Is there such a thing as a natural fish kill?

Yes. Although people frequently associate fish kills with a pollution event, natural fish kills are not uncommon. These can result from a variety of causes including:

1. Depletion of oxygen. In the summer, oxygen depletion can occur during periods of prolonged calm, cloudy, hot weather, especially when water levels are low. In the winter, oxygen depletions can occur when ice and snow reduce light penetration into the water. These depletions are usually associated with high concentrations of organic matter, abundant growths of rooted vegetation, or heavy algal blooms. Oxygen depletion occurs when photosynthesis by the algae and vegetation is unable to produce and maintain sufficient levels of available oxygen to meet the needs of the fish. Turnover of the thermal stratification in lakes can cause oxygen depletion when the anoxic (low oxygen) bottom water and decaying organic matter mix with the oxygen rich upper water. In shallow lakes, this can be caused by high winds, heavy cold rain, or severe hailstorms during a period of prolonged hot weather. When oxygen depletion occurs, large fish and those fish species with the highest oxygen requirements die first.
    
2. Hydrogen sulfide poisoning. Turnover of the thermal stratification of a lake as described above, can release large quantities of dissolved hydrogen sulfide. Even in the presence of adequate amounts of dissolved oxygen, hydrogen sulfide can interfere with the ability of fish blood to carry oxygen, resulting in death of the affected fish. Larger fish are the most affected by this condition. Signs of hydrogen sulfide poisoning include disoriented and dying fish; dark, decaying organic material; and the odor of hydrogen sulfide.
    
3. Toxic algal blooms. Under certain conditions, blooms of blue-green algae and dinoflagellates can produce toxins that are toxic to zooplankton, insects, and fish. Fish kills due to toxic algal blooms begin early in the morning and continue through early afternoon. Daily mortality continues until the toxic algae bloom ends.
    
4. Spawning related mortality. Spawning and post-spawning fish have reduced resistance to pathogens. Fish that have heavy infections by parasites, bacteria or viruses, and that are exposed to environmental stressors, such as abnormal and/or fluctuating water temperatures or depressed oxygen levels, can suffer significant spring mortality. These fish kills are usually restricted to the adult fish of the spawning species; however, multiple species may be involved if their spawning periods overlap. Avian botulism. Botulism is a paralytic disease caused by toxin produced by the bacteria Clostridium botulinum. Seven strains (A-G) of botulinum toxin have been identified. Type E toxin most commonly affects fish and fish eating birds. Fish and waterfowl die offs associated with botulism have occurred in the Great Lakes. Affected fish exhibit erratic swimming behavior and may have darkening of the skin. Although the incidence of Type E botulism in humans is very low, the toxin has the potential to cause death; therefore, fish from affected waters should not be eaten raw. You should never consume fish or waterfowl that appear to be sick or dying. Botulism toxin is destroyed by heating at 180 F for 10 minutes. When canning fish, a pressure cooker heated to 240 F is recommended.

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