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 change may place the species at further risk. Climate change may lead to a warming of streams in the summer and an increasing probability of floods in the winter. These changes are projected to lead to widespread loss of habitat for bull trout. This project, supported by the Northwest Climate Science Center, seeks to further elaborate how these climate-related threats influence bull trout across five western states (OR, WA, ID, MT, NV) that form the southern margin of the species’ range.
Researchers used predictions of temperatures in streams across this extent to map coldwater streams or “patches” suitable for spawning and early rearing of bull trout. The study results indicated that larger patches of cold water were much more likely to support the species. The team 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 impacts as measured by the Human Footprint index. In addition to elucidating the importance of local and climate-related threats, this 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. Climate projections based on these models have allowed researchers to identify where the species is most likely to persist in the future. This rangewide view of the species, in conjunction with local assessments of threats that cannot be evaluated at a broad extent, allows more effective prioritization for the conservation of bull trout in a changing climate.