Fish Kills

Fish kills are not uncommon in farm ponds.  Fish die as a result of a wide variety of natural and unnatural causes, including old age, starvation, body injury, stress, suffocation, water pollution, diseases, parasites, predation, toxic algae, severe weather, spawning stress and other reasons.  A few dead fish floating on the surface of a pond or lake is not necessarily cause for alarm, but when large numbers of fish are found dead and dying, it is necessary to determine the cause in order to prevent future occurrences.

Sudden, large-scale fish kills in ponds are often the result of oxygen depletion. Fish kills from oxygen depletion usually occur in the early morning hours in fertile ponds, and are frequently associated with (1) the die-off of a large algae bloom, (2) the decay of weeds after treatment with a herbicide, (3) the turnover of oxygen-poor bottom waters following a thunderstorm, (4) the runoff of livestock waste and other organics after a heavy rain.  Symptoms of oxygen depletion may include fish gulping at the water surface or at the pond inlet or edges. Large fish may die first, but all sizes and most species of fish are usually affected. The color and clarity of pond water may change and a foul odor may be released. 

In order to prevent fish suffocation in fertile ponds:

• Do not over-fertilize ponds, or fertilize when feeding fish.

• Do not stock more fish than recommended.

• Fence livestock from the pond and upstream waters.

• Prevent manure and animal waste runoff into the pond.

• Use herbicides only as recommended

• Use emergency surface aerators or pump-sprays if necessary

Fish Diseases and Parasites

Fish are constantly exposed to a wide variety of diseases and parasites that occur in surface waters. However, fish diseases and parasites seldom reach epidemic levels, and large fish kills in farm ponds are rarely caused by diseases or parasites. Fish suffering from diseases or parasites usually die slowly, a few fish each day. 

Fish exhibiting any unusual form of behavior should be closely examined for external signs of disease or parasites. Infected fish usually show visible sores, discoloration, bleeding, swollen areas, lumps, pop-eyes, small black or white spots, or other abnormal growths on the head, body, and fins. Sick fish look and act abnormal.

There are few practical methods for treating diseased or parasitized fish in natural pond, lake, or stream waters.  Therefore, pond owners should make every attempt to prevent fish diseases and parasites from becoming a problem by:

• Stock only healthy fish from disease-free commercial hatcheries

• Do not stock wild fish, and prevent wild fish invasion from water sources

• Follow the stocking recommendations to avoid overcrowding

• Preventing unintended nutrient or pesticide runoff into ponds

• Do not overfeed fish

Toxic Chemicals

It is very rare for the cause of fish kills to be chemical in nature.  However, if you have reason to suspect a specific chemical or incident, have this information available when you talk to an extension agent.

Help With Fish Kills: Who to Call

You should call your local county extension agent is you want information on pond problems including fish kills.  Be prepared to answer the following questions:

• What is the size and depth of the pond?

• Source of water?

• What is the water temperature?

• What type and how many aquatic plants?

• Any recent agricultural spraying or animal waste runoff?

• Any recent thunderstorm or hot, cloudy weather?

• What species of fish were killed?

• What species are in the pond?

• What sizes of fish were killed?

• How many fish were killed?

• Where were the fish found (surface, bottom, sides, inlet)?

• Are fish still dying?

• Are the fish gulping at the surface?

• Are the fish swimming erratically or behaving abnormally?

• Any visible body sores, hemorrhage, growths, tumors?

• Any dead fish upstream or downstream?