Unconsumed fish food apparently resulted in the high sulfide concentrations seen near the feeding area in May or June and September of both years. Dissolved sulfide concentrations decreased near the feeding area in July and August, when the water temperatures were over 28(DEGREES)C and the fish consumed nearly all of the food given. The direct effect of unconsumed food on dissolved sulfide was limited to the vicinity of the feeding area.

The highest dissolved sulfide concentrations found at shoreline areas 80 meters from the feeding area occurred in September, and apparently resulted from the die-offs and deposition of phytoplankton. At those areas, the highest sulfide concentrations were at the location that was sheltered from the wind, and where the deposition of suspended matter was the greatest.

Hydrogen sulfide dissolved in the interstitial water of the sediment of commercial catfish culture ponds in Oklahoma was measured from February, 1978, through November, 1979.

Sulfide-inhibited cytochrome oxidase recovered rapidly in vivo. Brain cytochrome oxidase rose from a 50% inhibition to control levels after six hours at 10(DEGREES)C. The recovery rate was similar in both the brain and gill.

A measurable reduction in cytochrome oxidase activity was seen after exposure of fish to 0.1 mg/l H(, 2)S at 10(DEGREES)C for only five minutes. Exposure of channel catfish to 0.5 mg/l H(, 2)S at 20(DEGREES)C resulted in hyperpnea, followed by apnea, and finally respiratory arrest. When exposed to 0.1 mg/l H(, 2)S at 20(DEGREES)C for 30 minutes, 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.

Mortalities of fish resulting from exposure to hydrogen sulfide on commercial fish farms can be avoided by; (1) reducing dissolved sulfide concentrations in the sediment through pond design, improved feeding practices, and the use of lime in the ponds, (2) landing fish in areas of the ponds with lower sulfide concentrations, (3) treating the water with potassium permanganate during harvesting to oxidize the sulfide in the water, (4) reducing the physical exertion of the fish after sub-lethal exposure to H(, 2)S, and (5) allowing for short-term recovery before transportation.

Hydrogen sulfide competitively inhibited cytochrome oxidase. The enzyme was inhibited in vitro 18% by 10('-7) M H(, 2)S, 64% by 10('-6) M, and 100% by 10('-4) M.

The maximum concentration of sulfide in the sediment was less than 1.2 mg(.)liter('-1) from December through February, and was as high as 18.1 mg(.)liter('-1) in the warmer weather. The maximum sulfide concentration usually occurred from 5.7 to 12.8 cm below the surface of the sediment from December through February, and within the top two centimeters of the sediment from July through September.