Juveniles typically use estuarine or sheltered coastal areas as nursery grounds, and
move to deeper waters with age. However, tarwhine exhibit atypical behaviour in the Swan
Estuary. Spawning occurs in the lower estuary during ebb tides, which allows the pelagic eggs
to be immediately transported from the estuary (Hesp et al. 2004). Larvae then settle out of the
plankton at a length of ~10 mm into sheltered sandy habitats in coastal waters. Juveniles move
into nearby seagrass beds at lengths of >40mm. At ~90 mm length, fish commence movement
to more exposed coastal sites and finally move to offshore reefs, where fish >3 y are common.
It has been suggested that tarwhine usually recruit back to the Swan Estuary at about 1 y of age
and ~140 mm (Hesp et al. 2004). However, small (23-90 mm) juveniles have been caught in
the lower, middle and upper estuary (Loneragan et al. 1989, Kanandjembo et al. 2001).
Spawning near the Swan Estuary occurs over an extended period from July to October, when
individual females spawn multiple batches of eggs (Hesp 2003).
Tarwhine attain a total length of 142, 216 and 254 mm at age 1, 2 and 3 years, respectively
(Hesp 2003). In the Swan Estuary, the size at 50% maturity of tarwhine is ~170 mm TL
(body weight of ~85 g), which occurs an age of 2 y (Hesp and Potter 2003, A. Hesp, Murdoch
University, unpubl. data). The maximum recorded size of tarwhine is 80 cm (Kailola et al.
1993). A maximum age of 13 y has been recorded in Shark Bay (Hesp 2003)
Fish kills and environmental impacts on stock: Harmful algal blooms and other factors
causing fish kills appear to be a low risk to tarwhine in the Swan Estuary. Major fish kills are
most likely to occur in summer/autumn in the upper estuary and the majority of the population
would be downstream or in ocean waters at this time and not be directly affected. However, fish
kills may have indirect effects on tarwhine, such as reducing the abundance of prey.
Tarwhine within the Swan Estuary are probably part of a larger stock that is distributed among
the estuaries and coastal waters of south-western Australia. Adults spawn at the mouth of the
estuary, but larvae are then transported to ocean waters where they presumably mix with larvae
from other spawning aggregations. Juveniles recruiting to the estuary are probably derived
from various spawning locations and so the annual rate of juvenile recruitment to the Swan
Estuary is not likely to be strongly dependent on population size within the estuary. Therefore,
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high mortality in the Swan Estuary during one year is likely to have a low impact on the total
stock and a low impact on subsequent fishery catches of tarwhine in the estuary.
On the other hand, most of the south-western estuaries and some of the coastal areas where this
species occurs are degraded and experience occasional or annual fish kills. While juveniles
and adults of this species are not totally dependent on estuaries (i.e. they also utilise marine
habitats), nevertheless many individuals of this species do occur in estuaries especially as
juveniles. It is possible that the cumulative effects of various anthropogenic impacts throughout
its range could impact on the stock.
Trophic links: Tarwhine are omnivorous. They mainly consume various benthic invertebrates
and algae (Blaber 1984).
Fishery: Tarwhine contributed approximately <1% by weight and <1% by value to recent
annual commercial finfish catches in the Swan Estuary (Tables 2, 3). Recent annual
commercial catches have been <500 kg. The most recent survey of the recreational fishery in
2000/01 estimated a retained catch of <1 t (Henry and Lyle 2003).
In the Swan Estuary, recreational fishers are subject to a daily bag limit of 16 tarwhine. The
minimum legal length is 230 mm TL, which equivalent to a body weight of ~195 g. From 1986
to 2003, the majority of tarwhine retained by the MAAC weighed 180-280 g, suggesting that
tarwhine retained by recreational fishers in the estuary are mature (Fig. 11b).
Stock assessment: Tarwhine have historically been a minor component of commercial and
recreational landings in WA. However, commercial and recreational landings have increased in
recent years throughout the south-west region, possibly reflecting an increase in abundance.
6.2.4 Yellowtail scad
Biology: Yellowtail scad (Trachurus novaezelandiae) are distributed around southern Australia,
from southern Queensland to Exmouth Gulf, WA, and also occur in New Zealand (Kailola et
al. 1993). They are most abundant on the south-east coast, where they are one of the most
common small pelagic fish species. Adults and juveniles are pelagic and form schools in
inshore marine and estuarine waters. Adults often occur over offshore reefs, whereas juveniles
often occur over shallow, soft substrates. Fish tend to move offshore with increasing age (Horn
1991).
The distribution of yellowtail scad in ocean waters appears to be related to temperature. In
the south-eastern Australia, commercial fishers report that fish disappear from coastal waters
in winter when temperatures fall below 13 0C and most reappear in spring/summer when
temperatures reach ~17 0C (Stewart and Ferrell 2001).
Spawning occurs in marine waters, probably over the inner shelf. Eggs and larvae are pelagic
and can disperse widely across the continental shelf, depending on currents (Smith 2003).
Hence, yellowtail scad along the Western Australian coast are probably a single stock due to
adult movement and widespread dispersal of marine larvae. Larvae transform to juveniles at
approximately 14-15 mm and then recruit to inshore and estuarine waters (Neira et al. 1998).
Yellowtail scad reach maturity at 220-250 mm TL, which corresponds to a body weight of
192-250 g and an age of ~3 years (Kailola et al. 1993, Stewart and Ferrell 2001). Growth rate
is variable between regions (Stewart and Ferrell 2001).
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In Australia, a maximum length of 33 cm fork length has been recorded, although 44 cm fork
length has been recorded from New Zealand (Horn 1993, Stewart and Ferrell 2001). Maximum
recorded age is 28 y (Horn 1991).
Fish kills and environmental impacts on stock: Any environmental factor (e.g. harmful
algal bloom) causing high mortality of yellowtail scad in the Swan Estuary is likely to have
a negligible impact on the total stock and a low impact on subsequent fishery catches of this
species in the estuary. Yellowtail scad within the estuary are mainly juveniles and are part of
a widespread stock that is distributed among the coastal waters and estuaries of south-western
Australia. Spawning occurs in ocean waters and the annual rate of recruitment to the Swan
Estuary is not dependent on the previous population size within the estuary.
Populations of yellowtail scad in eastern Australia are characterised by highly variable
recruitment. The environmental factor(s) influencing recruitment success are unknown,
but may include appropriate dispersal of eggs and larvae by ocean currents and adequate
abundance of planktonic prey.
Trophic links: Yellowtail scad are small, pelagic schooling fish. They are likely to be
consumed by numerous larger predators, including sea birds, dolphins, seals, sharks and
finfish. Yellowtail scad are planktivorous.
Fishery: Yellowtail scad have historically been a minor component of commercial and
recreational landings in WA, which reflects low market demand and their relatively low
abundance compared to their abundance in eastern Australia. Commercial landings have
increased in recent years, mainly in response to market demand (R.J. Bales, commercial fisher,
pers. comm.).
Yellowtail scad contributed approximately 1% by weight and <1% by value to recent annual
commercial finfish catches in the Swan Estuary (Tables 2, 3). The annual commercial catch
was <2 t in all years. Commercial landings of yellowtail scad can occur in all months but
are typically highest in February-March (Fig. 5). The most recent survey of the recreational
fishery in 2000/01 estimated a retained catch of <1 t (Henry and Lyle 2003).
Most yellowtail scad caught in the estuary are probably juveniles. The average size of
yellowtail scad retained by the MAAC was 80-120 g, indicating that virtually all retained fish
were immature (Fig. 11b).
In the Swan Estuary, recreational fishers are subject to a daily bag limit of 40 yellowtail scad.
There is currently no legal minimum length for yellowtail scad.
The vast majority of yellowtail scad landings in Western Australia are taken by the West
Coast Purse Seine Managed Fishery. A total Western Australian commercial catch of 15.6 t of
yellowtail scad was recorded in 2003/04 (Penn 2005).
Stock Assessment: Yellowtail scad is typically regarded as a ‘baitfish’, which implies a
schooling species that is fast-growing and short-lived, with highly variable stock abundance.
‘Baitfish’ fisheries are characterised by high variable annual landings and the potential for
sudden stock collapse, due to highly variable recruitment. While yellowtail scad meet some of
these criteria – they are fast growing and have variable recruitment – they are in fact relatively
long-lived (28 y) and require more conservative management than other, shorter-lived baitfish
species.
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Yellowtail scad move offshore with increasing age. Hence, fishery landings in estuaries and
coastal waters comprise relatively young fish. The breeding adults tend to occur offshore
where they are currently subject to moderate fishing pressure in WA. A future increase in
offshore landings would need to be monitored carefully.
Despite a being historically targeted as a ‘baitfish’, yellowtail scad are actually good quality
eating and there are potential domestic and export markets for human consumption. Yellowtail
scad are caught by line and net methods, and so are vulnerable to capture by recreational and
commercial fishers.
6.2.5 Mulloway
Mullloway (Argyrosomus japonicus) occur around southern Australia from North West
Cape, WA, to Bundaberg, Qld. They also occur widely across the Indo-Pacfic (Kailola et al.
1993). They are demersal and mainly inhabit coastal areas (estuaries, coastal reefs, beaches,
embayments) but also occur in continental shelf waters to 150 m. They can be solitary or form
loose schools. Juveniles tend to occur in estuaries and adults in ocean waters, although adults
also enter estuaries. West coast mulloway, including those in the Swan Estuary, are a distinct
population to those in the Great Australian Bight or further east (Kailola et al. 1993).
Mulloway are capable of extensive (100s of kilometres) movement between estuaries. However,
in South Australia, tagged fish have been found to leave the estuary in autumn and return to
adjacent beaches next summer, suggesting a restricted home range (Kailola et al. 1993).
The distribution of mulloway in estuaries tends to coincide with oceanic salinities, although
juveniles also tolerate brackish water. Older mulloway (age >2 y) mainly occur in the lower
and middle sections of the Swan Estuary, but leave the estuary at the onset of winter rains and
return during spring/summer. Younger fish appear to remain in the lower estuary in winter and
disperse to all sections of the estuary at other times of the year (Holt 1978, Loneragan et al.
1989).
Spawning aggregations tend to occur in/around the entrance of estuaries, including in surf zones,
although spawning fish have also been found away from estuaries (Hall 1986, Kailola et al.
1993). Spawning occurs in the Swan Estuary (B. Farmer, Murdoch University, unpubl. data).
In WA, spawning and spent fish have been caught in the lower parts of estuaries, including in
the Swan Estuary (Holt 1978, Lenanton 1977). In general, spawning occurs in spring/summer
and larvae occur in ocean waters during summer/autumn, suggesting a larval phase of several
months (Neira et al. 1998). Eggs and young larvae are pelagic but older larvae are demersal,
thus restricting dispersal in marine waters (Smith 2003). Larvae metamorphose to juveniles
at ~12 mm, but a low abundance of small juveniles suggests that they may not recruit to west
coast estuaries until 100-150 mm ((Neira et al. 1998, Potter et al. 1983). However, smaller
(<50 mm) fish have been caught in eastern Australian estuaries and so the lack of observations
may be due to the difficulty of capturing small demersal fish (Kailola et al. 1993).
In South Australia, mulloway reach ~46 cm (1.5 kg) by 2-3 y of age, and reach ~80 cm
(8 kg) by 5-6 y of age (Kailola et al. 1993). On the lower west coast, growth is relatively rapid
and fish attain 920 cm TL (male) or 950 cm (female) at 6 y (B. Farmer, Murdoch University,
unpubl. data). Maturity occurs at ~6 y of age. In Australia, the maximum reported size
and weight of mulloway is >2 m and >43 kg, respectively, but 71 kg has been recorded in
South Africa. Recent observations on the lower west coast include fish up to 31 years old and
1400 cm TL (B. Farmer, Murdoch University, unpubl. data).
Fisheries Research Report [Western Australia] No. 156, 2006
57
Fish kills and environmental impacts on stock: In South Australia, mulloway abundance
appears to have declined as a result of poor recruitment following consecutive years of low
Murray River flow. Successive years of low freshwater outflow from west coast estuaries,
due to low rainfall or diversion of river flows, may also have had a negative impact on the
stock(s).
Harmful algal blooms and other factors causing fish kills appear to be a low risk to mulloway
in the Swan Estuary. Major fish kills are most likely to occur in the upper estuary in summer/
autumn. Some juvenile mulloway occur in this area at this time but adults would be mostly
downstream or in ocean waters at this time and not be directly affected. However, fish kills
may have indirect effects on mulloway, such as reducing the abundance of prey. For example,
mulloway are known to feed on Perth herring, which is at high risk from fish kills.
Trophic links: Juveniles and adults feed throughout the water column on various fish species
(e.g. mullet, garfish, Australian herring, pilchards, yellowtail scad, smaller mulloway) and
larger benthic invertebrates (e.g. crabs, prawns, worms). Movement and foraging usually
occurs at night, with individuals often returning to the same resting site each day (M. Taylor,
University of NSW, pers. comm.).
Fishery: Mulloway contributed approximately <1% by weight and <1% by value to recent
annual commercial finfish catches in the Swan Estuary (Table 2). Recent annual commercial
landings have been <500 kg. The most recent survey of the recreational fishery in 2000/01
estimated a retained catch of ~1 t (Henry and Lyle 2003).
In the west coast bioregion, total commercial landings of mulloway were approximately 23 t in
2003 and 14 t in 2004. Most of these landings were taken by wetline and gill net fishers. The
total recreational catch in the west coast region was estimated to be about 65 t in 2000/01, most
of which was taken by boat-based fishers in inshore coastal waters (Henry and Lyle 2003).
In the Swan Estuary, recreational fishers are subject to a daily bag limit of 2 mulloway. The legal
minimum length is currently 500 mm, which is significantly less than the size at maturity.
Stock assessment: The status of mulloway stock(s) on the lower west coast is unclear. The
majority of landings on the lower west coast are taken by recreational sector. Apart from bag
and size limits, recreational landings are essentially unconstrained. Mulloway is inherently
vulnerable to overfishing because it is relatively slow-growing, long-lived and late-maturing.
6.2.7 Common blowfish
Biology: Blowfish (Torquigener pleurogramma) are common in estuaries and protected coastal
waters of southern Australia from Coral Bay, WA, to Adelaide, SA, and from Harvey Bay, Qld,
to Narooma, NSW. They do not occur in Bass Strait or Tasmania but do occur around Lord
Howe Island (Hutchins and Swainston 1986, Kuiter 1993). They form small to large schools,
usually over sand (Kuiter 1993).
In the Swan Estuary, blowfish are most abundant in the lower estuary, moderately abundant in
the middle estuary and rare in the upper estuary (Potter et al. 1988). They are most abundant
in shallow (<4 m) waters. Blowfish in the Swan Estuary probably belong to a single genetic
stock along the lower west coast, due to the mixing of planktonic larvae in ocean waters.
However, there is probably some degree of subdivision among populations within estuaries
because movement by adults appears to be limited and eggs are demersal, which would limit
dispersal and mixing.
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Fisheries Research Report [Western Australia] No. 156, 2006
Mature blowfish migrate from the Swan Estuary to spawn at shallow coastal sites immediately
outside the estuary entrance. Spawning occurs from October to January, peaking in November
and December (Potter et al. 1988). Many adults re-enter the estuary after spawning. Large
adult schools have been observed in summer moving from estuary to ocean, and large numbers
of dead blowfish have occasionally been observed along the coast in autumn, presumably
dying after spawning (Hutchins and Thompson 1983).
Juveniles spawned outside the Swan Estuary probably recruit to adjacent coastal nurseries
(e.g. Rockingham), as well as the Swan Estuary. No larvae have been caught in the estuary,
indicating that blowfish recruit to the estuary as juveniles. They typically enter in JulyAugust, at age 7-9 months, length 50-70 mm and weight 2-6 g (Potter et al. 1988). Growth is
apparently faster in estuaries than adjacent coastal areas. At age 1 y, total length is 85-100 mm
in the Swan Estuary but only 74 mm at Rockingham and 65 mm at Jurien Bay and Dongara
(Potter et al. 1988).
Maturity is reached at the end of the 2nd year, at length ~125 mm TL and weight ~39 g. A
maximum length and age of 230 mm TL and 6 y, respectively, has been recorded from the
Swan Estuary (Potter et al. 1988).
Fish kills and environmental impacts on stock: Spawning and recruitment success by
blowfish appear to be extremely variable. For example, Potter et al. (1988) observed very
strong year classes in 1980 and 1985. Limited data from the recreational fishery indicated that
blowfish were very abundant in the Swan Estuary from 1998/99 to 2004. The environmental
factors influencing recruitment success are unknown. Anecdotal reports from recreational
fishers suggest that abundances were low until the early 1970s, coinciding with a marked
decrease in rainfall. Current trends in climate change (lower river flows, warmer temperatures,
more saline estuarine water) may be favouring blowfish recruitment.
Trophic links: Blowfish are opportunistic and consumes a wide variety of benthic invertebrates,
especially polychaetes, amphipods and bivalve molluscs (Potter et al. 1988). Large predatory
fish, including tuna, are reported to consume blowfish.
Fishery: There is no fishery for blowfish in WA. The flesh is poisonous.
Fisheries Research Report [Western Australia] No. 156, 2006
59
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in Western Australian waters: their movement, exploitation, growth and mortality. Marine and
Freshwater Research 50:633-642.
Young G.C. and Potter I.C. 2003. Do the characteristics of the ichthyoplankton in an artificial and a
natural entrance channel of a large estuary differ? Estuarine, Coastal and Shelf Science 56:765779.
Fisheries Research Report [Western Australia] No. 156, 2006
65
8.0
Tables and figures
Table 1. Recent fish kills in Swan-Canning Estuary (communicated by T. Rose, Waters and Rivers
Commission).
Month
Date
Location
Probable cause
Species affected,
number (n) of fish
collected
Summer
Jan, 1999
Guildford, Swan River
Flush of organic matter &
fresh water into river
following heavy rains,
resulted in low oxygen.
n=40, species unknown
Feb 2000
Fremantle (Pier 21)
Microcystis bloom & low
oxygen.
n= <10, blowfish
Feb 2000
Mosman Bay
Possibly disease
n=<10, mullet
Mar 2000
Claisebrook Cove up to
Railway Bridge
Myxosporidiosis & low
oxygen
n=100, mullet
Mar 2000
Mill St drain, Canning
River, at Wilson
Chemical spill over small
area
n=1000, carp & other
species
Apr 2001
Guildford, Swan River
Unknown
n=<10 mullet
Apr-Jun
2003
Perth Water to Ron
Courtney Island &
throughout Canning
River
Karlodinium micrum blooms,
toxins, low oxygen
Nearly all collected
dead fish (approx 7.8 t)
were black bream, but
other species (including
herring, gobies,
flathead, flounder,
mulloway, mullet) also
killed.
Apr 2005
East Perth to Bassendean
Karlodinium micrum blooms,
low oxygen
N = >5000, including
blowfish, black bream,
flathead, Perth herring,
sea mullet, 6-lined
trumpeter &
gobbleguts.
May 2000
Black Creek, Welshpool
Chemical spill over small
area
n=40, carp
May 2001
Ascot
Chemical spill
n=12, bream & mullet
Jun 2004
Como Beach, Matilda
Bay
Karlodinium micrum blooms
24,000 blowfish, 8,000
gobbleguts
Winter
Jul 1998
Cockram St drain,
Canning River
Chemical spill over small
area
no fish collected but
dead observed
Spring
Oct 2001
Mosman Bay
Possibly low oxygen
n=50, seahorses
Nov 1997
Ascot
Chemical spill
n > 15,000, bream &
herring
Autumn
66
Fisheries Research Report [Western Australia] No. 156, 2006
60
Table 2. Average annual landings by Swan Estuary commercial fishery, 1995-2004. (*catch value
based on 2000/01 prices paid to fishers).
Average annual catch, 1995-2004
Species
Average annual
weight (kg)
% of total
catch
weight
% of total
catch
value*
*2000/1
price
$/kg
Blue swimmer crab
Mud crab
Portunus pelagicus
Scylla serrata
19329
1
34
<1
57
<1
4.25
Perth herring
Sea mullet
Black bream
Yellow-eye mullet
Tailor
Yellowtail scad
Cobbler
Flathead
Whiting, various
Australian herring
Tarwhine
Flounder
Other fish
Mulloway
Pilchard
Skates and rays
Yellowtail kingfish
Whitebait
Roach
Yellowtail trumpeter
Pink snapper
Shark, various
Samson fish
Trevally, various
Nematalosa vlaminghi
Mugil cephalus
Acanthopagrus butcheri
Aldrichetta forsteri
Pomatomus saltatrix
Trachurus novazelandiae
Cnidoglanis macrocephalus
Platycephalus endrachtensis
Sillaginidae
Arripis georgianus
Rhabdosargus sarba
Pseudorhombus jenynsii
13393
12258
3832
2828
936
627
470
462
403
285
225
208
177
173
129
113
112
104
91
50
49
30
12
4
24
22
7
5
2
1
1
1
1
1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
4
18
12
2
2
<1
1
1
1
<1
<1
1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
0.45
2.09
4.72
0.89
2.73
36968
19331
66
34
43
57
Total finfish
Total crabs
Argyrosomus japonicus
Sardinops sagax
Elasmobranchi
Seriola lalandi
Hyperlophus vittatus
Gerres subfasciatus
Amniataba caudavittata
Pagrus auratus
(mainly Carcharhinidae)
Seriola hippos
Pseudocaranx spp.
Fisheries Research Report [Western Australia] No. 156, 2006
4.35
2.30
3.55
5.00
67
68
Acanthopagrus butcheri
Torquigener pleurogramma
Arripis georgianus
Pomatomus saltatrix
Sillaginidae
Platycephalus endrachtensis
Sillago schomburgkii
Pseudorhombus jenynsii
Rhabdosargus sarba
various
Hemiramphidae
Amniataba caudavittata?
Cnidoglanis macrocephalus
Trachurus novazelandiae
Pagrus auratus
Argyrosomus japonicus
Pseudocaranx spp.
Pentapodus vita
Aldrichetta forsteri
Portunus pelagicus
Penaeidea
Annelida
Mytilidae
Black bream
Blowfish
Australian herring
Tailor
Whiting, various
Flathead, various
Yellow-finned whiting
Flounder
Tarwhine
Small baitfish
Garfish
Trumpeter
Catfish
Yellowtail scad
Pink Snapper
Mulloway
Trevally
Butterfish
Yellow-eye mullet
Blue swimmer crab
Prawns
Blood worms
Mussels
Species
20176
-
900
793
843
2306
2153
849
775
181
107
107
160
681
minor
699
-
699
3544
831
406
182
302
minor
minor
1049
minor
minor
minor
minor
20875
0
0
0
1599
4337
1674
2712
2335
1151
0
>775
>181
0
0
1156
0
>1
0
107
>160
>681
>1
1998/99 catch (no. of fish)
BoatShorebased
based TOTAL
130985
116366
7399
1632
3532
1558
2030
666
17357
6163
55470
173882
33703
26159
12597
10511
7869
7092
2707
2057
1999
1143
1100
1005
816
696
662
148341
122528
55470
181281
35334
26159
16128
12069
9899
7092
2707
2057
1999
1143
1100
1005
816
696
666
662
-
2000/01 catch (no. of fish)
BoatShorebased
based TOTAL
33080
2083
n/a
n/a
16273
n/a
2312
2420
2079
1605
434
407
411
91
202
143
533
56
307
975
0
0
0
0.22
0.02
n/a
n/a
-
0.46
n/a
0.14
0.20
0.21
0.23
0.16
0.20
0.21
0.08
0.16
0.14
0.65
0.08
0.46
1.47
2000/01 catch (weight)
TOTAL*
kg per fish
(approx)*
(kg)
Table 3. Total estimated catch retained by Swan Estuary recreational fishery in 1998/99a and
2000/01b ( aMalseed and Sumner 2001: creel survey of lower and middle estuary only ;
bHenry and Lyle 2003: national phone survey).
Fisheries Research Report [Western Australia] No. 156, 2006
Table 4. Total number of captured (retained or discarded) fish reported by competitors at
‘Swanfish’, an annual public fishing competition on the Swan Estuary held in February
each year. (*blowfish not fully reported in any year).
Number of fish
2000 2001 2002
Species
Blowfish*
Black bream
Yellowtail trumpeter
Flathead, various
Tarwhine
Yellow-finned whiting
Six-lined trumpeter
Tailor
Flounder
Cobbler
Australian herring
Pink Snapper
Mulloway
Trevally
Yellow-eye mullet
Gobbleguts
Butterfish
Blue swimmer crab
Yellowtail scad
Snook
Whiting, various
Wrasse/groper
Sea garfish
King George whiting
Long-finned pike
Trumpeter whiting
Samson fish
Leatherjacket
Ray
Unidentified
Torquigener pleurogramma
Acanthopagrus butcheri
Amniataba caudavittata
Platycephalus endrachtensis
Rhabdosargus sarba
Sillago schomburgkii
Pelates sexlineatus
Pomatomus saltatrix
Pseudorhombus jenynsii
Cnidoglanis macrocephalus
Arripis georgianus
Pagrus auratus
Argyrosomus japonicus
Pseudocaranx spp.
Aldrichetta forsteri
Apogon rueppellii
Pentapodus vita
Portunus pelagicus
Trachurus novazelandiae
Sphyraena novaehollandiae
Sillaginidae
Labdridae
Hyporhamphus melanochir
Sillaginodes punctata
Dinolestes lewini
Sillago maculata
Seriola hippos
Monocathidae
Elasmobranchi
Number of competitors surveyed
Fisheries Research Report [Western Australia] No. 156, 2006
22
701
124
103
28
39
25
10
22
46
7
14
12
1
3
6
0
3
2
0
3
0
0
1
0
0
107
821
429
306
135
205
109
114
70
30
36
24
10
5
4
1
0
9
2
7
0
1
0
0
1
0
0
0
27
439
2003 2004
0
0
130
369
788
392
471
289
260
48
81
93
40
17
18
20
8
29
4
6
1
12
0
0
1
0
0
0
0
1
0
1
181
67 1541
1610
889
527
547
474
250
392
192
220
267
142
132
82
115
64
29
52
7
29
18
22
0
12
11
28
1
2
4
2
13
8
11
2
10
0
0
0
0
0
1
1
1
1
2
1
0
0
0
1
0
0
0
1
0
0
0
327
108
695
824
1119
976
69
Table 5. Number of fish reported at the monthly Estuary Field Day ‘weigh-in’ by the Melville
Amateur Angling Club (MAAC), 1987 to 2003 (numbers do not include discards).
Species
Australian herring
Black bream
Flathead, various
Yellow-finned whiting
Tarwhine
Six-lined trumpeter
Trevally
Yellowtail trumpeter
Tailor
Wrasse/groper
Sea garfish
King George whiting
Yellowtail scad
Flounder
Mackerel, Blue
Leatherjacket
Butterfish
Yellow-eye mullet
Scaly mackerel
Long-finned pike
Trumpeter whiting
Mulloway
Longtom
Cobbler
Blue weed whiting
Lizardfish/grinner
Banded sweep
Common scalyfin
Footballer/stripey
Snook
Pomfret
Sweetlip
Mackerel/bonito
Australian salmon
Roach
Zebra fish
Redfin perch
Bony herring
Gurnard
Gummy shark
70
Arripis georgianus
Acanthopagrus butcheri
Platycephalus endrachtensis
Sillago schomburgkii
Rhabdosargus sarba
Pelates sexlineatus
Pseudocaranx spp.
Amniataba caudavittata
Pomatomus saltatrix
Labdridae
Hyporhamphus melanochir
Sillaginodes punctata
Trachurus novazelandiae
Pseudorhombus jenynsii
Scomber australasicus
Monocathidae
Pentapodus vita
Aldrichetta forsteri
Sardinella lemura
Dinolestes lewini
Sillago maculata
Argyrosomus japonicus
Belonidae
Cnidoglanis macrocephalus
Haletta semifasciata
Synodontidae
Scorpis georgianus
Parma mccullochi
Microcathus strigatus
Sphyraena novaehollandiae
Schuettea woodwardi
Haemulidae
Scombridae
Arripis truttaceus
Gerres subfasciatus
Girella zebra
Perca fluviatilis
Elops machnata
Chelidonichthys kumu
Mustelus antarcticus
Years
1987-1991 1992-1995
123
230
131
295
128
214
103
730
113
11
168
92
92
210
176
278
162
133
15
24
54
259
64
39
87
37
2
2
330
2
38
1
679
210
251
17
2
2
1
2
1
1
1
89
1
2
1996-1999 2000-2003
105
406
159
259
122
193
185
185
45
175
93
119
81
112
117
105
100
99
14
85
59
7
27
30
21
48
20
7
12
12
1
11
47
8
6
4
5
25
4
8
2
2
58
1
3
1
2
1
1
1
1
1
1
1
4
12
3
4
3
2
1
TOTAL
864
844
657
1203
344
472
495
676
494
138
59
152
90
406
73
16
14
644
6
9
67
13
2
948
4
3
3
1
1
1
1
1
93
14
7
5
3
3
1
1
Fisheries Research Report [Western Australia] No. 156, 2006
Table 6. Abundance and status of key fishery stocks in Swan Estuary.
C = commercial fishery; R = recreational fishery; 1 = primary species; 2 = secondary species; 0 = minor/not
caught; O = marine/estuarine opportunist (spawns at sea but common in estuaries, particularly as juveniles); E
= estuarine (only found in estuaries); EM = estuarine and marine (can spawn in estuaries or ocean, often forms
discrete populations in estuaries); A = semi-anadromous (spends part of life at sea but migrates to upper estuary
to spawn); ↑ = catch trends suggest increase in estuary abundance; ↓ = catch trends suggest decrease in estuary
abundance; N/A = insufficient data. ‘Fish kill’ risk (High/Medium/Low) = risk to stock from major fish kill in
Swan Estuary.
Species
Recent
Fishery
Importance
Life
History
Category
‘Fish kill’ risk
Estuary catch trend
since 1990
Black bream
C1, R1
E
H
↑
Yellowtail scad
C2, R2
O
L
↑
Tarwhine
C2, R2
O
L
↑
Blue swimmer crab
C1, R1
O
L
↑
Australian herring
C2, R1
O
L
↑ (longer-term trend is ↓)
Flathead
C2, R1
EM
M
↑ (longer-term trend is stable)
Flounder
C2, R2
O
L
↓ (longer-term trend is stable)
Six-lined trumpeter
C0, R2
O
L
Stable/fluctuating
Tailor
C1, R1
O
L
Stable/fluctuating
Yellowfin whiting
C1, R2
O
L
↓
Sea mullet
C1, R0
O
L
↓
Yellow-eye mullet
C1, R0
O
L
↓
Perth herring
C1, R0
A
H
↓
Cobbler
C2, R2
EM
M
↓
Yellowtail trumpeter
C2, R2
E
M
↓
School prawn
C0, R2
E
M
↓
Trevally
C0, R2
O
L
N/A
Mulloway
C2, R2
O
L
N/A
Garfish
C0, R2
O
L
N/A
Pink snapper
C0, R2
O
L
N/A
Fisheries Research Report [Western Australia] No. 156, 2006
71
Figure 1. Map of Swan-Canning Estuary. ‘Lower’ estuary = entrance to Blackwall Reach;
‘Middle’ estuary = Blackwall Reach to Heirisson Island, and lower half of Canning River;
‘Upper’ estuary = upstream of Heirisson Island and upper half of Canning River. (Note:
commercial fishery is located in middle estuary.
72
Fisheries Research Report [Western Australia] No. 156, 2006
Number of boats
60
2a) Effort
40
20
0
Annual catch (t), finfish
300
2b) Catch
1960
1980
40
Finfish
Crabs
Prawns
30
200
20
100
10
0
0
1920
16000
1940
1960
1980
6000
2c) CPUE
12000
4000
8000
2000
4000
0
Catch rate (kg/boat),
crabs and prawns
Catch rate (kg/boat), finfish
1940
Annual catch (t), crabs and prawns
1920
0
1920
1940
1960
1980
Year
Figure 2. Annual a) total fishing effort (number of registered boats), b) catch and c) CPUE of total
finfish, crabs and prawns in the Swan Estuary commercial fishery from 1912 to 1999.
Some incomplete catch and effort data prior to 1939 and 1952, respectively. (Data from
2000 to 2004 not shown because catches were taken by <5 fishers in these years).
Fisheries Research Report [Western Australia] No. 156, 2006
73
Annual catch (t)
2000
Landed weight
Live weight
1500
1000
500
0
1975
1980
1985
Year
1990
1995
Figure 3. Comparison of annual landings by the Swan Estuary commercial fishery reported as ‘live
weight’ and ‘landed weight’ in CAES database, 1975 to 1999. (Data from 2000 to 2004
not shown because catches were taken by <5 fishers in these years)
Average monthly number of boats
25
40
30
20
10
0
10000
8000
6000
4000
20
4000
15
3000
10
2000
5
1000
0
0
16000
15000
CPUE (kg/active boat
12000
Mean monthly number of active boats
Number of gear days
Number of fishing days
Number of registered boats
kg/active boat
kg/gear day
kg/fishing day
kg/registered boat
4b) Catch rate
100
90
14000
80
13000
70
12000
60
11000
50
10000
9000
1975
CPUE (kg/fishing or gear day)
CPUE (kg/registered boat)
14000
5000
4a) Effort
Number of days
Number of registered boats
50
40
1980
1985
1990
1995
Year
Figure 4. a) Comparison of the various available fishing effort units and b) their effect on the
calculation of total annual CPUE in the Swan Estuary commercial fishery, 1976 to 1999.
(Data from 2000 to 2004 not shown because catches were taken by <5 fishers in these
years)
74
Fisheries Research Report [Western Australia] No. 156, 2006
1200
600
Australian herring
600
300
0
0
20000
Perth herring
1200
Flathead
Cobbler
800
10000
400
0
0
8000
Tailor
Black bream
2000
4000
1000
0
Monthly catch (kg)
0
Whiting (mainly S. schombergkii)
6000
Yellow-eye mullet
400
4000
200
2000
0
0
1600
Flounder
Yellowtail scad
400
800
200
0
0
Sea mullet
Blue swimmer crab
20000
40000
10000
20000
0
0
J
F M A M J
J
J A S O N D
F M A M J
J A S O N D
Month
Figure 5. Mean (+ s.d.) monthly landings of key species in the Swan Estuary commercial fishery,
1996-2002.
Fisheries Research Report [Western Australia] No. 156, 2006
75
160
1200
Black bream
6
800
400
2
0
0
Cobbler
2000
20
1000
20
1000
500
10
0
0
0
Perth herring
8000
120
80
4000
40
0
0
6
300
Flathead
4
400
0.8
200
2
100
0
20
0
1200
Tailor
800
12
8
40
0.4
400
Yellowtail trumpeter
300
16
80
0
4
1
Flounder
800
6
100
0
120
1200
4
8
200
0
1.6
1600
0
3
2
8
0
Mulloway
CPUE (Kg/boat)
12000
CPUE (Kg/boat)
Catch (t)
1500
30
10
1.2
2000
Yellow-eye mullet
40
30
5
0
50
3000
40
160
2000
40
0
60
200
4000
80
4
50
6000
Sea mullet
120
Catch (t)
8
400
4
0.0
0
0
1920
1940
Year
1960
0
1920
1980
1940
1960
1980
Year
Figure 6. Annual catch (solid line) and CPUE* (dotted line) of key finfish species in Swan Estuary
commercial fishery from 1912 to 1997 (*CPUE derived by using ‘number of registered
boats’ as the unit of effort). (Data from 1998 to 2004 not shown because catches were
taken by <5 fishers in these years).
100
200x103
4000
80
150x103
3000
100x103
2000
50x103
1000
Effort
CPUE
0
0
1975
1980
1985
1990
60
40
20
Cpue (kg/gear day)
5000
Catch
Effort (gear days)
Catch (kg)
250x103
0
1995
YEAR
Figure 7. Recent total annual catch, effort and CPUE in Swan Estuary commercial fishery from
1975 to 1999. (Data from 2000 to 2004 not shown because catches were taken by <5
fishers in these years).
76
Fisheries Research Report [Western Australia] No. 156, 2006
Finfish
Elasmobranchs
Crabs
a) Catch
150x10
300
100x103
200
50x103
100
0
30x103
20x103
10x103
0
100
1980
1985
1990
1995
0.8
80
0.6
60
0.4
40
0.2
20
0
1975
0.0
1980
1985
1990
1995
30
20
10
Crab CPUE (kg/gear day)
b) CPUE
0
Elasmobranch CPUE (kg/gear day)
1975
Crab catch (kg)
3
Finfish CPUE (kg/gear day)
40x103
400
Elasmobranch catch (kg)
Finfish catch (kg)
200x103
0
YEAR
Figure 8. Recent annual a) catch and b) CPUE of major taxonomic groups in Swan Estuary
commercial fishery from 1975 to 1999. (Data from 2000 to 2004 not shown because
catches were taken by <5 fishers in these years).
Fisheries Research Report [Western Australia] No. 156, 2006
77
Effort (fisher days)
250
1000
250
600
150
400
100
200
50
1988
1990
1992
1994
1996
1998
2000
0
2004
2002
b)
25
0.6
0.5
20
0.4
15
0.3
10
0.2
number of species
mean fish weight
catch rate
5
0
1986
1988
1990
1992
1994
1996
1998
2000
0.1
2002
0.0
2004
150
100
50
5
4
3
2
1
0
Annual mean catch rate (fish/angler/day)
30
200
0
Annual mean fish weight (kg/fish)
Number of species in annual catch
300
800
200
0
1986
1200
Annual catch (no. of fish)
300
effort (angler days)
total catch (no. of fish)
total catch (kg)
a)
Annual catch (weight of fish, kg)
350
Year
Figure 9. Melville Amateur Angling Club (MAAC) Swan Estuary fishing competition data. a) Annual
catch and effort. b) Annual number of retained species, mean weight of retained fish and
mean CPUE, 1986 to 2003. (Some fish weights not available in 1998 and 1999).
78
Fisheries Research Report [Western Australia] No. 156, 2006
0.6
1.0
Black bream
0.5
0.8
Tarwhine
0.4
0.6
0.3
Catch rate (mean (+s.e.) number of fish per angler per day)
0.4
0.2
0.2
0.1
0.0
0.0
1988
1992
1996
2000
2004
1988
1992
1996
2000
2004
1992
1996
2000
2004
1996
2000
2004
1996
2000
2004
1.2
0.8
Flathead
Cobbler
0.6
0.8
0.4
0.4
0.2
0.0
0.0
1988
1992
1996
2000
2004
1988
1.2
Flounder
Australian herring
0.4
0.9
0.6
0.2
0.3
0.0
0.0
1988
1992
1996
2000
2004
1988
1992
1.0
Yellow-eye mullet
0.8
Yellowtail scad
0.8
0.6
0.4
0.4
0.2
0.0
0.0
1988
1992
1996
2000
2004
1988
1992
Financial Year
Figure 10a. Mean annual CPUE (+ s.e.) of key species retained by Melville Amateur Angling Club
during their Swan Estuary fishing competition, 1986 to 2003.
Fisheries Research Report [Western Australia] No. 156, 2006
79
1.0
0.8
Tailor
Trevally
0.8
0.6
0.6
0.4
0.4
0.2
0.2
Catch rate (mean (+s.e.) number of fish per angler per day)
0.0
0.0
1988
1.6
1992
1996
2000
2004
1988
1992
Six-lined trumpeter
1996
2000
2004
Yellowtail trumpeter
0.8
1.2
0.8
0.4
0.4
0.0
0.0
1988
1992
1996
2000
2004
1988
1992
1996
2000
2004
1996
2000
2004
1996
2000
2004
1.0
King George whiting
0.8
0.4
Trumpeter whiting
0.6
0.4
0.2
0.2
0.0
0.0
1988
1992
1996
2000
2004
1988
1992
2.0
1.6
Yellow-finned whiting
0.3
1.2
Wrasse/groper
0.2
0.8
0.1
0.4
0.0
0.0
1988
1992
1996
2000
2004
1988
1992
Financial Year
Figure 10b. Mean annual CPUE (+ s.e.) of key species retained by Melville Amateur Angling Club
during their Swan Estuary fishing competition, 1986 to 2003.
80
Fisheries Research Report [Western Australia] No. 156, 2006
0.4
Black bream
1.0
Tarwhine
0.3
0.8
0.6
0.2
0.4
0.1
0.2
0.0
0.0
1988
0.8
1992
1996
2000
2004
Cobbler
1988
0.6
1992
1996
2000
2004
1992
1996
2000
2004
1996
2000
2004
1996
2000
2004
Flathead
0.6
0.4
Average (+s.e.) weight of retained fish (kg)
0.4
0.2
0.2
0.0
0.0
1988
1992
1996
2000
2004
1988
Flounder
0.4
Australian herring
0.2
0.3
0.2
0.1
0.1
0.0
0.0
1988
1992
1996
2000
2004
1988
0.12
0.3
Yellow-eye mullet
1992
Yellowtail scad
0.08
0.2
0.04
0.1
0.0
0.00
1988
1992
1996
2000
2004
1988
1992
Financial Year
Figure 11a. Mean (+ s.e.) annual body weight of key species retained by Melville Amateur Angling
Club during their Swan Estuary fishing competition, 1986 to 2003.
Fisheries Research Report [Western Australia] No. 156, 2006
81
0.4
0.4
Tailor
Trevally
0.3
0.3
0.2
0.2
0.1
0.1
0.0
0.0
1988
1992
1996
2000
2004
0.20
1988
1992
0.3
1996
2000
2004
Yellowtail trumpeter
0.15
0.2
Average (+s.e.) weight of retained fish (kg)
0.10
0.1
0.05
0.00
Six-lined trumpeter
1988
1992
0.0
1996
2000
2004
0.3
1988
1992
1996
2000
2004
1996
2000
2004
1996
2000
2004
0.15
Trumpeter whiting
0.2
0.10
0.1
0.05
0.0
King George whiting
1988
1992
0.00
1996
2000
2004
1988
1992
0.3
0.8
0.2
Wrasse/groper
0.6
0.4
0.1
0.2
Yellow-finned whiting
0.0
0.0
1988
1992
1996
2000
2004
1988
1992
Financial Year
Figure 11b. Mean (+ s.e.) annual body weight of key species retained by Melville Amateur Angling
Club during their Swan Estuary fishing competition, 1986 to 2003.
82
Fisheries Research Report [Western Australia] No. 156, 2006
9.0
Appendices
Appendix 1. Pre- July 1989 format of commercial fisher compulsory monthly catch returns.
APPENDICES.
79
Appendix 1. Pre- July 1989 format of commercial fisher
compulsory monthly catch returns
Fisheries Research Report [Western Australia] No. 156, 2006
83
Appendix 2. 2003 format of commercial fisher compulsory
monthly catch returns
Original post to Department of Fisheries Duplicate - retain for your records
Netting: catch and effort return
Office
Use
Fish Resources Management
Regulations 1995 Regulation 64
LFB
Year
Month
2005
Boat name
Boat registration
LFB
SEPT
A 199
BLUEFIN
Fishing Boat Licence
FBL
Managed fishery licence(s)
MFL
Anchorage
Master’s CFL No.
Master’s name (Authorisation holder or agent)
Phone no.
Address
Fuel purchased (litres)
I certify that the information on this form is
correct (Master, authorisation holder or agent)
1432
SCEP 23
ALBANY
1000
Months you propose not to fish
OCT
9845 3210
Crew number
(inc master)
No. days fished.
12
3
Fishery eg. SGL, SCPS, SCEP
(one fishery per column)
If applicable
J. CITIZEN
1200
SCEP
1 HIGHWAY RD ALBANY
SCEP
If applicable
(one zone per column)
(one method per column)
Block number
(one block per column)
Days fished
Hours fished per day
GN
GN
Block number
Days fished
6
WL
Zone fished
Other methods eg. HR
9507 8511
4
4
8
4/10/05
Fishery eg. WL, EXEM
Zone fished
Netting methods eg. PS
Date signed
Hours fished per day
HR
9603
4
4
Pots/traps pulled per day
Hooks per day
1
Shots/pulls per day
Net length (m) per shot
Mesh size range (mm)
Species
(include all retained catch)
YE MULLET
WHITING KG
WS WHITING
COBBLER
BLACK BREAM
Dealer/processor
Shots/pulls per day
1000 800
64-102 102
kg
WH 123
WH 14
WH 10
HG
6
WH
kg
Net length (m) per shot
Mesh size range (mm)
kg
No
kg
Species
(include all retained catch)
WHITING KG
PINK SNAPPER
Condition
codes
kg
kg
WH 150
GG 105
e
l
p
251
m
a
x
E
Crew names
BAYFISH PROCESSORS
Have you had an interaction
with a protected species?
Yes
Condition
codes
1
6
A.FISHER
O.SHUN
Species, location and other comments
15/9 STOKES INLET - PELICAN TANGLED, CUT OUT OF NET, FLEW AWAY.
If yes, was animal released
Alive
Dead
Notification of months when no fishing occurred is required on this form. A signed facsimile of this form may be submitted
84
Fisheries Research Report [Western Australia] No. 156, 2006
List of Fisheries Research Reports
Not all have been listed here, a complete list is available online at http://www.fish.wa.gov.au
83 The Western Rock Lobster fishery 1985/86.
Brown, R.S. and Barker, E.H. (1990).
84 The Marine open shelf environment: review of
human influences. Hancock, D.A. (1990).
85 A Description of the British United Trawlers /
Southern Ocean Trawlers operation in the Great
Australian Bight during the period 19.11.77 to
28.5.79. Walker, M.H., Blight, S.J. and Clarke, D.P.
(1989).
86 The Demersal trawl resources of the Great
Australian Bight as indicated by the fishing
operations of the stern trawlers Othello, Orsino and
Cassio in the period 19.11.77 to 28.5.79. Walker,
M.H. and Clarke, D.P. (1990).
87 The recreational marron fishery in Western
Australia summarised research statistics, 1971–
1987. Morrissy, N.M. and Fellows, C.J. (1990).
88 A synopsis of the biology and the exploitation of
the Australasian pilchard, Sardinops neopilchardus
(Steindachner). Part 1: Biology. Fletcher, W.J.
(1990).
89 Relationships among partial and whole lengths
and weights for Western Australian pink snapper
Chrysophrys auratus (Sparidae). Moran, M.J. and
Burton, C. (1990).
90 Unpublished.
91 A synopsis of the biology and the exploitation of
the Australasian pilchard, Sardinops neopilchardus
(Steindachner) Part II : History of stock
assessment and exploitation. Fletcher, W.J. (1991).
92 Spread of the introduced yabbie Cherax albidus
Clark, 1936 in Western Australia. Morrissy, N.M.
and Cassells, G. (1992).
93 Biological synopsis of the black bream,
Acanthopagrus butcheri (Munro) (Teleostei:
Sparidae). Norriss, J.V., Tregonning, J.E., Lenanton,
R.C.J. and Sarre, G.A. (2002).
94 to 98 No reports were published under these
numbers.
99 An Investigation of weight loss of marron (Cherax
tenuimanus) during live transport to market.
Morrissy, N.; Walker, P.; Fellows, C.; Moore, W.
(1993).
100 The Impact of trawling for saucer scallops and
western king prawns on the benthic communities in
coastal waters off south-western Australia. (FRDC
final report 90/019 ) Laurenson, L.B.J., Unsworth,
P., Penn, J.W. and Lenanton, R.C.J. (1993).
101 The Big Bank region of the limited entry fishery for
the western rock lobster Panulirus cygnus. Chubb,
C.F., Barker, E.H. and Dibden, C.J. (1994).
102 A Review of international aquaculture development
and selected species in environments relevant to
Western Australia. Lawrence, C.S. (1995).
103 Identifying the developmental stages for eggs of
the Australian pilchard, Sardinops sagax. White,
K.V. and Fletcher, W.J. (Warrick Jeffrey) (1998).
104 Assessment of the effects of a trial period of
unattended recreational netting in selected
estuaries of temperate Western Australia.
Lenanton, R.C., Allison, R. and Ayvazian, S.G.
(1996).
112 Final report, FRDC project 94/075: enhancement
of yabbie production from Western Australian farm
dams. Lawrence, C., Morrissy, N., Bellanger, J. and
Cheng, Y. W. (1998).
113 Catch, effort and the conversion from gill nets
to traps in the Peel-Harvey and Cockburn Sound
blue swimmer crab (Portunus pelagicus) fisheries.
Melville-Smith, R., Cliff, M. and Anderton, S.M.
(1999).
114 The Western Australian scallop industry.
Harris, D.C., Joll, L.M. and Watson, R.A. (1999).
115 Statistical analysis of Gascoyne region recreational
fishing study July 1996. Sumner, N.R. and Steckis,
R.A. (1999).
116 The western rock lobster fishery 1993/94 to
1994/95 Chubb, C.F. and Barker, E.H. (2000).
117 A 12-month survey of coastal recreational boat
fishing between Augusta and Kalbarri on the
west coast of Western Australia during 1996-97.
Sumner, N.R. and Williamson, P.C. (1999).
118 A study into Western Australia’s open access
and wetline fisheries. Crowe, F., Lehre, W. and
Lenanton, R.J.C. (1999).
119 Final report : FRDC project 95/037 : The biology
and stock assessment of the tropical sardine,
Sardinella lemuru, off the mid-west coast of
Western Australia. Gaughan, D.J. and Mitchell,
R.W.D. (2000).
136 Assessment of gonad staging systems and other
methods used in the study of the reproductive
biology of narrow-barred Spanish mackerel ,
Scomberomorus commerson, in Western Australia.
Mackie, M. and Lewis, P. (2001).
137 Annual report on the monitoring of the recreational
marron fishery in 2000, with an analysis of longterm data and changes within this fishery. Molony,
B. and Bird, C. (2002).
138 Historical diving profiles for pearl oyster divers in
Western Australia. Lulofs, H.M.A. and Sumner, N.R.
(2002).
139 A 12-month survey of recreational fishing in the
Gascoyne bioregion of Western Australia during
1998-99. Sumner, N.R., Willimson, P.C. and
Malseed, B.E. (2002).
121 Synopsis of the biology and exploitation of the blue
swimmer crab, Portunus pelagicus Linnaeus, in
Western Australia. Kangas, M.I. (2000).
142 Identifying the developmental stages of preserved
eggs of snapper, Pagrus auratus, from Shark Bay,
Western Australia. Norriss, J. V. and Jackson G.
(2002).
122 Western rock lobster mail surveys of licensed
recreational fishers 1986/87 to 1998/99. MelvilleSmith, R. and Anderton, S.M. (2000).
123 Review of productivity levels of Western Australian
coastal and estuarine waters for mariculture
planning purposes. CDRom in back pocket has title
“Chlorophyll-a concentration in Western Australian
coastal waters - a source document. by S. Helleren
and A. Pearce” (document in PDF format) Pearce,
A., Helleren, S. and Marinelli, M. (2000).
124 The Evaluation of a recreational fishing stock
enhancement trial of black bream (Acanthopagrus
butcheri) in the Swan River, Western Australia.
Dibden, C.J., Jenkins, G., Sarre, G.A., Lenanton,
R.C.J. and Ayvazian, S.G. (2000).
125 A history of foreign fishing activities and fisheryindependent surveys of the demersal finfish
resources in the Kimberley region of Western
Australia. [Part funded by Fisheries Research and
Development Corporation Project 94/026] Nowara,
G.B. and Newman, S.J. (2001).
126 A 12 month survey of recreational fishing in the
Swan-Canning Estuary Basin of Western Australia
during 1998-99. Malseed, B.E. and Sumner, N.R.
(2001).
127 A 12 month survey of recreational fishing in the
Peel-Harvey Estuary of Western Australia during
1998-99. Malseed, B.E. and Sumner, N.R. (2001).
106 Environmental and biological aspects of the mass
mortality of pilchards (Autumn 1995) in Western
Australia. Fletcher, W.J., Jones, B., Pearce, A.F. and
Hosja, W. (1997).
129 Morpholgy and incidence of yabby (Cherax albidus)
burrows in Western Australia. Lawrence, C.S.,
Brown, J.I. and Bellanger, J.E. (2001).
109 The western rock lobster fishery 1991/92 to
1992/93. Chubb, C.F. and Barker, E.H. (1998).
135 The western rock lobster fishery 1995/96 to
1996/97. Chubb, C.F. and Barker, E.H. (2002).
140 The western rock lobster fishery 1997/98 to
1998/99. Chubb, C.F. and Barker, E.H. (2003).
128 Aquaculture and related biological attributes of
abalone species in Australia - a review. Freeman,
K.A. (2001).
108 Aspects of the biology and stock assessment
of the whitebait, Hyperlophus vittatus, in south
western Australia. Gaughan, D.J., Fletcher, W.J.,
Tregonning, R.J. and Goh, J. (1996).
134 Towards an assessment of the natural and human
use impacts on the marine environment of the
Abrolhos Islands. Volume 1, Summary of existing
information and current levels of human use.
CDRom in back pocket has the title “Abrolhos
Habitat Survey”. Webster, F.J., Dibden, C.J., Weir,
K.E. and Chubb, C.F. (2002). Volume 2, Strategic
research and develoment plan. Chubb, C.F.,
Webster, F.J., Dibden, C.J. and Weir, K.E. (2002).
120 A 12 month survey of recreational fishing in
the Leschenault Estuary of Western Australia
during 1998. Malseed, B. E., Sumner, N.R. and
Williamson, P.C. (2000).
105 The western rock lobster fishery 1986/7 to
1990/91. Chubb, C.F., Barker, E.H.and Brown, R.S.
(1996).
107 Chemical composition of yabbies, Cherax albidus
Clark 1936 from Western Australian farm dams.
Francesconi, K.A. and Morrissy, N.M. (1996).
the potential application of results. Molony, B. and
Parry, G. (2002).
130 Environmental requirements and tolerences of
rainbow trout (Oncorhynchus mykiss) and brown trout
(Salmo trutta) with special reference to Western
Australia : a review. Molony, B. (2001).
131 Pilchard (Sardinops sagax) nursery areas and
recruitment process assessment between different
regions in southern Western Australia. Gaughan,
D.J., Baudains, G.A., Mitchell, R.W.D. and Leary, T.I.
(2002).
110 A Research vessel survey of bottom types in the
area of the Abrolhos Islands and mid-west trawl
fishery. Dibden, C.J. and Joll, L.M. (1998).
132 A review of food availability, sea water
characteristics and bivalve growth performance
occuring at coastal culture sites in temperate and
warm temperate regions of the world. Saxby, S.A.
(2002).
111 Sea temperature variability off Western Australia
1990 to 1994. Pearce, A., Rossbach, M., Tait, M.
and Brown, R. (1999).
133 Preliminary assessment and seasonal fluctuations
in the fish biota inhabiting the concentrator ponds
of Dampier Salt, Port Hedland, with options for
141 A guide to good otolith cutting. Jenke, J. (2002).
143 Methods used in the collection, preparation and
interpretation of narrow-barred Spanish mackerel
(Scomberomorus commerson) otoliths for a study
of age and growth in Western Australia. Lewis P. D.
and Mackie, M. (2003).
144 FRDC Project 1998/302 – Rock Lobster
Enhancement and Aquaculture Subprogram:
Towards establishing techniques for large scale
harvesting of pueruli and obtaining a better
understanding of mortality rates. Phillips B. F.
(2003).
145 The western rock lobster fishery 1999/2000 to
2000/01. Chubb, C.F. and Barker, E.H. (2004).
146 Catch composition of the Western Australian
temperate demersal gillnet and demersal longline
fisheries, 1994 to 1999. McAuley, R. and
Simpfendorfer, C. (2003).
147 Quantification of changes in recreational catch and
effort on blue swimmer crabs in Cockburn Sound
and Geographe Bay, FRDC Project No 2001/067.
Sumner, N.R. and Malseed, B.E. (2004).
148 Historical distribution and abundance of the
Australian sea lion (Neophoca cinerea) on the west
coast of Western Australia. Campbell, R. (2004).
149 The western rock lobster fishery 2001/02 to
2002/03. Chubb, C. F. and Barker, E. H. (2004).
150 Unpublished.
151 Biology and stock assessment of the thickskin
(sandbar) shark, Carcharhinus plumbeus, in
Western Australia and further refinement of
the dusky shark, Carcharhinus obscurus, stock
assessment, Final FRDC Report – Project
2000/134. McAuley, R., Lenanton, R. Chidlow, J.,
Allison, R. and Heist, E. (2005).
152 Development of a DNA Database for Compliance
and Management of Western Australian Sharks,
Final FRDC Report – Project 2003/067. McAuley,
R., Ho, K. and Thomas, R. (2005).
153 A 12-month survey of recreational fishing in the
Pilbara region of Western Australia during 19992000. Williamson, P.C., Sumner, N.R. and Malseed
B.E. (2006).
154 The development of a rigorous sampling program
for a long-term annual index of recruitment for
finfish species from south-western Australia Final
FRDC Report – Project 1999/153. Gaughan, D.,
Ayvazian, S., Nowara, G, Craine, M. and Brown, J.
(2006).

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