Survival and Growth of Juvenile Snake River Fall Chinook Salmon Exposed to Constant and Fluctuating Temperatures

Abstract

AbstractThe incipient lethal temperature (ILT) and critical thermal maximum (CTM) methods are used to set temperature limits for fish. However, the standard application of these methods does not always match the temperature regimes that fish experience in the wild. We used alternative methods to determine the thermal tolerance thresholds of juvenile fall‐run Chinook salmon Oncorhynchus tshawytscha exposed to the temperature regimes that are common in the entrapment pools that form along the shoreline of the Snake River when flows are altered to meet electric power demand. A modified CTM test with a steady temperature rise (1.5°C/h) showed that one‐half the fish died when temperatures reached 27.4–27.9°C and that survival at 25°C was highly variable; the average time to the first death was 9.1 h, varying from 1.7 to 22.5 h. Over 30 d, 99.8% of the fish in the constant temperature regimes (14–22°C) survived. In the fluctuating temperature regimes (which varied from 10–14°C to 22–27.5°C), overall survival was 97.3%; however, only 83.0% and 88.9% survived in the groups that reached daily maximums of 27°C and 27.5°C, respectively. Growth over 30 d in the constant thermal regimes was nearly twice as high as that in the fluctuating regimes, even when daily average temperatures were similar. The maximum growth was 1.9%/d in terms of fork length [FL] and 11.2%/d in terms of weight (WT) at a constant 20°C. The lowest growth occurred in the two groups exposed to daily temperatures of 27°C or more, namely, 0.7–0.8% (FL) and 2.7–3.4% (WT). The results of this study suggest that thermal tolerance tests that expose juvenile fall Chinook salmon to thermal regimes that match the field conditions in entrapment pools along the shoreline of the Snake River provide higher temperature criteria than the standard ILT and CTM methods.