Upper Thermal Tolerance of Mountain Whitefish Eggs and Fry

Some populations of Mountain Whitefish Prosopium williamsoni, a widely distributed native Rocky Mountain salmonid, have experienced catastrophic declines while other populations remain robust. To assess the possibility that the declines have arisen from climate-related factors, several experiments were conducted to determine the upper thermal tolerances of embryos and fry and the effects of temperature on growth. Hatching of eggs was measured at 5.7, 6.4, 8.4, and 10.4 degrees C (three replicates each). Survival and growth of fry were measured for 33 d at 5.7, 8.2, 11.8, 16.5, 19.1, 22.2, and 25.2 degrees C (three replicates each). Mean hatching success was 96.7% (SD, 2.9) at 5.7 degrees C, 98.3% (2.9) at 6.4 degrees C, 90.0% (8.7) at 8.4 degrees C and 38.3% (10.4) at 10.4 degrees C. The ultimate upper incipient lethal temperature for fry was 23.6 degrees C (95% CI, 23.5-23.7 degrees C) after 7 d, which decreased with time to 22.6 degrees C (22.1-23.0 degrees C) after 33 d. The critical thermal maximum was 26.7 degrees C (SD, 0.8) for fry acclimated to 13.4 degrees C. The temperature for maximum growth was 13.8 degrees C (95% CI, 10.8-16.8 degrees C). Growth rates above and below the temperature for maximum growth decreased sharply to about 40% of the maximum at 5.7 degrees C and 22.2 degrees C. Based on these experimental data, the criteria for the protection of Mountain Whitefish fry are 21.6 degrees C for acute water temperature and 16.8 degrees C for chronic water temperature. The acute criterion for reproductionbased on the maximum temperature for successful incubationis 8.4 degrees C. The low upper thermal tolerance of Mountain Whitefish relative to those of other salmonids suggests that they will be very vulnerable to the increasing stream temperatures resulting from climate change. Received September 13, 2012; accepted January 8, 2013