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FISH KILLS DUE To NATURAL CAUSES 43 with dark, chocolate-colored gill filaments. Acidify- ing a sample of the brown blood with acetic or hydrochloric acids will release a distinctive H2S or rotten-egg odor. Signs of an oxygen depletion may be observed, but are not always present. The dis- solved oxygen is sometimes above 3 ppm in cases of H2S poisoning. Toxic Natural Substances Problems occasionally develop because of thermal stratification. In areas where manganese is abun- dant in soils of the watershed, dissolved manganous oxide may accumulate in the anoxic, acid hypolim- nion to levels that are toxic to fish. Generally, because no fish are in the anoxic zone, the potential hazard usually goes unrecognized. However, if the stratification is disturbed (e.g., by a cold rain, a turn- over, or an internal seiche), a fish kill may occur. If a turnover or internal seiche brings the toxic water to the surface or above the intake of penstocks in a dam, a fish kill may result in the river or at a hatchery below the dam. Such fish kills are par- ticularly difficult to diagnose because the mortality is sporadic, environmental characteristics appear normal, and there are no lesions on affected fish. Diagnosis is based on the detection of toxic levels of manganese in the water. Gas Supersaturation The solubility of gases in water is inversely related to temperature and directly related to atmospheric and hydrostatic pressures. As illustrated by the bub- bles that form in a glass of cold water set in the sun, warm water holds less gas than cold water. When a container of carbonated beverage is opened, pres- sure is released and gas bubbles form (the beverage fizzes). If a diver surfaces too rapidly from a deep dive, bubbles of nitrogen form in the blood vessels because the solubility of nitrogen in the blood decreases as the hydrostatic pressure is reduced. This results in a condition known as "the bends" that can be lethal. Fish can suffer from the same condi- Supersaturation of water with dissolved gases may be lethal to fish. This yellow perch shows typical lesions associated with gas bubble disease. Note the presence of large gas bubbles around and behind the eye. I