Local habitat, watershed, and biotic features associated with bull trout occurrence in Montana streams

Abstract.—We evaluated the association of local-habitat features, large-scale watershed factors, the presence of nonnative brook trout Salvelinus fontinalis, and connectivity to neighboring popu-lations with patterns of occurrence of threatened bull trout S. confluentus in 112 first-order to fourth-order streams in the Bitterroot River drainage in western Montana. Species presence or absence was estimated via single-pass electrofishing, local-habitat features were measured in 500-m sampling reaches, watershed variables were obtained from topographic maps, and potential demographic sup-port from nearby bull trout populations occupying larger main-stem streams was estimated from electrofishing data records. We defined a set of nine candidate models that represented various combinations of these four main factors and used an information-theoretic approach to evaluate the relative plausibility of competing models. Models combining local habitat (width, gradient, and woody debris) with brook trout presence and the main-stem abundance of bull trout and a global model (all variables) were the best approximating models. In contrast, watershed models based on elevation, basin area, and tributary slope and models with local-habitat or biotic variables alone were poor predictors of bull trout occurrence. Bull trout occurrence was positively associated with channel width, large woody debris, and the presence of a ‘‘strong’ ’ neighboring main-stem population an