My Fish Are Dying! - AgriLife Extension Wildlife &

My Fish Are Dying!
Billy Higginbotham
Professor and Extension Wildlife and Fisheries Specialist
Texas AgriLife Extension Service
Fisheries biologists and county Extension Agents will hear these words countless times
throughout the year, especially in the summer months. As a general rule, small ponds intensively
managed for catfish are the most susceptible to die-off problems. How do you determine the
cause? In most cases, asking the right questions will lead you to the cause or causes. Here are
the questions I ask and the assessments made based on answers received to help a frantic
How many different species are dying? What you are trying to determine with this
question is that if more than one species of fish is dying, you probably are faced with a
water quality problem (i.e., oxygen, ammonia, nitrites). If only one species of several
species present are dying, it may be a disease/parasite problem, but not always because
different species may have different tolerance levels for water quality parameters. If only
one species of fish is in the pond and a die-off is occurring, you need more information.
Have any pesticides been used recently that were introduced into the pond? This could
include pasture insecticides washing into ponds or even cattle that were treated with an
insecticide standing in the pond to escape the summer heat.
How big is the pond? The pondowner usually thinks the pond is 2x to 3x larger than it
really is? Walk them through the process of estimating surface area in acres by dividing
square footage by 43,560. Remember, excessive depth does not make up for lack of
surface area when it comes to fish production!
How many pounds of fish are present? Once you know the surface acreage, try to
determine the pounds of fish present by asking: (1) for an estimate of fish stocked
(minus those caught out) and (2) the average weight of the fish present. This will help
you estimate the total poundage of fish present.
STOP RIGHT HERE — if the total pounds of fish exceed 1,000 pounds per surface acre
(that’s only 100 pounds in a 0.10 acre pond), you are probably dealing with an oxygen
depletion. This accounts for about 85% of all fish die-offs in Texas farm ponds!!!
When the fish standing crop exceeds the 1,000 lbs/acre carrying capacity during the hot months,
the stage is set for a die-off. Why the summer months? Because warm water cannot hold as
much oxygen as cool water, yet the fish need more because their metabolism (and therefore their
oxygen requirements) increases as water temperature increases. A farm pond that could easily
carry 2,000 to 3,000 pounds of fish per surface acre through the winter months won’t stand a
chance once the dog days of summer arrive. Remember, oxygen is usually lowest right at
daylight, so that’s a good time to check and see if fish are swimming at or near the surface. In
many cases, the larger fish will be the first to exhibit signs of oxygen stress. Now that the
“hammer is cocked”, additional events that could “pull the trigger” and further contribute to an
oxygen depletion include:
A couple of hot, still, cloudy days in succession that reduce photosynthesis and therefore
oxygen production,
Aquatic herbicide treatments in hot weather that kill too much vegetation in too short a
period of time resulting in an oxygen debt and,
Overfeeding/overfertilization resulting in nutrient decomposition or phytoplankton dieoff.
So, how do you correct for low oxygen?
Reduce the fish load present to well below 1,000 lbs of fish/acre,
Aerate by backing a boat on a trailer into the pond and running the motor in a fixed
position to circulate the water and increase oxygen,
Add fresh well water, but aerate it well before it enters the pond, and
Circulate water with a pump, but set the intake near the pond surface (pumping water off
the pond bottom and spraying it back over the surface only compounds the problem!)
Other Common Water Quality Problems
Ammonia can also kill fish and can occur in ponds with heavy loads of fish (near or over 1,000
lbs/acre). Heavy fertilization in the watershed followed by run-off rains washing into the ponds
can cause toxic levels of ammonia. This includes chicken litter. Ammonia becomes more toxic
as pH and temperature increase. A water test is required to determine if ammonia is present.
The only cure is to add fresh water, remove dead water from the bottom and reduce the load of
fish present.
Nitrites are converted from un-ionized ammonia by the Nitrosomonas bacteria. Nitrites can
cause brown blood disease, so called because the fish affected have chocolate colored blood.
Nitrites bind to the blood’s hemoglobin and form methhemaglobin, which is ineffective at
removing oxygen from the water - resulting in the fish’s inability to breathe. Ponds suffering
from nitrite poisoning are usually heavily fed catfish ponds, and nitrites usually don’t show up
until late summer or early fall. Heavy pasture fertilization with run-off into a pond can also
cause elevated levels of nitrites. Nitrites can be “neutralized” by adding stock salt. The amount
to add varies with the level of nitrites. However, blanket recommendations of “adding salt to the
pond,” without the benefit of a water test is not recommended and tends to give a pondowner a
false sense of security. If possible, remove some bottom water from the pond to reduce organic
So, what if it’s not a water quality problem killing my fish?
Well, then your pondowner is in the vast minority. Even many disease problems are triggered by
stress brought on by poor water quality. Diseases and parasites normally only affect one species,
even if there are many species present. Proper identification is a must, but typically the most
common diseases are:
Fatal – Common Parasites/Diseases
Ich - typically occurs when water temperatures are below 70-75◦ F. Fish may have tiny
white pimples covering the surface of the skin - only a problem between November and
March. Consult a specialist for verification and control options.
Bacterial Disease - occurs during periods of transition (summer to fall or winter to spring)
when water temperatures are changing or any time fish may be subject to stress. Fish
may have eroded fins, irregularly shaped discolored areas on their backs and sides or
even eroded holes in their heads or backs. Consult a specialist for verification and
control options.
Non-Fatal – Common Parasites/Diseases:
Grubs - Once fishing season arrives, anglers catch a few fish and during cleaning
(normally filleting) notice white, yellow or black “worms” encysted in the skin or flesh.
These grubs have an interesting yet complex life cycle involving fish, snails and wading
birds, but seldom kill fish by themselves. Breaking the cycle can be difficult, although
some pondowners achieve success by controlling plants (food for snails), or stocking
redear sunfish (feeds on snails).
Contracaecum - This roundworm looks like a watch spring and is often found attached to
the mesentery outside various internal organs. It’s more of a curiosity to anglers cleaning
fish than anything else.
For more detailed information on diseases/parasites, consult the Southern Regional aAquaculture
website at Also, Dr. Ken Hasson’s and Dr. Patricia Varner offer fish
diagnostic services via the Vet Diagnostic Lab by calling him at 1-888-646-5623. As always,
you can also contact one of the Extension Fisheries specialists for additional advice.