EFFECTS OF ONE-YEAR EXPOSURES TO GAS SUPERSATURATION ON LAKE TROUT

Lake trout (Salvelinus namaycush) were reared for 1 year in water with one of six levels of incoming differential gas pressure (DELTAP): 4, 17, 33, 43, 58, or 75 mm Hg. Growth and survival of fish were evaluate d as measures of response to the potential long-term stress of elevate d dissolved gases in rearing water. Mortality rates were not different among fish held in supersaturated water up to DELTAP 58 (108% of tota l gas saturation), but mortality at DELTAP 75 (110% saturation) was si gnificantly higher after day 28. From days 21 to 35, the number of non feeding, moribund fish increased with increasing gas level. By day 56, the length and weight of fish were significantly greater at DELTAP 4 than at higher gas levels. Growth rate reductions were evident for lak e trout in DELTAP 17 and above for more than 252 d. Feed conversion ef ficiency was significantly better in fish held at DELTAPs 4 and 17 tha n at higher pressures. Lake trout grew fastest and most efficiently at DELTAP 4 for 252 d, but DELTAP 58 was also a safe rearing level in te rms of mortality. There were no signs of ps bubble formation in fish h eld at or below DELTAP 58 and only 3% of the fish at DELTAP 75 exhibit ed emphysemas after 269 d of exposure. For optimum growth of juvenile lake trout, total dissolved ps levels should be less than DELTAP 17, p robably near DELTAP 0.