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 14-16degreesC, 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 normative 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.