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Fish Kills

Watch a video about fish kills, their causes, remedies and prevention.

Fish kills may occur if toxic substances enter the pond, if dissolved oxygen is low, if parasites or diseases attack the fish, or for other reasons. When a fish kill occurs, a fishery biologist should be contacted immediately to determine the cause and to develop a plan to reduce or eliminate additional losses. The numbers, weights and sizes should be estimated for each species of fish that dies. Mortality estimates will be helpful to a fishery biologist in determining the severity of the kill and whether the fish population will recover.

Care should be exercised when using pesticides on or near the watershed of ponds. Pumps used to spray pesticides and then cleaned in the pond may contain enough toxic material to kill fish. In addition, cattle sprayed with pesticide should not be allowed to access ponds. Many pesticides are not toxic to fish and should be used when practical.

Low oxygen levels can cause major fish kills.Low oxygen may result in fish kills, particularly during the hot summer months as warm water contains less oxygen. The primary source of oxygen in most ponds is from photosynthesis, a process through which phytoplankton (green algae) produce oxygen in the presence of sunlight. Since sunlight is required, oxygen production occurs during daylight hours. After dark, oxygen concentrations tend to decline as it is consumed by aquatic organisms (fish, insects, algae, etc.) through respiration. Oxygen level is typically above 8-10 ppm (or higher) during the day. If concentrations fall below 2-3 ppm, fish mortalities may occur.

The decay of plant material is another common cause of oxygen kills. Since decomposition of organic matter consumes oxygen, chemical treatment of plants should be done in early spring when water temperatures are lower and plants are less abundant.  No more than a third of the plants should be treated at a time. Several days of cloudy weather or a rapid change in temperature following a thunderstorm may also cause an oxygen deficiency through a sudden die-off of plankton. Over fertilization increases the likelihood of an oxygen problem.

Symptoms of oxygen depletions may include a change in water color (from green to black or brown), a pungent odor, fish that suddenly stop biting or taking pelleted feed, or fish are swimming near the surface and gulping for air.  When any of these symptoms are noticed, the pond should be aerated immediately. Usually, a pump is the most practical method for aeration. The intake should be placed just beneath the pond’s surface and the water sprayed into the air and allowed to fall back on the pond. The stream of water should be diffused so that it will collect more oxygen from the air. Aeration is especially critical at night when oxygen levels are lowest. An application of 20 pounds per acre of triple superphosphate should also be made to quickly improve oxygen production by stimulating plankton growth. (Note: In ponds greater than 3 acres, small pumps will have little or no effect in improving oxygen.)

Fish parasites or diseases seldom cause extensive kills in bass-bream ponds. Symptoms may include external sores or bloody lesions. Chemical treatments, though expensive, are available for certain fish parasites and diseases. If the fish are taking a pelleted ration, many diseases may be treated with medicated feed. Consult a fishery biologist or the Auburn University Fish Parasite and Disease Lab (phone 334-844-4786).

During the spring, kills may also occur when fish, primarily bream, gorge themselves on swarming fire ants. Fire ant kills can be often determined by examining the stomach contents of a dead or dying fish. Fire ants may also be suspected if large numbers of ants are floating on the pond.

The above information came from the Alabama Wildlife and Freshwater Fisheries Division's booklet Sportfish Management in Alabama Ponds, which is available as a PDF.

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