Assessing Temperature Tolerance of Bonneville Cutthroat Trout Based on Constant and Cycling Thermal Regimes

We assessed the response of Bonneville cutthroat trout Oncorhynchus clarki utah to constant and cycling thermal regimes in the laboratory. The 7-d upper incipient lethal temperature (LT50) based on a constant thermal regime was 24.2°C, and no fish survived temperatures greater than 25°C. All cutthroat trout survived a 7-d exposure to a diel cycle of 16–26°C, despite a 6-h daily exposure to temperatures (>24.2°C) that would be lethal under chronic exposure. However, declines in feeding and activity were observed during the 16–26°C cycle, indicating that long-term exposure to such a thermal regime would be detrimental. Mortality occurred when diel temperature cycles were escalated to18–28°C. Throughout the natural geographic range of Bonneville cutthroat trout, diel temperature cycles in streams can attain peaks above 24°C, suggesting that some populations likely experience thermal stress during part of the summer.