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Aquaculture 319 (2011) 484–492
Contents lists available at ScienceDirect
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a q u a - o n l i n e
Growth performance and condition of oysters (Crassostrea gigas and Ostrea edulis)
farmed in an offshore environment (North Sea, Germany)
B. Pogoda a, b,⁎, B.H. Buck b, c, d, W. Hagen a
University of Bremen, Marine Zoology, p.o. Box 330440, 28334 Bremen, Germany
Alfred Wegener Institute for Polar and Marine Research (AWI), Am Handelshafen 12, 27550 Bremerhaven, Germany
Institute for Marine Resources (IMARE), Bussestrasse 27, 27570 Bremerhaven, Germany
Bremerhaven University of Applied Sciences, Applied Marine Biology, An der Karlstadt 8, 27568 Bremerhaven, Germany
a r t i c l e
i n f o
Article history:
Received 4 March 2011
Received in revised form 12 July 2011
Accepted 14 July 2011
Available online 23 July 2011
Offshore aquaculture
Crassostrea gigas
Ostrea edulis
Site selection
a b s t r a c t
Aquaculture production plays an increasingly important role to meet the global demand for aquatic products and
expands continuously. Most mariculture organisms are produced in coastal areas, where space is scarce and user
conflicts exist. For extensive cultures farming off the coast at offshore sites could be a solution to eliminate these
problems and facilitate further expansion of environmentally sustainable aquaculture. The aim of this study was to
examine the biological adequacy of two candidate species for ostreiculture, the Pacific oyster Crassostrea gigas and
the European flat oyster Ostrea edulis. Growth rates, condition and mortality in offshore environments were
investigated by transferring oyster spat of both species to 4 different sites in 2004 and 2007. Samples were taken
every six to eight weeks from April to October and length, width, height and dry mass were measured as well as the
Condition Index (CI = dry mass meat ∙100/dry mass shell) was calculated. Results show that both oyster species
grow successfully in a high-energy environment. Mean growth rates are similar to those measured in individuals
from coastal habitats (wild banks and cultures) and the CI shows seasonal variation in both species. The survival
rate for both species was N 99% in 2007. However, in the previous trial in 2004 a high mortality rate was observed
for O. edulis at one single site at the end of the experiment. Differences were observed in the increase of shell length
and dry mass between sites and size classes. Taking these results into account site-selection criteria for different
offshore locations are presented. We conclude that offshore cultivation of oysters will be successful if site-selection
criteria are examined carefully when choosing a location for offshore aquaculture.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Aquaculture is the fastest-growing sector of food production
worldwide (FAO, 2009). The production of marine organisms mainly
takes place in sheltered areas or embayment of the coastal sea (Buck
et al., 2003; Firestone et al., 2004). However, in coastal areas space for
all kinds of utilization is scarce, thus user conflicts exist and
permanently increase (Buck et al., 2004; Wirtz et al., 2002). Intensive
seafood production (e.g. fish or shrimp) often generates environmental stress caused by chemicals, eutrophication and impacts on benthic
communities (McElwee, 1998). Therefore, in most nearshore areas the
ecological carrying capacity sets limitations to massive expansions of
intensive aquaculture activities (Troell et al., 2009). Extensive
cultivation methods and extractive culture species with modest
service needs offer the chance to move to offshore areas, away from
socio-economic conflicts and coastal pollution (Krause et al., 2003)
and still have an economic potential (Buck et al., 2010). Offshore or
⁎ Corresponding author at: University of Bremen, Marine Zoology, p.o. Box 330440,
28334 Bremen, Germany. Tel.: + 49 421 21863037; fax: + 49 421 21863055.
E-mail address: [email protected] (B. Pogoda).
0044-8486/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
open-ocean aquaculture stands for the move of aquaculture operations from sheltered nearshore areas to more exposed environments
where high wave action and strong currents exist (Ryan, 2005).
However, due to the strong exchange and dilution effects of the water
column in these high-energy environments, the water quality, the
major element in aquaculture operations, is regarded to be very good
(BSH 2006; Takayanagi, 1998).
Less user conflicts and higher wind speeds in offshore regions also
enhance growing interests in offshore wind farming. Worldwide wind
farms in offshore areas are being planned or already under construction (Gierloff-Emden, 2002). Following the multi-use concept of the
Alfred Wegener Institute for Polar and Marine Research (AWI) and the
Institute for Marine Resources (IMARE) foundations of offshore wind
turbines can be used for additional purposes. Therefore, aquaculture
installations can be attached to or installed between turbines within a
wind farm (e.g. Buck & Krause 2011; Buck et al., 2008). Following these
concepts of multifunctional use, sustainable seafood production shows
an enormous economic potential in offshore areas (Buck et al., 2008).
Shellfish aquaculture, particularly oyster and mussel cultivation, is a
good example for extensive production and has become very attractive
(Gibbs, 2004). These candidates do not require artificial feeding
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B. Pogoda et al. / Aquaculture 319 (2011) 484–492
2. Materials and methods
2.1. Origin of test animals and description of study sites
Fig. 1. Map of the German Bight showing all test sites: Butendiek (BD), Helgoland (HE),
Wurster Arm (WA), and Nordergründe (NG).
(Ferreira et al. 2009; Garen et al., 2004) and as essential bio-extractive
organisms can even improve water quality in marine systems (Ferreira
et al., 2009; Rose et al., 2010). A feasibility study on open-ocean
aquaculture by Buck (2002) identified the following extractive
candidates as most suitable for the North Sea: sugar kelp (Laminaria
saccharina/Saccharina latissima), dulse (Palmaria palmata), blue mussel
(Mytilus edulis) and two oyster species, the Pacific oyster (Crassostrea
gigas) and the European flat oyster (Ostrea edulis). As oysters represent
high-value products and realize high prices on the market, compared to
other shellfish (FAO 2011; Troell et al., 2009), these organisms seem to
be ideal candidates for offshore aquaculture. However, techniques for
offshore cultivation of oysters have not been developed yet, as over
centuries, different forms of oyster cultures were only developed in
coastal areas (Burbridge et al., 2001).
Consequently, no information is available on growth performance
and fitness of oysters farmed in offshore areas, which experience
high-energy environments. From the biological perspective the
present study aims to assess whether offshore oyster cultivation
offers a promising potential regarding growth performance in rough
This is the first study focusing on cultivation and subsequent
performance characteristics of the Pacific oyster (C. gigas) and the
European flat oyster (O. edulis), exposed to offshore farming
conditions. In two experiments (2004 and 2007) we examined
growth performance, condition and survival of oysters and analyzed
site-specific criteria for the cultivation at offshore sites in the North
Sea (Germany).
Juvenile oysters were obtained from commercial hatcheries. For
the sampling season 2004 O. edulis was produced at the Danish
Shellfish Centre (Denmark) in September 2003 (observed as veliger)
and C. gigas at Guernsey Seafarms (UK) in July 2003. For the sampling
season 2007 oysters of O. edulis were produced in April 2006
(observed as veliger) at Bømlo Skjell A. Musling (Norway). C. gigas
spat (produced in July 2006) was obtained from Guernsey Seafarms
as well. Initial size classes (shell length) of juvenile oysters were 10–
20 mm in 2004 and 20–30 mm in 2007. For the cultivation under
offshore conditions juveniles of both species were transferred to the
test sites in April 2004 (preliminary experiment for site selection) as
well as in April 2007 (main experiment).
In the preliminary experiment oysters were cultivated in three
different areas of the German Bight (Fig. 1). These study sites are
characterized by various hydrographic features, such as depth,
condition of the sea bottom, wave exposure, current velocity and
significant wave heights (BSH 2011; Mittelstaedt et al., 1983), which are
shown in Table 1. The site-specific conditions of these areas were chosen
to fulfill the offshore-criteria in terms of a high-energy environment
following the definition of Ryan (2005). Two test sites were set up
within the areas of projected wind farms to underline the idea of the
multiple-use concept. Furthermore, accessibility to sample and service
the sites was also taken into account but of secondary relevance.
The test site Butendiek (BD) was located 15 nautical miles (nmi)
west of the northern tip of the North Frisian island of Sylt (54° 59,1´ N;
007° 54,4´ E). Water depth was approximately 16 m at mean high
water (MHW). This site was chosen because it is located within the
area of the planned wind farm “Butendiek”. The rounded metal piles
of a former research platform of the Federal Maritime and Hydrographic Agency (Buck et al. 2008) provided an adequate fixed position
within an offshore (high-energy) environment, and were used as a
holding device for the oysters. A permit for the utilization of the study
site was obtained in March 2004. Cultures were mounted at 4 m depth
(high tide). The test site Helgoland (HE) was set up northeast of the
island of Helgoland (54° 11,4´ N; 007° 53,0´ E). Water depth at site was
approximately 6 m at MHW. Cultures were mounted to a metal frame
at 4 m depth of water (high tide). The metal frame was moored on the
seabed and co-used as a test site for the settlement of Mytilus edulis
post-larvae. Despite the fact that this test site was about 150 m off the
island of Helgoland, it can be considered as an offshore site due to the
typical conditions that characterize this location (strong currents,
high waves, good water quality, deep enough for submerged cultures
(Barnaby, 2006)) and the fact that Helgoland is defined as an offshore
island at all. The test site Wurster Arm (WA) was located in the outer
Weser estuary (53° 40,7´ N; 008° 24,5´ E). Water depth at the site was
approximately 8.5 m at MHW. Cultures were mounted at 1.5 m depth
below a navigation buoy of the Water and Shipping Agency (WSA)
Bremerhaven and were therefore independent of tidal influence. The
main experiment started in 2007 and cultivation was carried out at a
Table 1
Site-specific conditions and classification of the four test sites.
Test site
Distance to
coast [nmi]
Depth at
Sediment type
at sea bottom
Sediment load
of water column
wave height
[m] a
[m/s] a,b
Max. daily tidal
current velocity
[m/s] a
Wurster Arm
Mud (clay)
nmi: nautical mile, MHW: mean high water.
BSH 2011.
Mittelstaedt et al. 1983.
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forth test site called Nordergründe (NG), which was located in the
outer Weser estuary (53° 51,0´ N; 008° 04,0´ E), approximately 17 nmi
northwest of Bremerhaven. It was situated in an official research
testing area (Buck, 2007), which was established for offshore
aquaculture as a multi-use in offshore wind farms. The planned
offshore wind farm “Nordergründe” (Energiekontor, 2011) is located
about one nmi off the test site. Experimental oysters were permanently submerged at all four testing sites.
2.2. Experimental design and sampling
In temperate regions both oyster species show an inactive period
during winter and start feeding in spring (Løfstedt, 2010; Matthiessen, 2001). Therefore, our experiments were conducted over one
complete growing season (first cultivation year) from April to October
(Matthiessen, 2001; Robert et al., 1991; Walne & Mann, 1975). Due to
the sensitivity of oysters in their early juvenile stage, the first year of
cultivation can be considered as the most critical period (Newkirk
et al., 1995). Therefore, the present study focused on this period to
elucidate survival and robustness of the two species.
The oysters were reared in small oyster lanterns (60 cm length,
16 cm diameter, 4 compartments arranged in vertical levels, mesh
size 10 × 10 mm), which were specifically produced for offshore
aquaculture experiments (Fig. 2A). In both experiments 600 individuals were reared in each lantern (150 per compartment, two
compartments with O. edulis and two with C. gigas), respectively. In
the preliminary experiment (2004) two lanterns were installed at
each site, attached to a metal frame, which acted as a rigid holding
device. In the main experiment (2007) two lanterns were fixed to
rigid steel rings, which were welded to large offshore marker buoys
(Fig. 2B). These test buoys were constructed specifically for offshore
aquaculture research on shellfish candidates (e.g. C. gigas, O. edulis,
Mytilus edulis) (Brenner et al., 2007). Samples (80 individuals of each
species) were taken by SCUBA-divers in June, August and October in
2004 and 2007. To ease sampling and maintenance and to avoid the
loss of test animals the complete lantern was detached from the
holding device and transferred to a zodiac nearby. Each lantern was
then opened and oysters collected. In contrast to usual procedures in
aquaculture the animals were not sorted into different size classes.
Maintenance was performed by cleaning the lantern nets from
biofouling and by removing empty shells. Oyster lanterns were
returned to the holding device right after sampling to avoid stress for
the animals. Sampled oysters were carefully stored in containers and
transferred to the research vessel.
2.3. Analysis of growth, condition index and mortality
On board the research vessel length, width and height were
measured to the closest 0.1 mm, before animals were stored deepfrozen (−80 °C) and transferred to the lab facilities at the AWI in
Bremerhaven as well as to the University in Bremen. Empty shells
were counted to estimate mortality rates. To determine dry mass
(DM) and condition index (CI) soft tissue and shell of the test animals
were freeze-dried (48 h, LYOVAC GT2, Leybold-Heraeus) and weighed
(R200D, Sartorius). CI was then calculated by using Eq. (1), as
suggested by Davenport & Chen (1987) and Walne & Mann (1975):
CI = DM meat ðgÞ•100 = DM shell ðgÞ
Selecting a condition index that is based on dry mass is expected to
be quite accurate as it is independent of the high variability of
intervalval fluid in oysters (Beninger & Lucas, 1984).
2.4. Environmental parameters
Oceanographic parameters (current velocities, wave heights) were
extracted from the Operational Circulation Model BSHcmod (BSH,
2011; Dick, 2001). Data describing temperature, salinity, chlorophyll
a, phaeophytin a, nitrate and nitrite (NOX), particulate organic carbon
(POC), and particulate organic nitrogen (PON) were obtained from
the Marine Environmental Data Base (MUDAB, BSH, 2009) and
Brenner (2009).
2.5. Statistical analysis
Means, standard deviations, standard errors of the mean (mean ±
SE) and confidence intervals of growth rates and condition indices
were calculated using MS-Excel software. Differences in growth
and condition index between sites and species were tested by t-test
(MS-Excel software) and considered to be significant when p b 0.05.
Fig. 2. (A) Oyster lanterns containing juveniles of Ostrea edulis and Crassostrea gigas. (B) Marker buoy, modified for offshore aquaculture research. Steel rings were used as rigid
attachment device for the fixation of oyster lanterns.
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3. Results
The results of the preliminary experiment acted as the basis to
determine the site for the main experiment: The site Nordergründe
(NG) was chosen as it represents an offshore aquaculture location
class 4 (Ryan, 2005) plus a similar situation as the site Butendiek (BD)
but was easier to access due to its shorter distance from the city of
Bremerhaven. Additionally, NG is situated within the official testing
area mentioned above.
3.1. Environmental conditions
Environmental conditions at the test sites are displayed in Table 2.
Temperature measurements showed comparable conditions at all
three test sites in 2004. In spring and autumn 2007 temperature was
slightly lower than in spring and autumn 2004. Salinity ranged
between 29.4 and 33.6 at the offshore sites and was lower at the
nearshore site (WA): Depending on the direction of the tidal current it
ranged between 11.8 and 22.1. Chlorophyll and nutrient concentrations characterizing the suitability for shellfish cultivation at the sites
were extracted from the Marine Environmental Data Base (BSH, 2009)
and Brenner (2009) and are also shown in Table 2.
3.2. Growth
The average length of O. edulis was 11.2 ± 0.2 mm at the beginning
of the preliminary experiment (2004). At the end of the experiment it
reached 27.9 ± 0.4 mm at WA, 38.6 ± 0.5 mm at HE, and 42.0 ±
0.9 mm at BD. The average length of C. gigas at the beginning of the
experiment was 22.9 ± 0.2 mm. At the end of the experiment it
reached an average length of 29.0 ± 0.6 mm at WA, 47.1 ± 0.9 mm
near HE, and 50.5 ± 1.1 mm at BD. C. gigas and O. edulis showed similar
growth patterns in terms of shell length. Growth of young oysters was
moderate during spring 2004 and increased in summer and autumn.
Compared to the sites Butendiek (BD) and Helgoland (HE) a
significantly slower growth (p b 0.01) was observed for both species
at the site Wurster Arm (WA) (Fig. 3A-B).
At the beginning of the main experiment (2007) the average
length of O. edulis was 38.5 ± 0.5 mm and 24.7 ± 0.5 mm of C. gigas. At
the end of the experiment the average length of O. edulis was 58.9 ±
0.8 mm and 55.3 ± 1.1 mm of C. gigas, respectively. Both species
showed rapid growth in spring, which slowed down in summer and
increased again during autumn. Seasonal differences of growth rates
were more pronounced in C. gigas than in O. edulis, which showed a
Table 2
Environmental parameters at the four test sites.
Test site
Temperature Salinity Chl a Pheo a POC
[μg/l] [μg/l]
n. d.
n. d.
n. d.
n. d.
HE April
WA April
NG April
n. d.
n. d.
n. d.
n. d.
n. d.
n. d.
BD: Butendiek, HE: Helgoland, WA: Wurster Arm, NG: Nordergründe, Chl a: Chlorophyll a,
Pheo a: Phaeophytin a, POC: particulate organic carbon, PON: particulate organic
nitrogen, NOX: nitrate + nitrite, n.d.: no data available.
Fig. 3. Shell growth of C. gigas (A) and O. edulis (B) at test sites: Butendiek (BD),
Helgoland (HE) and Wurster Arm (WA) during the preliminary experiment (2004).
n ≈ 50 per species, sampling date and site. (C) Shell growth of O. edulis and C. gigas
during the main experiment (2007) at test site Nordergründe (NG). n ≈ 80 per species
and sampling date. Graphs displayed include the mean increase in shell length (mm) ±
confidence interval.
rather constant increase in shell length (Fig. 3C). Daily growth rates at
the different sites (and years) are shown in Table 3. They were very
similar for both species at sites BD (C. gigas 0.17 mm/d, O. edulis
0.18 mm/d), HE (C. gigas 0.15 mm/d, O. edulis 0.16 mm/d), and in the
same range at NG (C. gigas 0.19 mm/d, O. edulis 0.13 mm/d). Only
at site WA growth rates were significantly lower (C. gigas 0.02 mm/d,
O. edulis 0.06 mm/d) (p b 0.01). Both species showed considerable
intraspecific variability in growth success at all sites. To highlight
Table 3
Growth rates of Crassostrea gigas and Ostrea edulis presented as daily increase of shell
length (SL) and dry mass (DM) at the four test sites.
Test site
Increase of SL/d
Increase of DM/d
C. gigas
O. edulis
C. gigas
O. edulis
Wurster Arm
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B. Pogoda et al. / Aquaculture 319 (2011) 484–492
Fig. 4. Growth range (minimum and maximum growth of shell length) of C. gigas (A) and O. edulis (B) during the main experiment (2007). Species-specific shell growth (length,
width, thickness) of C. gigas (C) and O. edulis (D) in the main experiment (2007).
these differences the range of shell lengths of C. gigas and O. edulis in
the experiment 2007 is presented in Fig. 4A–B. The two oyster species
showed clear differences in terms of shell length, width, height and
absolute size because of their species-specific morphology (Newkirk
et al., 1995; Walne, 1979). O. edulis is rather round, whereas C. gigas
shows a more elongated morphology (Fig. 4C-D).
3.3. Dry mass, condition index and mortality
Dry mass of both species increased during cultivation time at all
sites in 2004 as well as in 2007 (Fig. 5A–C). In 2004 low growth rates
in terms of increasing dry mass were observed for both species at site
WA (C. gigas 0.86 mg/d, O. edulis 0.25 mg/d) and significantly higher
rates (p b 0.01) at HE (C. gigas 1.93 mg/d, O. edulis 0.95 mg/d), and BD
(C. gigas 2.31 mg/d, O. edulis 1.14 mg/d) (Table 3). In 2007 both
species showed a similar increase in dry mass during springtime.
Afterwards different performances were observed for the two species:
O. edulis individuals grew slightly slower in summer than in spring but
growth increased again in autumn. C. gigas individuals showed no
length changes during summer, however, marginal increases in dry
mass in during autumn were observed (Fig. 5C). C. gigas obtained the
highest daily increase in dry mass at sites BD and NG (same growth
rates in 2004 and 2007). Clearly lower increases in dry mass were
observed in 2004 for individuals of O. edulis. In the main experiment of
2007 (site NG) growth rates of O. edulis (4.57 mg/d) were almost two
times higher than those of C. gigas (2.33 mg/d) (Table 3).
Condition index (CI) of the Pacific oysters ranged between 3.4 ±
0.12 and 11.8 ± 0.37 at all sites (Fig. 5E). Corresponding values for the
European oysters ranged between 1.8 ± 0.02 and 4.8 ± 0.13 in the
preliminary experiment (2004) and were higher (between 5.2 ± 0.26
and 7.2 ± 0.22) in the main experiment (2007) (Fig. 5F). In the
preliminary experiment values for both species increased during
cultivation at sites BD and HE. C. gigas obtained the highest CI at BD
(10.0 ± 0.31) and similar characteristics at HE but with lower values
(7.7 ± 0.29) at the end of the cultivation period. Data for O. edulis
show similar inclines for BD and HE with maximum values (4.8 ± 0.13
and 4.6 ± 0.07) at the end of the cultivation period. Only at WA a
different pattern was observed: In June C. gigas obtained values
similar to those at BD and HE. Subsequently the CI remained constant
and after August even a slight decrease in CI was observed. O. edulis
obtained values similar to those at BD and HE in June and August at
WA but no values exist for October due to mortality after the sampling
in August. In the main experiment C. gigas showed a high CI after the
spring period (11.8 ± 0.37) which decreased during the rest of the
sampling season with faster rates in summer and slightly slower rates
in autumn (Fig. 5E). O. edulis also showed the highest CI after the
spring period (7.2 ± 0.2) and a decrease during summer. In contrast to
C. gigas a slight increase was observed for O. edulis in autumn (Fig. 5F).
In 2004 mortality was low (b3.2%) for both species at sites BD and
HE. However, at site WA O. edulis individuals died after August. In
2007 (NG), mortality was low (b1%) for both species during the entire
sampling period (Fig. 5D).
4. Discussion
Offshore cultivation of oysters is mentioned and recommended in
various publications (e.g. Chávez-Villalba et al., 2010; Delmendo,
1998; Goulletquer, 1998; Handley & Jeffs, 2003), however, none of
these publications give attention to “real” offshore conditions as
defined by Ryan (2005) and Buck (2007) for open-ocean or offshore
aquaculture, namely a high-energy environment with strong currents
and high waves. Therefore, no data on offshore farming of oysters
among those conditions were available so far to allow comparison of
the results of our test animals.
In this study we investigated the biological response of Pacific and
European oysters in high-energy environments in terms of growth
performance, condition and survival rates.
4.1. Growth
The results of the length increase in this study show that both C.
gigas and O. edulis can grow successfully at exposed conditions in an
offshore environment. O. edulis and C. gigas were reared outside their
natural coastal habitat, which is normally located on seabeds of the
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Fig. 5. Dry mass of C. gigas (A) and O. edulis (B) at test sites: Butendiek (BD), Helgoland (HE) and Wurster Arm (WA) during the preliminary experiment (2004). (C) Dry mass of
O. edulis and C. gigas during the main experiment (2007) at test site Nordergründe (NG). Graphs present the mean increase in shell length (mm) ± confidence interval. Mortality
(D) and condition index of C. gigas (E) and O. edulis (F) in 2004 and 2007 at the different test sites. n ≈ 50 per species, sampling date and site. (F) Please note: Lines WA, HE and BD are
overlapping from April to June, and Lines WA and HE from June to August. Line WA ends in August.
coastal sea. The animals obtained positive growth rates in terms of
shell length and dry mass. However, the duration of the project did
not allow the farming of oysters to market size. Nevertheless, this time
period allows an estimation of oyster growth over a certain life span.
Accepting the lack of growth data of offshore grown oysters, mean
growth rates are still comparable to those measured of individuals
from nearshore coastal areas (Diederich, 2006; Robert et al., 1991,
1993; Schmidt et al., 2008; Valero, 2006; Walne & Mann, 1975).
Following Valero (2006) there is evidence that juvenile oysters show
high length growth in their first year which means that they primarily
invest in shell development before flesh biomass starts to increase. In
the second and third year they change to higher growth of their body
mass. Oysters of this study still grew in the first year of the typical
oyster cultivation period. They showed an accelerated length
increment, which in turn underlines the projected development.
Seasonal differences in growth performance were observed for the
Pacific oyster in the main experiment. The European oyster showed a
more constant increase in shell length and dry mass at the same site
(NG). The experimental set-up at NG was only conducted in 2007 and
no data of other seasons at the same site were available for
comparison. However, we assume a higher ability of dietary
assimilation, when food availability is low in summer (Rick et al.,
2006) and a better adaptation of the native European oyster to this
region (Matthiessen, 2001; Newkirk et al., 1995) instead of its
counterpart, the introduced Pacific oyster (Andrews, 1980; Chew,
1990; Ruesink et al., 2005).
Oysters of the present study produced an esthetic shell and
achieved their species-specific morphology, which plays an important
role for their market value in Europe (Matthiessen, 2001). Only
oysters grown at Wurster Arm (WA) developed a different shape:
Thicker shells and a very compact appearance indicate that animals
strongly invested in shell growth to withstand the high currents as
well as to prevent shell abrasion (Newkirk et al., 1995).
Mean growth rates in terms of shell length and dry mass strongly
depended on the different culture sites. Both oyster species preferred
the same sites: At the sites Butendiek (BD) and Nordergründe (NG),
both species showed the highest growth rates. As the Pacific oyster
showed very similar daily increases in dry mass at both sites, we
assume that the sites BD and NG offer similar good conditions for
cultivation. However, the European oyster performed differently:
Daily increase in dry mass was four times higher at NG (main
experiment, 2007) than at BD (preliminary experiment, 2004). We
assume that the reason for this different growth development
originates in the condition of the spat regarding its larval source. In
the preliminary experiment of 2004 European oysters were obtained
from the Danish Shellfish Centre (DSC), which started land-based
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B. Pogoda et al. / Aquaculture 319 (2011) 484–492
cultivation of O. edulis in 2002. Løfstedt (2010) indicated poor
conditions for some of the first generations of oyster spat produced
at DSC, thus, data on growth rates and mortality of O. edulis in the
preliminary experiment have to be interpreted with caution. This
could explain the deviating results for the European oyster at sites BD
and NG. As O. edulis of DSC shows positive growth rates in 2004, the
question arises, if growth rates could have been even higher with spat
animals, which had optimum condition initially. We conclude that O.
edulis individuals used for the preliminary cultivation experiment in
2004 were in poor condition and too small to move off the coast. After
Buck (2002) only those organisms are appropriate for offshore
cultivation that are healthy and comprise a certain size class, which
is characterized by sufficient robustness. According to typical size
classes used in commercial aquaculture farms, larger O. edulis spat
from a Norwegian producer was used in the main experiment (Buck,
2002; Matthiessen, 2001; Newkirk et al., 1995; Draver, pers. comm.).
These animals yielded higher growth rates, even exceeding those of
Pacific oysters. The better condition of these animals is reflected in the
higher condition index of European oysters in the main experiment.
period in August. This mass die-off could have been caused by the high
sediment-load in the water column at this site (BSH, 2011), since natural
O. edulis oyster beds are never found on muddy sediments (OSPAR,
2009). The presence of high quantities of silt in the water can block the
digestive and respiratory tracts, causes stress to the animal and results in
the death of the oyster after a certain time (Laing et al., 2005; Partridge,
1981). Additionally, due to the short distance of this site to the port of
Bremerhaven, urban sewage or contaminants could have affected the
water quality. In general, high mortality of oysters can be correlated to
energy expenditure during spawning, high water temperatures in
summer as well as the result of multiple stressors (e.g. low salinity, low
dissolved oxygen, pathogens) creating physiological pressure (Costil
et al., 2005; Soletchnik et al., 2006).
The high survival rates presented in this study indicate that water
conditions at the other offshore cultivation sites have no negative
impact on the survival of the tested oyster species. This is supported
by the fact that natural banks of the European oysters also occurred in
deeper waters and offshore down to 50 m, e.g. in the North Sea and
the eastern English Channel (OSPAR, 2009).
4.2. Condition
4.4. Site-specific performance
The condition index of various oyster species such as O. edulis,
C. gigas, C. virginica etc. is commonly used to evaluate the effects of the
surrounding environment on these organisms (Rheault & Rice, 1996).
It is an adequate parameter to describe the commercial quality and
physiological state (health) of bivalve molluscs (Dridi et al., 2007). In
shellfish production, the most commonly applied condition index (CI)
is the ratio of flesh mass to shell mass (Davenport & Chen, 1987). The
data on the condition of both oyster species support the positive
results observed for the growth performance at offshore sites.
Condition index values for the Pacific oyster indicate good condition
(Linehan et al., 1999) in both experiments. Values for the European
oyster also indicate good condition in the 2007 experiment. In
contrast, condition indices were quite low at the beginning of the
preliminary experiment in April 2004, confirming the poor condition
of the DSC oyster spat. But the CI increased with cultivation time and
the European oyster showed a better condition (Linehan et al., 1999;
Walne & Mann, 1975) at the end of the experiment in October 2004.
Apparently, O. edulis was capable of an excellent recovery at sites BE
and HE. The condition index showed seasonal variations for both
species. Similar variations have also been reported for cultured
oysters in coastal areas of Western Europe (Abad et al., 1995; Linehan
et al., 1999; Ruiz et al., 1992; Soletchnik et al., 2006) and South
Australia (Li et al., 2009). These variations are generated by changing
food supply and in adult individuals by gametogenesis and spawning
(Li et al. 2009; Soletchnik et al., 2006). In summer 2007 only slow
growth in terms of dry mass was observed for the Pacific oyster. This
resulted in a decline of the CI, which has also been reported for late
summer/autumn in South Australia (Li et al., 2009). Similarly, a slight
decline of the CI could be observed for the European oyster. Whereas
the CI of O. edulis showed a little increase again after August, C. gigas
did not recover. This is another indication for a better adaptation of
the European oyster to this region.
In the experiments of 2004 and 2007 oysters were cultivated at
four different sites. Due to the varying conditions from site to site,
differences in growth performance, condition and survival were
observed. To evaluate these sites regarding site selection criteria,
biotic and abiotic parameters (e.g. temperature, salinity, nutritional
situation) and their impact on the overall condition of the candidates
were examined.
No significant differences in temperature and salinity were
detected between the three offshore cultivation sites. The growing
season of oysters depends on temperature and naturally proceeds
from April until late September in the North Atlantic (Korringa, 1952;
Matthiessen, 2001; Walne & Mann, 1975). As C. gigas tolerates a wide
range of temperatures and cultivated O. edulis individuals belong to
the well-adapted cold-water population (OSPAR, 2009), no seasonal
temperature abnormalities were observed. Additionally, salinity is
known to be very stable at offshore sites and both species prefer the
marine and fully saline conditions that characterize those areas (Laing
et al., 2005).
Oysters are supplied with food and oxygen and relieved from waste
products through the action of hydrodynamic currents. Since filtration
rate and food availability increase with flow rate (Walne, 1972; 1979),
oysters were expected to grow better at cultivation sites with stronger
water currents (Valero, 2006). Although offshore areas are known to
offer lower plankton concentrations compared to coastal regions
(Ducrotoy et al., 2000), the higher hydrodynamic flow offshore can
compensate for this limitation. However, the offshore test sites in the
present study resemble special conditions as the North Sea shows
symptoms of eutrophication. Additionally, turbid areas, like estuaries
and their run-offs, can be classified as hyper-nutrified systems where
food supply is regarded to be sufficient (Ducrotoy et al., 2000). The
combination of current speed and food amount implies high food
availability at the offshore sites BD and NG with high current speed.
WA, also a site with high currents, deviated from that scheme. As
described above, oysters did not show successful growth rates and the
shell morphology indicated mechanical stress. Former studies have
shown that filtration rates increase with increased flow rates until a
maximum value when oysters are unable to remove more particles
from the water passing by (Walne, 1979). Apparently, current forces at
WA exceeded this maximum. Furthermore, low salinity values at this
site could have resulted in physiological stress for the oysters and
affected their growth success (Laing et al., 2005).
In the preliminary experiment, test site Butendiek (BD) showed the
best results. The distribution of nutrients and plankton in the German
Bight follows the current system, a counter-clockwise gyre parallel to
4.3. Survival rates
Survival rates for both oyster species (N99% in 2007) were very high
and appropriate for the culture of marine organisms and are rarely
achieved in commercial oyster aquaculture production (Chávez-Villalba
et al., 2010; Costil et al., 2005; Soletchnik et al., 2006; Valero, 2006). High
survival or low mortality rates of Pacific oysters have also been reported
for wild banks in the German Wadden Sea (Diederich, 2006; Schmidt
et al., 2008) and for oyster cultures in South Australia (Li et al., 2009).
However, in 2004 a high mortality rate was only observed for O. edulis
at one single site: At site WA, animals died some time after the sampling
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B. Pogoda et al. / Aquaculture 319 (2011) 484–492
the coast (Ducrotoy et al., 2000, Mittelstaedt et al., 1983). Hence,
nutrients from river run-offs and the coast are transported to the
north (BSH, 2004). Subsequently, high plankton production can be
observed parallel to the coast of the North Frisian Wadden Sea and
around the area of the test site BD (Ducrotoy et al., 2000; WeigeltKrenz et al., 2010). This situation provides optimal nutritional
conditions for the cultivated oysters. Results of similar quality in
terms of shell growth were found at test site Helgoland (HE).
However, the area around Helgoland is located further offshore,
outside of the nutrient-rich zone of the German Bight (Weigelt-Krenz
et al., 2010). Correspondingly, slightly lower growth rates (dry mass)
of the Pacific oyster indicate a different food situation at this site. At
site Wurster Arm (WA) low salinity and a high sediment load of water
originating from the Wadden Sea (BSH, 2004) may explain the low
growth success. Poor growth performances of both species and the
high mortality of the European oyster lead to the conclusion that this
site is not adequate for aquaculture production.
Consequently, for the main experiment in 2007 it was aimed for a
cultivation site, which fulfills the described site selection criteria and
offers the same quality as site BD. De facto, growth performance,
condition and survival of both oyster species showed the expected
success at the selected site Nordergründe (NG).
5. Conclusions and outlook
This study shows that juvenile oysters (1) exhibit a convincing
growth performance at offshore sites, (2) are in strong condition, and
(3) cope well with rough conditions. Subsequently, from an oyster
farmer perspective, these data attest the suitability of open ocean
environments for oyster cultivation and encourage aquaculture
activities at offshore sites. Both, C. gigas and O. edulis, respectively,
play a significant contributor to the European economy, especially on
the oyster half shell market in France. Generally, the oyster trade is of
enormous commercial importance with wholesale average prices for
the European oyster commonly being 3 to 5 times higher than for the
Pacific oyster. Following Goulletquer (2004), O. edulis fills an economic
niche as a premium seafood item. Therefore, oysters as high-value
products would be potential candidates for offshore cultivation since
an economic analysis for mytiliculture by Buck et al. (2010) have
calculated higher costs when moving far off the coast.
Our experiments clearly reveal indications that the native species
is very well adapted (Matthiessen, 2001; OSPAR, 2009) and shows an
adequate growth performance. Therefore, the cultivation of the
European flat oyster in the North Sea is recommended in particular.
Assuming appropriate site selection criteria, we conclude that both
oyster species, C. gigas and O. edulis, are successful candidates for
offshore cultivation. A multiple-use of potential offshore locations
together with other users, e.g. offshore wind farms is promising and
synergistic effects may offer technical and financial advantages to
oyster farmers.
Future studies will focus on physiological parameters and fitness
indicators of oysters cultivated at offshore sites. Additionally,
complete grow-out periods during various seasons should be
examined to increase knowledge about oyster culture experiments
off the coast.
We thank the Water and Shipping Authority, namely Mr. Boening
and the Captain and Crew of the MV Bruno Illing and MV Mellum for
their support in maintaining the test sites Wurster Arm and
Nordergründe. Thanks are expressed to the captain and crew of the
research vessel Uthörn for their assistance. We also thank the AWIdiving group for technical support and all divers involved in this
project. Thanks go to Dr. Matthias Brenner (IMARE) who provided
environmental data from the support program of the Mytifit-Project
and to Dr. Susanne Tamm from the German Oceanographic Data
Centre (DOD) for her help with the databases of the BSH. The research
carried out for this manuscript has been funded by the University of
Bremen and the Alfred Wegener Institute for Polar and Marine
Research and is further a partial fulfillment for a Doctoral degree of
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