Fine‐Scale Factors Affecting the Abundance and Thermal Sensitivity of Nonnative Brook Trout in Coldwater Rocky Mountain Streams

The Brook Trout ( Salvelinus fontinalis ) is a globally distributed nonnative species that threatens native salmonids in coldwater streams. Understanding the factors regulating Brook Trout densities in coldwater refugia is critical to effective management, so we investigated (a) fine‐scale drivers of Brook Trout abundance and (b) factors affecting their thermal sensitivity in 49 coldwater streams in Idaho. Among the 14 candidate variables evaluated, Brook Trout density was most closely associated with mean August water temperature (dome‐shaped response peaking at 11.0°C–11.1°C), wetted width (negative association), gradient (negative association), run habitat (positive association), and substrate size (negative association). Simulations indicated that densities were most sensitive to climate warming in low‐velocity habitats, with the largest increases occurring in areas with baseline temperatures of 8.9°C and largest decreases in areas with temperatures of 12.2°C. We discuss our results in the context of Brook Trout behavior, effects on native salmonids, climate warming, and habitat restoration.