Abstract

Bull Trout is the most cold-adapted fish in freshwaters of the Pacific Northwest. The species is listed as threatened under the U.S. Endangered Species Act, but climate warming may place the species at further risk. Climate warming may lead to warming of streams in summer and increasing probability of floods in winter, leading to widespread loss of habitat projected for Bull Trout.

This project sought to further elaborate how these climate-related threats influence Bull Trout across five western states (OR, WA, ID, MT, NV). These states form the southern margin of the species’ range. We used predictions of temperatures in streams across approximately two-thirds of this extent to map coldwater streams or “patches” suitable for spawning and early rearing of Bull Trout.

Our results indicate that larger patches of cold water were much more likely to support the species. We also found that Bull Trout were more likely present in patches with extremely cold (<10C or 50F) temperatures in summer (August), fewer floods in winter, and low human influences as measured by the Human Footprint index. In addition to elucidating the importance of local and climate-related threats, our work has identified dozens of places where Bull Trout may exist, but have not yet been detected, as well as other places where Bull Trout have been observed recently, but may be at high risk of local extinction. Future work will focus on completing these analyses across the range of Bull Trout in the conterminous United States.