Fish Kills

Fish Kills

Fish populations commonly have high mortality rates. In some ponds, one-quarter to one–half of all fish present will die of natural causes each year. This mortality takes place throughout the year. Many fish succumb to predation. Fish dying from other causes are usually quickly eaten by scavengers, so dead fish are seldom seen. On some occasions in some ponds, noticeable mass mortalities of fish do occur. Once dead fish are seen, it is usually too late to do anything, but knowing the possible causes can sometimes help the pond owner prevent fish kills from recurring or at least reduce their severity.


A variety of chemicals are being introduced into our environment, and those used in agriculture can gain access into ponds. Some pesticides are extremely toxic to fish, and others are low in toxicity. Most herbicides used today have a low toxicity to fish, and most persistent insecticides have now been banned from use. Many of the currently used insecticides are short-lived, especially when exposed to water and are usually broken down and non-toxic by the time they get into ponds. Problems can, however, occur when someone carelessly sprays a pond while spraying a field, or when heavy rains wash pesticide-loaded silt into a pond immediately following application on a nearby field. Washing out a spray tank and equipment in a pond can also cause fish mortalities.

It is difficult to establish with certainty that a fish mortality was related to chemical use. Analysis of water samples is expensive and time consuming, and chemicals will break down by the time analysis is possible. Circumstantial evidence can be used in determining whether chemicals caused a fish kill. The pattern of mortality is usually the best clue. In a chemical poisoning, small fish die sooner than large fish, and all species of vertebrates including turtles and frogs are affected.

In addition to massive fish kills, pesticides can have longrange effects on fish production if sub-lethal dosages are continuous or repeated. Pesticides may affect food organisms; they may alter fish reproduction, or they may be an added stress, causing decreased resistance to low oxygen levels and diseases.


Fish kills are common during the winter in Kansas. Mass mortalities are noticed in late winter when ice cover disappears. This type of mortality is caused by oxygen depletion under the ice. A long period of snow cover on the ice is usual43 ly responsible for a winterkill. Ice is usually clear enough to allow sunlight penetration so that plants can produce oxygen, but snow cover greatly reduces the amount of light penetration so plants are unable to produce oxygen. Instead, there is a steady decline in oxygen due to the decay of organic matter and respiration by bacteria and other organisms. If snow persists long enough, complete oxygen depletion will occur. Winterkill ponds are typically shallow and have a high organic matter content commonly in the form of decaying vegetation or livestock wastes.

Winterkills can often be prevented by controlling aquatic vegetation and reducing the amount of livestock or other wastes that get into the pond. Water depth in Kansas should be at least 8 feet going into the winter to hold enough oxygen to carry fish through a normal period of ice cover. Ponds that rely on surface runoff should be built at least 10 feet deep in eastern Kansas and 15 feet deep in the western part of the state. Removal of even a strip of snow from the ice may prevent winterkill. Another effective way to prevent winterkill is to place an aeration device on the pond bottom or to install a water circulator to keep an area free of ice. Much of the water is then exposed to the air for oxygen absorption. Just cutting a hole in the ice is not effective since too little water gets exposed to the air.


Summerkills are massive fish mortalities which occur during the summer due to oxygen depletion. The chain of events leading to summerkills was described in the “Aquatic Vegetation” section. Fish mortality due to summerkill usually occurs early in the morning, at which time the dissolved oxygen in the pond is at its lowest level. The mortality pattern is different than occurs due to pesticide poisoning, with larger fish dying first and frogs and turtles not affected.

Summerkills can be prevented by keeping aquatic vegetation from becoming too abundant. Excessive nutrients should also be prevented from entering the pond. This will reduce heavy algae blooms. If a fish kill is beginning or about to begin (fish are gulping for air at the surface), heavy mortality can often be prevented by pumping fresh water into the pond or by installing an aeration device.

Summerkills are common in fish feeding programs where high densities of fish are crowded into small shallow ponds. The addition of the organic matter in the form of feed can deplete the oxygen content in the pond.

Diseases and Parasites

Fish are affected by a wide variety of diseases and parasites just like any other group of animals. Diseases can be caused by viruses, bacteria, or fungi. Fish are most susceptible to diseases in early spring when their resistance is low coming out of the winter. In most cases, mortality is not extensive in pond fish populations. Diseases are a greater problem where fish are crowded as in hatcheries and commercial operations. Disease diagnosis is difficult, and treatment is expensive and usually not feasible except in large investment situations such as fish farming.

Most fish will have at least a few parasites. Parasites may be protozoa, flukes, tapeworms, roundworms, leeches, or crustaceans. A healthy fish can tolerate some parasites and show no ill effects. It is difficult to rid a pond of parasites, since there are a variety of parasites that can be readily introduced from a variety of sources. The best way to keep fish populations healthy is to maintain good water quality and prevent overpopulation.

“Black spot” and “yellow grub” are the fish parasites people most commonly encounter. Black spot (or black grub) consists of small, round, black grains (about pinhead size) embedded in the skin and flesh. Sunfish and minnows are commonly affected. Yellow grubs appear as small yellow or white nodules under the skin and in the flesh, especially near the base of fins and the tail. These parasites are found in many species of fishes, but are most noticeable in largemouth bass. The yellow grub is an immature stage of a parasitic flatworm which has a complicated life cycle. The adult worms live in fish-eating birds such as kingfishers and herons. The eggs are expelled into the water and hatch, producing larvae which enter snails. They then undergo massive asexual reproduction and numerous freeswimming individuals are released. These penetrate fish and become embedded. This is the stage that is readily observed by anglers. When the fish-eating bird eats the fish, the grubs have a chance to become adults and complete the cycle.

Most fish diseases and parasites are specifically found in fishes and are not harmful to man, especially if the fish flesh is properly cooked before being eaten.