Local‐Habitat, Watershed, and Biotic Features Associated with Bull Trout Occurrence in Montana Streams

Abstract

AbstractWe evaluated the association of local‐habitat features, large‐scale watershed factors, the presence of nonnative brook trout Salvelinus fontinalis, and connectivity to neighboring populations 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 support 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 and negatively associated with channel gradient and the presence of brook trout. Our findings suggest that bull trout have increased resistance to invasion by brook trout in streams with high habitat complexity and connectivity. Consideration of abiotic and biotic factors at multiple scales, along with a means for ranking their relative importance, is needed to perform more comprehensive assessments of landscape and local influences on species distribution patterns.