Thermal tolerance in cutthroat trout of the southern Rocky Mountains

With temperatures expected to rise across the southern Rocky Mountains, the ability of native fishes to tolerate stream warming has become a critical concern for those tasked with preserving coldwater species. We used common garden experiments to evaluate the thermal tolerance of cutthroat trout ( Oncorhynchus clarkii) fry from five populations important to managers representing three sub-species. Critical thermal maxima (CTMs) were evaluated through traditional exposure trials, while optimal growth and ultimate upper incipient lethal temperatures (UUILTs) were examined over the course of 21-day trials at six static temperature treatments. Whereas CTMs differed among populations (mean = 27.91 °C, SD = 0.35 °C), UUILTs did not (mean = 24.40 °C, SD = 0.04 °C). Comparison of cubic temperature-growth functions to the traditional quadratic functions showed that adding a third-order term for temperature can improve model fit, and revealed substantial differences in optimal growth temperatures (15.4–18.3 °C). Knowledge of these thermal tolerance thresholds will help to predict the consequences of a warming climate, identify suitable habitats for repatriation, and inform water quality temperature standards established to protect these fish into the future.