Effects of Hydrostatic Pressure on Steelhead Survival in Air-Supersaturated Water

Abstract

Juvenile steelheads (Salmo gairdneri) were placed in cages and suspended at various depths in water supersaturated with air at levels from 120 to 140% of normal atmospheric gas pressure. Survival times of fish held at 10, 50, and 100 cm depth increased with increasing depth at a given level of supersaturation. When the hydrostatic pressure (7.4 mm Hg per 10 cm of water depth) was subtracted from the excess gas pressure (relative to surface barometric pressure), mortality curves (times to 50% mortality versus excess gas pressure) for fish at all three depths essentially coincided. The significant measure of supersaturation appears to be the pressure of dissolved gases in excess of the sum of barometric and hydrostatic pressures. Steelheads held near the surface in supersaturated water for a near-lethal period and then lowered to a depth providing total hydrostatic compensation appeared to recover completely in about 2 hours. The longer fish remained at depth, the longer their survival time when they subsequently were reexposed to surface conditions.