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I bioaccumulate a pesticide in their energy reserve (fat) to levels that are much higher than a single acutely toxic dose. As long as the food intake meets or exceeds their daily energy requirements, the fish will continue to function normally. However, when seasonal water temperatures fall below the feeding range, the fish must rely on stored energy to sur- vive. In fish with a high pesticide residue in their fat, mobilization of the energy reserve may release lethal levels of pesticide into the blood stream. Although signs of toxicosis are sometimes seen, the fish usually seem weak or lethargic, or unconscious. Diagnosis of such a cause requires the analysis of blood samples or brain tissue for pesticide residues. Although analysis of the fat is helpful, the results can be misleading because stored residues may be unrelated to the kill. Under certain circumstances, selenium, a required element, enters the food chain in excessive amounts. Runoff from coal mining areas can cause fish kills. Low pH in the acid mine drainage is usually the causative agent in such kills. Note the reddish iron precipitate and the conspicuous lack of life in the water. Toxic SUBSTANCES 25 Waterborne concentrations exceeding 3 µg/L have been detected in lentic systems-for example, in power plant cooling reservoirs and certain agricul- tural drainage waters. Selenium bioaccumulates in the ovaries of sensitive fish species such as centrar- chids. Although selenium-laden ova can be fertilized, the young fail to survive, leading to an eventual col- lapse of the fish population (Lemly 1985; Baumann and Gillespie 1986). The EPA (1986) published brief summaries of acute and chronic toxicity information for fresh- water and marine species for all contaminants for which the agency has developed criteria recommen- dations. These criteria, which are summarized in Appendix B, are expected to be adequate to protect aquatic life. The summaries are updated to reflect recent changes in EPA's recommendations on acceptable limits for the protection of aquatic life and human health. More detailed information on I