A Review of Bull Trout Habitat Associations and Exploratory Analyses of Patterns across the Interior Columbia River Basin

Abstract

An understanding of habitat relationships for bull trout Salvelinus confluentus remains an important component for identifying future restoration, management, and recovery efforts. We examined past efforts through a comprehensive synthesis of peer-reviewed articles evaluating bull trout habitat relationships, and we used field data within classification tree analysis (CTA) to improve our understanding of the consistency of bull trout habitat use patterns. We performed CTA using reach-level habitat data collected from currently occupied stream networks (i.e., those within the current distribution of bull trout) and unoccupied stream networks where hierarchical filters (i.e., area and temperature) were met and from occupied areas where temperature criteria were exceeded. Results from the literature review demonstrated consistent results at the microhabitat and channel unit scales; indicated the importance of slow-velocity, deeper habitats; and, together with observed diel shifts, highlighted the importance of complex habitat regardless of scale or season. At the reach scale, however, our results indicated substantially less consistency in bull trout habitat relationships. Including maximum stream temperature into the CTA did not result in changes to the overall structure of the CTA; results from the CTA indicated that bull trout were found in reaches with larger substrate, deeper pools, and more cover, but the specific criteria differed along a gradient of stream depth. Our results indicated (1) important gaps in our knowledge regarding the role of substrate size in juvenile bull trout habitat use, (2) the need for elucidation of habitat use patterns in downstream reaches, which may act as critical overwintering habitat or migratory corridors, and (3) the need to incorporate sampling efficiencies in future bull trout habitat evaluations.