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RESOURCE PUBLICATION 177, 1990 41 CHAPTER 5 Fish Kills Due to Natural Causes Roger L. Herman and Fred P. Meyer Introduction Mortality from natural causes is the largest single cause of death of individual fish in a population. Unless fish are killed by some disturbance of the environment, by angling, or by other human inter- vention, they are most likely to die as a result of predation or old age. Although natural phenomena can lead to fish kills, the most common effect of environmental changes in natural waters is the stress imposed on the fish. If the stress level is high enough, a weakening of the immune response may predispose affected fish to infectious diseases. If fish are carrying a signifi- cant burden of parasites, harboring a subclinical bacterial infection, or are already weakened by mal- nutrition, the resultant effect of an environmental stressor is sometimes a fish kill. The magnitude of the kill may far exceed the losses that might be ex- pected from the pathogen observed; the primary cause is then the environmental stressor-not the apparent pathogen or parasite. Fish kills do, however, occur as a direct result of natural causes. Causative agents that have been identified are oxygen depletion, gas supersaturation, toxic algal blooms, turnovers, toxic gases, natural toxic substances, sudden or excessive temperature changes, lightning, bacterial infections, fungi, viruses, parasites, and others. Usually there is suf- ficient evidence at the site to help the investigator accurately determine if the kill was due to a natural cause. Some of the common natural causes are discussed here. Oxygen Depletion Perhaps the most common natural cause of fish kills is oxygen depletion. It occurs when the total demand for oxygen by biological and chemical proc- esses exceeds the oxygen input from aeration and photosynthesis or when the water is unable to hold sufficient dissolved oxygen to maintain aquatic life through the night. Oxygen depletion is usually asso- ciated with abundant growth of rooted vegetation, heavy algal blooms, or high concentrations of or- ganic matter. The oxygen required during the decay of plants and breakdown of organic matter by the bacterial flora, coupled with consumption by fish and other biota, may exceed the oxygen available in the water. Circumstances that foster development of natural oxygen depletion include calm, cloudy, hot weather or low water levels, as may occur during a drought or an extended period without rainfall. Oxygen depletion is highly seasonal in occurrence unless there is extreme eutrophication (a high release of organic nutrients) such as that resulting from untreated or partly treated sewage. Oxygen depletion in natural waters is most common during June, July, and August, but may also occur in December, January, or February. The environmental evidence associated with sum- mer oxygen depletion may include the following: 1. Kill occurred abruptly in early morning, usual- ly between 2:00 a.m. and sunrise. If the kill is incomplete, it usually subsides soon after sun- rise, but then may resume the following night. 2. Large fish of a species died first; small fish may still be alive, attempting to gulp air in shallow water. 3. Species selectivity is evident; species with the highest oxygen requirements die first. 4. Dissolved oxygen concentration is low-usually between 0 and 1 ppm. 5. The pH is between 6.0 and 7.0. 6. Concentration of free carbon dioxide is high. 7. Color of the water changes from light green to pea-soup green, brown, gray, or black. 8. The site and water have a sour-cabbage odor. 9. Decaying vegetation (black and odorous) may be abundant, or many dead and dying algae can be detected under a microscope. 10. Zooplankters are dead or dying.