A sudden appearance of dead fish in a lake or pond causes considerable concern and alarm for most people, whose first reaction is to suspect someone of poisoning the water body. Most fill kills, however, result from natural events, although people can influence their frequency and severity. Fish kills usually result from too little oxygen in the water, according to biologists with the Florida Fish and Wildlife Conservation Commission. The Southwest Regional Office of the Commission covers 13 southwest Florida counties and receives between 100 and 150 fish kill reports each year. While some result from spills or illegal discharges of toxic substances, most kills occur when oxygen dissolved in the water drops to levels insufficient for fish survival.
For a dissolved oxygen or DO related fish kill to occur, a combination of environmental conditions must occur simultaneously. Weather patterns, water temperature, depth and quality, amount and type of plant growth, fish community structure, along with the presence of viruses and bacteria, are all factors that are necessary to trigger a fish kill. Lakes, ponds, and canals located in residential areas are particularly vulnerable to DO-related fish kills. Developed areas create runoff that contains high amounts of nutrients from septic tanks; street and yard drainage that enters water bodies and causes water quality problems. High levels of nutrients from fertilizers applied to lawns, golf courses and farms cause aquatic plants to thrive.
Ponds with high nutrient levels produce dense growths of microscopic plants called algae. When sunlight is available, algae use nutrients and produce oxygen through the process of photosynthesis. Most oxygen available to fish comes from algae. During nighttime and cloudy weather, low sunlight causes algae to switch from photosynthesis to respiration, thereby consuming oxygen needed by fish. During severe events, fish can suffocate from low DO. Most frequently, however, fish become stressed during a low DO period and become susceptible to viral or bacterial infections.
Most times, fish can tolerate temporary lags in DO levels. Fish kills occur only when several contributory factors occur simultaneously. Prolonged cloudy weather, drought conditions, overcrowded fish populations, excessive algal or other plant growths and high water temperatures are the most common factors that combine to cause fish kills. In Florida, most DO-related fish kills occur in the warmer months from May through September, although winter cold fronts can also trigger DO lags. A typical scenario occurs when fish are observed at the water surface appearing to gasp for breath. Fish usually continue to die from viral or bacterial infections for 3-4 days. Most of the time, this occurs after a period of rainy or cloudy weather.
During the spring, kills involving only one species can occur and these are caused from stress brought on by spawning activities. Along coastal areas of Florida, surface and groundwater inflows of salt water can kill freshwater fishes. Decomposing vegetation from aquatic plant control operations may reduce oxygen levels in the water. If this occurs during times when other environmental conditions cause low levels of dissolved oxygen, fish kills may occur. It is essential that only herbicides that have been approved for use in aquatic sites by the US Environmental Protection Agency (EPA) and the Florida Department of Agriculture and Consumer Services (FDACS) be utilized in strict accordance with the label and that consideration be given to weather conditions and DO levels prior to application.
Application of pesticides to control lawn and crop insects can enter a pond during heavy rains and cause a fish kill. Use of any type of chemical pesticide should be done with extreme caution around all water bodies.
Very few kills result in total loss of the population. Remaining fish can usually reproduce and quickly restore the population. Occasionally, undesirable species will tolerate low DO and dominate the population. Many people incorrectly assume that these "trash fish" have out competed sport fishes; however, this is actually a situation where hardy "trash fish" species have thrived in a situation of poor water quality that other species find intolerable. A good fishing pond can sometimes be ruined by a fish kill while others may benefit. Repeated kills may necessitate the installation of an aeration system that will maintain DO levels.
People can only prevent fish kills by maintaining good water quality. Once a kill starts, there is nothing that can be done. State and most county agencies cannot clean up dead fish and private landowners must undertake the task themselves. Kills occurring on city-maintained lakes are often cleaned up by city crews. Concerned individuals can report fish kills to the Commission, especially if they suspect that a kill is a result of toxic spills. Discussions with pond owners often lead to determinations of cause and Commission biologists can provide recommendations to prevent future kills. On-site investigations are done on water bodies with public access and when environmental laws have been broken. Should anyone suspect that a fish kill be a result of unnatural causes, they should call the Commission listed in the inside cover of their telephone directory. Private pond owners can obtain the advice of a Commission biologist by calling a local Regional Office.