Fish Kills

Fish kills are the mass death of the fish population in any given body of water. Fish die as a result of a wide variety of natural and unnatural causes. They may die of old age, starvation, body injury, predation, stress, suffocation, water pollution, diseases, parasites, toxic algae, severe weather or other reasons. A few dead fish floating on the surface of a pond or lake is not necessarily cause for alarm. However, when large numbers of fish of all sizes are found dead and dying over a long period of time, there is cause for concern. It is important to determine the causes of a fish kill to determine if future kills are preventable and to suggest the best protocol for prevention. The main causes include:

• Low oxygen - Fish swimming near the surface of the water, and appearing to be gulping for air, indicates a low oxygen problem. The amount of oxygen in a body of water will vary with water temperature, degree and type of bottom muck, algae and aquatic plant densities, and amount of sunlight. But unfortunately, as water warms and holds less oxygen, fish, bacteria, algae and other aquatic life forms become more active and require more oxygen. In Florida, most oxygen related fish kills occur in the warmer months from May through September.
• Storm water runoff - can lead to fish kills by washing large amounts of organic material, nutrients and fertilizers into lakes, accelerating plant growth that can lead to oxygen depletion. Proper watershed management includes diverting or eliminating the sources of excessive nutrients (phosphorus and nitrogen) so they do not threaten fish life in lakes.
• Chemicals - including herbicides and pesticides, entering a lake can lead directly to fish kills. Ammonia, which comes from animal wastes, is highly toxic to fish. Toxic chemicals usually affect all species and all sizes of fish.
• Seasonal temperature changes - Turnover occurs in the spring and fall when surface water mixes with water near the bottom. That water may contain little or no oxygen. Carbon dioxide and hydrogen sulfide gasses that are potentially lethal to fish can build up in the lake’s deepest water and are quickly circulated throughout the lake during turnover. Turnover is usually not harmful to fish. However, if strong winds, cold rains or rapid temperature changes accelerate the turnover process, fish can die as a result of suddenly being exposed to low quality water.

Your lake does not have to be a victim of a fish kill. People can prevent most fish kills by maintaining good water quality. However, once a kill starts, there is little that can be done to save the existing fish. The only option is to eliminate the original causes and restock the lake with new fish.