Effects of gas supersaturated water on juvenile brown and rainbow trout

Abstract

Six bioassays were conducted at the Bozeman Fish Technology Center using juvenile brown trout (Salmo trutta) and rainbow trout (Salmo gairdneri) to determine the influence of fish size and species to gas supersaturated water. Juveniles of both species were exposed for 30-d to 104 + 1.2% (control), 112 ± 1.0% (medium treatment), and 125 + 0.7% (high treatment) atmospheric gas supersaturated water. No significant mortality occurred in the 112% treatment except during two tests when percent total gas pressure exceeded 113%, indicating that a critical threshold exists above this pressure. As fish grew they became more susceptible to 125% gas supersaturation. Total mortality and mortality rate were less when fish were small. The frequency at which emphysema occurred by body region also varied with fish size and smaller fish died with different external symptoms than larger fish. Larger fish were more likely to die faster than smaller fish and with no external symptoms. Daily mortality of brown trout exposed to 125% gas supersaturation was always significantly greater than for rainbow trout of similar size. Juvenile brown trout that survived exposure to 112% supersaturation showed greater incidence of external symptoms of GBT than juvenile rainbow trout of similar size. Juvenile rainbow and brown trout were exposed to supersaturations from 112% -118% total gas pressure to test for recovery. Recovery varied according to external symptom severity. Fish that did not recover usually had severe exopthalmia. Juvenile brown trout, repeatedly exposed to 118% gas supersaturated water, given 30-d to recover between exposures, developed more severe symptoms with each exposure. Sublethal effects of exposure to gas supersaturated water on growth, predation, and microbial infection were tested using fish exposed to supersaturations from 112 - 118% total gas pressure. Surviving fish developed new symptoms more often than recurring symptoms. Growth of survivors of 30-d exposure to 112% total gas pressure was not different from control fish. There was no difference in vulnerability to predation in circular tanks between juvenile brown and rainbow trout caused by exposure to gas super-saturation. Predation tests conducted in an artificial stream suggested some difference in susceptibility may be present, but data were not conclusive. Bacterial challenges indicated that juvenile brown trout exposed to 118% gas supersaturated water for 5-d were more susceptible to infection by Aeromonas hvdrophila than non-exposed fish.