Effects of temperature on survival and growth of westslope cutthroat trout and rainbow trout : implications for conservation and restoration

Abstract

Westslope cutthroat trout Oncorhynchus clarkii lewisi have declined throughout their native range in the Northern Rockies and were considered for listing under the federal Endangered Species Act. Water temperature is widely regarded as playing a key role in determining their persistence, but specific lethal levels and thermal optima for this cutthroat trout subspecies had not been precisely defined. This laboratory study used the acclimated chronic exposure method to determine tolerances and thermal optima of westslope cutthroat trout and rainbow trout Oncorhynchus mykiss, a potential nonnative competitor now occupying much of the former range of westslope cutthroat trout. Rainbow trout had a distinct survival advantage over westslope cutthroat trout at warmer temperatures. The ultimate upper incipient lethal temperature (temperature at which 50% of the population survives for 60-d) of rainbow trout (24.3°C; 95% CI, 24.0 - 24.7°C) was 4.7°C higher than that of westslope cutthroat trout (19.6°C; 95% CI, 19.1 - 19.9°C). In contrast, the optimum growth temperature for westslope cutthroat trout (13.6°C; 95% CI, 10.3 - 17.0°C) over the 60-d test period was very similar to that of rainbow trout (13.1°C; 95% CI, 6.8 - 18.2°C), although rainbow trout grew better over a wider range and at higher temperatures than did westslope cutthroat trout. The upper lethal and optimum growth temperatures for westslope cutthroat trout are in the lower range among most salmonids. The higher upper temperature tolerance of rainbow trout and its greater ability for growth at warmer temperatures may account for its increased occurrence at lower elevations than cutthroat trout. Water quality standards setting maximum daily temperatures from 13-15°C, near the optimum growth temperature, would ensure suitable thermal habitat to maintain the persistence of westslope cutthroat trout populations. In addition, survival and growth parameters indicated in this study can be used with stream temperature modeling to predict suitable habitat for westslope cutthroat trout, as they may be particularly susceptible to increases in stream temperature associated with climate change. Such predictions of habitat suitability will be vital in prioritizing conservation efforts with respect to reintroduction and translocation of westslope cutthroat trout.