Influences of Temperature and Environmental Variables on the Distribution of Bull Trout within Streams at the Southern Margin of Its Range

—The bull trout Salvelinus confluentus is believed to be among the most thermally sensitive species in coldwater habitats in western North America. We conducted a comprehensive field assessment of thermal habitat associations throughout the southern margin of the species’ range. We developed models of thermal habitat associations using two data sets representing a geographically diverse range of sites and sampling methods. In both data sets, maximum temperature was strongly associated with the distribution of bull trout. In spite of the potential biases in these data sets, model predictions were similar. In both cases, the probability of the occurrence of bull trout exceeded 50% when the maximum daily temperature was less than 14–168C, a result that is consistent with recent laboratory-based thermal tolerances. In one data set, we modeled the association between the distribution of bull trout and environmental variables, including temperature, instream cover, channel form, substrate, and the abundance of native and nonnative salmonid fishes. Only temperature was strongly associated with the distribution of bull trout. Our results and related studies of landscape habitat associations suggest that conservation efforts for bull trout would benefit from a focus on maintaining and restoring large and interconnected coldwater habitats. In western North America, the bull trout Salvelinus confluentus is believed to be among the most thermally sensitive species in coldwater habitats (Rieman and McIntyre 1993; Buchanan and Gregory 1997; Haas 2001; Selong et al. 2001). The bull trout is listed as threatened under the U.S. Endangered Species Act and occupies a broad range across western North America (U.S. Fish and Wildlife Service 1999; Haas and McPhail 2001). Species with a narrow thermal ‘‘niche’’ (Magnuson et al. 1979) are most likely to be affected by alterations in water temperature regimes. In particular, species that are tied to coldwater habitats may be especially vulnerable to the increases in temperature that commonly result from human influences on such regimes (Poole and Berman 2001). Accordingly, issues regarding the sensitivity of bull trout to increases in temperature and that of temperature to human influences are of great interest to those involved in land management and species recovery efforts (Poole et al. 2001). Information on the thermal tolerance of bull trout has come from a variety of indirect lines of evidence and localized studies in the field (e.g., Pratt 1992; Rieman and McIntyre 1993, 1995; * Corresponding author: [email protected] Received February 11, 2002; accepted January 8, 2003 Bonneau and Scarnecchia 1996; Buchanan and Gregory 1997; Dunham and Rieman 1999; Zurstadt 2000; Haas 2001; Gamett 2002) and laboratory (Selong et al. 2001). These studies provide insight into the thermal requirements of bull trout, but with the exception of analyses of climatic gradients (Rieman et al. 1997), a comprehensive analysis of the thermal habitat associations of bull trout in the field has not been conducted throughout the species’ range. Of particular interest is the southern margin of the range, where temperature should be most important (e.g., Flebbe 1994). The present southern margin of this range is delineated by the Klamath basin in the west and the headwater tributaries to the upper Snake River downstream of Shoshone Falls in the east (Moyle 1976; Cavendar