Comparative thermal requirements of westslope cutthroat trout and rainbow 11-out: Implications for species interactions and development of thermal protection standards

Water temperature appears to play a key role in determining population persistence of westslope cutthroat trout Oncorhynchus clarkii lewisi, but specific thermal performance and survival criteria have not been defined. We used the acclimated chronic exposure laboratory method to determine upper thermal tolerances and growth optima of westslope cutthroat trout and rainbow trout O. mykiss, a potential normative competitor that occupies much of the former range of westslope cutthroat trout. Rainbow trout had a distinct survival advantage over westslope cutthroat trout at water temperatures above 20 degrees C. The ultimate upper incipient lethal temperature of rainbow trout (24.3 degrees C; 95 % confidence interval [CI] = 24.0-24.7 degrees C) was 4.7 degrees C higher than that of westslope cutthroat trout (19.6 degrees C; 95% CI = 19.1-19.9 degrees C). In contrast, both species had similar growth rates and optimum growth temperatures (westslope cutthroat trout: 13.6 degrees C; rainbow trout: 13.1 degrees C) over the temperature range of 8-20 degrees C, although rainbow trout grew over a wider range and at higher temperatures than did westslope cutthroat trout. The rainbow trout's higher upper temperature tolerance and greater growth capacity at warmer temperatures may account for the species' displacement of westslope cutthroat trout at lower elevations. Our results indicate that maximum daily temperatures near the optimum growth temperature of 13-15 degrees C would ensure suitable thermal habitat for westslope cutthroat trout populations. The low upper temperature tolerance and optimum growth temperature of westslope cutthroat trout relative to those of other salmonids suggest that this subspecies may be particularly susceptible to stream temperature increases associated with global warming and anthropogenic habitat disturbance.