HIGH-TEMPERATURE TOLERANCES OF FLUVIAL ARCTIC GRAYLING AND COMPARISONS WITH SUMMER RIVER TEMPERATURES OF THE BIG HOLE RIVER, MONTANA

Critical thermal maximum (CTM) and resistance time to high temperature were determined for juvenile Arctic grayling Thymallus arcticus from the fluvial population of the Big Hole River, Montana. Grayling were t ested after acclimation to 8.4, 16.0, and 20.0 degrees C. Thermal tole rances increased with acclimation temperatures; mean CTM was 26.4 degr ees C for the 8.4 degrees C acclimation group, 28.5 degrees C for the 16.0 degrees C group, and 29.3 degrees C for the 20.0 degrees C group; median resistance time at given test temperature also increased with acclimation. The upper incipient lethal temperature (UILT) was 23.0 de grees C for fish acclimated to 8.4 degrees C and 16.0 degrees C, and w as 25.0 degrees C for those acclimated to 20.0 degrees C, temperatures that were similar to the median tolerance limits of Arctic grayling i n Alaska. Comparisons of mean CTM and UILT for juvenile Arctic graylin g with levels and durations of maximum river temperatures recorded dur ing summers 1992-1994 indicated that resident fish may occasionally be subjected to potentially lethal temperatures in the warmest reaches o f the Big Hole River.