Dead and dying fish are an ugly sight. Truth is, most species of fish are relatively short-lived and have a high rate of mortality. Even large fish, too large to be eaten by predators such as bass and pike, experience a death rate of approximately 50% per year. Fortunately, the deaths are usually spread-out over the year and are rarely observed or become a problem except when concentrated as a "fish kill". The condition called "fish kill" occurs when a number of fish in a given body of water die from a specific cause. Most of the time, fish kills are due to natural causes over which we have no control, such as weather. Only occasionally is death directly related to pollution or improper use of herbicides or other chemicals. Only a fraction of the dead fish are ever observed because many decompose on the bottom or are eaten by scavengers such as turtles and crayfish. Natural fish kills are of three basic seasonal types:
  • WINTER KILL, which occurs in late winter but may not be seen until early spring.
  • SPRING KILL, which is occurs in late May to early June
  • SUMMER KILL, which occurs on the hottest days of mid summer.

Winter kill is the most common type of fish kill. When severe, it has devastating effects on fish populations and fishing quality. Winter kill occurs during especially long, harsh winters. Shallow lakes with excess amounts of aquatic vegetation and mucky bottoms are prone to this problem. The results of a winter kill are seldom noticed until spring when the ice melts. Then the dead fish, often the larger ones, are seen washing up along the edge. Because they require more oxygen, the large fish suffocate and die first. Winter kill begins with distressed fish gasping for air at holes in the ice and ends with large numbers of dead fish which bloat as the water warms in early spring. Dead fish may appear fuzzy because of secondary infection by fungus, but the fungus was not the cause of death.

Actually, the fish suffocated from lack of dissolved oxygen. Trace amounts of dissolved oxygen (measured in parts per million, ppm) are required by fish and all other forms of aquatic life. Even living plants and the bacteria that decompose organic materials on the bottom of the lake require oxygen. As a rule of thumb, the critical level of oxygen is about 2 ppm for most game fish native to warmwater lakes, and levels below 1 ppm for extended periods of time are lethal.

But species of fish vary in their tolerance of low oxygen. Trout are most sensitive; walleye, bass, and bluegill have intermediate sensitivity; and northern pike, yellow perch, and pumpkinseed are relatively tolerant. Bullheads and certain minnows are very tolerant. Lakes prone to periodic winterkill can often be detected from the composition of their fish populations - tolerant species predominate, sensitive species are rare, and prey greatly outnumber predators. Fortunately, usually enough fish survive, either in the lake or in connecting waters, to repopulate the lake in a couple of years. Only for extreme die-offs is fish restocking necessary.

The dissolved oxygen content of water depends primarily on three variables. These are the amount of mixing with the air above the lake, the rate of oxygen production by plants, and the rate of oxygen consumption (respiration) by living aquatic organisms. During periods of prolonged ice cover, the lake is sealed off from the atmosphere and cannot be recharged with oxygenated air. Furthermore, ice and snow reduce the amount of sunlight reaching aquatic plants, thereby reducing photosynthesis and oxygen production. (During photosynthesis, living plants use sunlight energy and carbon dioxide to make plant tissue and dissolved oxygen). Meanwhile, on-going consumption of oxygen depletes the supply of oxygen stored in the lake when the lake froze over. Shallow, productive lakes are at a disadvantage because they have a low storage capacity and high rates of oxygen-consuming decomposition.

In northern NY, January is usually a critical period and is the best time to check the oxygen content of lakes prone to winterkill. A good midwinter thaw about then often recharges the lake's oxygen supply by means of photosynthesis and melt water. Conversely, a prolonged winter, with continuous snow cover and late ice-out, increases the chance of winterkill.

The only long-term solution for winterkill lakes is to reverse the natural process of filling and enrichment (eutrophication). Dredging or sucking bottom sediments can increase the volume of water, reduce the nutrient-rich sediment, and reduce the growth of nuisance plants. However, such projects are extremely costly, require a site for disposing of the bottom material, and may require a permit. Lake residents can help slow down the rate of eutrophication by keeping all types of plant fertilizers out of the lake.


Spring kill occurs in lakes and rivers when fish survive the winter but die as the water warms rapidly in May and June. It rarely claims many fish and is usually over in a couple of weeks. Spring kill is almost always due to natural causes beyond our influence. The usual victims are large bluegills and crappies, and other fish which spawn in the spring such as perch, bass, pike and suckers.

A combination of stresses is usually responsible. Fish come through the winter in a weakened condition because they've been eating at a reduced rate. As the water warms, their metabolism increases and they divert much energy to strenuous spawning activities. In lakes, additional stress may be added during "turnover", which is when wave action stirs up bottom water low in oxygen and high in noxious gases. Diseases and parasites also become more active and on a few occasions have been implicated in fish kills.


Summer kill The most common cause of summer fish kill is the same as winter kill oxygen depletion. But the causes of depletion may be more complex than with winter kills. Summer depletion can result from oxygen removal by excessive quantities of plants, animals or decaying organic mater. This can be caused by overstocking, overfeeding, over fertilization, pollution from barns, feedlots, improper septic drainage, or chemical treatment of aquatic weeds during the critical months June through September.

Another common cause of summer fish kills occurs when a dense growth of submerged aquatic plants or algae in a pond dies suddenly from natural causes or from herbicides.
The decay process from the dead plants may use up the oxygen in the water. This type of summer fish kill almost always happens about sunrise, when the dissolved oxygen is at its low point for the day.
Another condition leading to summer fish kill, and related to the plant die-off, is high water temperatures. During July, and especially in August, water temperatures in ponds may reach 85°-95°F. Water can hold much less oxygen when its temperature is above 80°F.
When overcast skies persist for several days, while temperatures are high and winds are calm, a fish kill may occur. Plants cannot produce sufficient oxygen at reduced light levels and calm weather reduces the exposure of water to oxygen in the air. Therefore, the dissolved oxygen may disappear entirely.


Lightning strike A very unique type of fish kill is caused by a lightning strike on water. Death occurs immediately. Large fish, which draw more electricity than small fish, may be killed selectively.

In conclusion, the risk of some types of fish kills can be reduced by keeping as many nutrients out of the water as possible. Sources of nutrients include septic fields, fertilized lawns and farm fields, and wastes from livestock and waterfowl (including tame geese). Reducing nutrient input starts the following favorable chain reaction: production by aquatic plants is reduced, less decomposition is required, and oxygen will not become depressed to critical levels.

Natural fish kills are obnoxious, and may affect fishing and predator-prey "balance" for years. However, they are often not serious in the long run because lakes contain thousands of fish per acre. They may be thought of as nature's way of thinning out fish populations. Usually, fish kills indicate that the habitat is of marginal quality for certain species because of a broad range of weather conditions. Infrequently, fish kills indicate habitat or pollution problems we may be able to correct. And sometimes, fish kills beneficially reduce over-populated, slow-growing panfish and actually increase growth rates and improve fishing.

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