Physiological and biochemical effects of acute exposure of fish to hydrogen sulfide.

1. Exposure of channel catfish to 0.5 mg/l H2S at 20 degrees C resulted in hyperpnea, followed by apnea, and finally respiratory arrest. 2. Catfish brain cytochrome oxidase activity was inhibited in vitro 18% by 10(-7) M H2S, 64% by 10(-6) M H2S, and 100% by 10(-4) M H2S. 3. When channel catfish were exposed to 0.1 mg/l H2S at 20 degrees C for 30 min, the cytochrome oxidase activity of the brain was inhibited 40% and that of the gill was inhibited 74%, while blood lactate rose from 11.6 to 38.1 mg/100 ml. 4. Catfish with nitrite-induced methemoglobinemia showed less inhibition of brain and gill cytochrome oxidase when exposed to hydrogen sulfide than did control fish. 5. Sulfide-inhibited brain cytochrome oxidase recovered from a 50% inhibition to control levels in channel catfish after 6 hr in freshwater at 10 degrees C. 6. Management practices are recommended which may help the commercial fish farmer alleviate the problem of the toxicity of sulfide to fish during harvest operations.