Metabolic performance and thermal preference of westslope cutthroat trout (Oncorhynchus clarkii lewisi) and non-native trout across an ecologically relevant range of temperatures

The physiology and behaviour of fish are strongly affected by ambient water temperature. Physiological traits related to metabolism, such as aerobic scope (AS), can be measured across temperature gradients, and the resulting performance curve reflects the thermal niche that fish can occupy. We measured AS of westslope cutthroat trout (Oncorhynchus clarkii lewisi) at 5, 10, 15, 20, and 22 degrees C and compared temperature preference (Tpref) of the species with non-native brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), and rainbow trout (Oncorhynchus mykiss). Intermittent-flow respirometry experiments demonstrated that metabolic performance of westslope cutthroat trout was optimal at similar to 15 degrees C and decreased substantially beyond this temperature, until lethal temperatures at similar to 25 degrees C. Adjusted Tpref across species were comparatively high, ranging from 17.8 to 19.9 degrees C, with the highest Tpref observed for westslope cutthroat trout. Results suggest that although westslope cutthroat trout is considered a cold-water species, they do not prefer or perform as well in cold water (<= 10 degrees C) and thus can occupy a warmer thermal niche than previously thought. The metabolic performance curve (AS) can be used to develop species-specific thermal criteria to delineate important thermal habitats and guide conservation and recovery actions for westslope cutthroat trout.