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70 • Nebraska Game and Parks Commission Pesticides can enter a pond from an agricultural field, golf course, or lawn. Some pesticides can be very toxic to fish. Luckily, many are short-lived and usually break down and become non-toxic to fish before they enter a pond. Many pesticides can cause a fish kill when applicators are careless and allow the spray or its drift to enter the pond directly. A fish kill can also occur when storm runoff carrying pesticide-laden soil particles enters a pond immediately after an application. Fish kills resulting from chemical poisoning can be extensive and affect all fish species. They are also characterized by small fish dying sooner than larger fish. Many species of aquatic vertebrates, such as turtles, frogs and tadpoles, and other aquatic organisms, can also be killed or adversely affected. If sub-lethal pesticide dosages continue to enter a pond for an extended time period, they can affect fish food production, alter fish reproduction, or become an additional stressor that decreases fish resistance to low dissolved oxygen levels and diseases. By choosing a proper site for pond construction, many of these problems can be avoided. Landowners also need to consider pond health when choosing chemicals to use in the watershed. Fish Kills due to Diseases and Parasites Fish kills can also result from disease and parasites. Although viruses, bacteria, and fungi can all cause disease outbreaks, they usually do not result in massive fish die-offs. Mortality can occur during early spring, when a fish's disease resistance is low due to winter and pre-spawning stressors. If fish populations are out of balance and there is a high density of one species of fish, particularly crappies, a disease outbreak can result in a substantial fish kill. Angler caught fish that are mishandled during the release process are more susceptible to infections and diseases. Environmental conditions, such as prolonged periods of low dissolved oxygen, extreme pH levels, and high temperatures, can also stress fish and make them more susceptible to diseases. Although most fish have some parasites, they are normally not a problem for healthy fish. Like diseases, parasites can cause mortality if fish are already stressed from other factors. Maintaining good water quality and balanced fish populations will keep fish healthy and less susceptible to disease and parasite problems. See page 73 for additional information on fish parasites and diseases. Consequences of Fish Kills Most fish kills do not result in elimination of the entire fish community. The severity of a fish kill depends on environmental conditions, the size and depth of the pond, and the type of kill. The adverse conditions that caused the kill may not have occurred throughout the entire pond, and unaffected areas will provide a refuge until conditions improve. For example, a partial kill caused by pesticides associated with storm runoff may occur in the area near the inlet and nowhere else, as a result of dilution. Depending on the severity of the fish kill and the species involved, the remaining fish populations may not return to previous levels or provide the same quality of fishing. If undesirable fish species, such as carp, bullheads, or green sunfish, were present prior to the kill and were not affected by it, they may then overpopulate the pond due to reduced competition and lack of predation by largemouth bass. Diagnosing and Preventing Fish Kills The following information can be used as a guide for troubleshooting fish kills in ponds. Knowing what can cause a fish kill will help you prevent or lessen the severity of one. Fish populations that have suffered die-offs should be assessed to determine what needs to be done to restore the pond's fishery. See page 50 for further details on assessing fish populations.