Good productivity of fish and aquatic life are dependent upon clear, clean water at favorable temperatures and with sufficient concentrations of needed dissolved gases and solids. The number of individuals and species of bottom animals or plankton present in streams and lakes are important criteria of water quality. Siltation is one of the most damaging and widespread pollutants; it causes reduction of light penetration, destruction of shelter, and smothering effects on eggs. For short periods fishes tolerate turbidities up to 100,000 parts per million, but under long-term exposure, concentrations of 100-200 ppm can be directly harmful. Fishes may tolerate dissolved solids up to 3,000 ppm or more if they are nontoxic earth metals and physiologically balanced. Bass and bluegill eggs and fry can survive in salt water up to about 10 percent sea strength. Temperatures of 93° to 96° F. represent the critical level for most species of warmwater fishes. Trout require a maximum summer water temperature of about 68° F. for good production. The effects of cooling waters from steam electric plants and towers of industrial plants may be detrimental to fish, imposing temperature blocks on spawning runs or reducing desirable food organisms. They may, however, provide places where anglers can harvest fishes or (in the south) places where threadfin shad, a desirable forage fish, can winter north of their normal range. Oxygen levels should be high enough to permit growth and reproduction. This level is about 5 ppm for warmwater fishes and 6 ppm for salmonoid fishes. Oxygen requirements of fishes may be affected by the presence of carbon dioxide. The pH of streams of the United States generally ranges between pH 7.4 and 8.5. The acid death point for pond fishes is pH 4.0 and the alkaline death point pH 11.0. Levels of pH from 6.5 to 9.0 are most suitable for culturing pond fish. Organic wastes from domestic sewage and paper mills deplete oxygen supplies. Five-day BOD's above 10 ppm in streams and 3.5 ppm in lakes indicate pollutional effects.
