Comparative Thermal Requirements of Westslope Cutthroat Trout and Rainbow Trout: 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 nonnative 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 208C. The ultimate upper incipient lethal temperature of rainbow trout (24.38C; 95% confidence interval [CI]1⁄424.0–24.78C) was 4.78C higher than that of westslope cutthroat trout (19.68C; 95% CI 1⁄4 19.1–19.98C). In contrast, both species had similar growth rates and optimum growth temperatures (westslope cutthroat trout: 13.68C; rainbow trout: 13.18C) over the temperature range of 8–208C, 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 13–158C 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. Westslope cutthroat trout Oncorhynchus clarkii lewisi historically occupied a wide range of habitats, from small, headwater streams to large rivers and mountain lakes within drainages in western Montana, Idaho, northwestern Wyoming, eastern Oregon and Washington, and southern Alberta (Liknes and Graham 1988; Behnke 1992; Thurow et al. 1997; Shepard et al. 2005). However, as with many other native salmonids in western North America (Behnke 1992), this subspecies now exhibits a fragmented distribution over large portions of its range (Thurow et al. 1997; Shepard et al. 2005). The subspecies was recently evaluated for listing as a federally threatened species under the Endangered Species Act (USFWS 2003). Although listing was deemed not warranted, leading threats to the persistence of westslope cutthroat trout populations remain, including habitat degradation, hybridization with nonnative rainbow trout O. mykiss, and displacement or replacement by nonnative species (Shepard et al. 1997, 2005; Shepard 2004; Rubidge and Taylor 2005). The current distribution of westslope cutthroat trout is restricted primarily to cold, high-elevation, high-gradient streams, whereas mid-to-lower elevation portions of drainages are occupied by nonnative rainbow trout, rainbow trout 3 westslope cutthroat trout hybrids, brook trout Salvelinus fontinalis, and brown trout Salmo trutta (Thurow et al. 1997; Paul and Post 2001; Sloat et al. 2001, 2005; Shepard 2004). The current restricted distribution of native cutthroat trout to headwater reaches in many drainages indicates that water temperature is probably a key factor influencing their persistence (Paul and Post 2001; Sloat et al. 2001; de la Hoz Franco and Budy 2005). High temperature in lower elevations can cause direct mortality of salmonids that are more adapted to cold water, whereas sublethal temperature increases can alter metabolism, growth, and competitive interactions that may favor more warmwater-adapted salmonids (Taniguchi et al. 1998; Taniguchi and Nakano 2000). Field studies attest to the likelihood that temperature has a major influence on the growth potential and distribution of westslope cutthroat trout (Paul and Post * Corresponding author: [email protected] 1 Present address: Wyoming Game and Fish Department, 528 South Adams Street, Laramie, Wyoming 82070, USA Received March 28, 2006; accepted February 3, 2007 Published online July 2, 2007 1113 Transactions of the American Fisheries Society 136:1113–1121, 2007 Copyright by the American Fisheries Society 2007 DOI: 10.1577/T06-072.1 [Article]