PNNL

Abstract

Management agencies presently require operators to release water at Hells Canyon Dam on the Snake River when daily average water temperatures in solar-heated entrapment pools below the dam exceed 16°C for 3 d or if the instantaneous temperature is greater than 18°C for 4 h. The purpose of this laboratory study was to determine if these temperature guidelines protect sub-yearling fall Chinook salmon (Oncorhynchus tshawytscha) that become entrapped in shoreline pools below the dam when flows are fluctuated to meet electric power demand. To determine upper thermal tolerance, water temperatures of juvenile Chinook salmon were increased 1.5°C/h from 10, 12, and 14°C until fish lost equilibrium and died. An additional test exposed fish to a similar increasing thermal regime up to 25°C and held them there for 70 h. Finally, survival and growth was monitored for 30 d in a separate group of juvenile salmon exposed to one of 9 constant (14 to 22°C) or 11 fluctuating (daily fluctuations from 10-14°C to 22-27.5°C) thermal regimes. The thermal tolerance test with steady temperature rise showed that half the fish died when temperatures reached 27.4 to 27.9°C. The thermal tolerance test with temperature rise and hold showed that survival at 25°C was highly variable with the average time to first death 9.1 h (range 1.7 to 22.5 h). Survival in the constant temperature regimes was 99.8% over the 30-d exposure period. In the fluctuating temperature regimes, short-term exposure to temperatures equal to or greater than 27°C in the daily cycle resulted in significant mortality, however, if the daily maximum temperature did not go above 26°C and was followed by a rapid decline in temperature (3°C/h), then the survival rates were nearly 100% over the 30-d exposure period. The optimum growth of juvenile salmon when fed to satiation for 30 d occurred at a constant 20°C (range 19.7 to 20.7°C). Growth in the constant thermal regimes was nearly twice as high as in the fluctuating regimes, even when daily average temperatures were similar. The results of this study suggest that guidelines for managing temperatures could be revised to be protective of juvenile fall Chinook salmon while providing additional flexibility to dam operations.