Anticipated climate warming effects on bull trout habitats and populations across the interior Columbia River basin

A warming climate could profoundly affect the distribution and abundance of many fishes. Bull trout Salvelinus confluentus may be especially vulnerable to climate change given that spawning and early rearing are constrained by cold water temperatures creating a patchwork of natal headwater habitats across river networks. Because the size and connectivity of patches also appear to influence the persistence of local populations, climate warming could lead to increasing fragmentation of remaining habitats and accelerated decline of this species. We modeled the relationships between (1) the lower elevation limits of small bull trout and mean annual air temperature and (2) latitude and longitude across the species' potential range within the interior Columbia River basin of the USA. We used our results to explore the implications of the climate warming expected in the next 50 or more years. We found a strong association between the lower elevation limits of bull trout distributions and longitude and latitude; this association was consistent with the patterns in mean annual air temperature. We concluded that climate does strongly influence regional and local bull trout distributions, and we estimated bull trout habitat response to a range of predicted climate warming effects. Warming over the range predicted could result in losses of 18-92% of thermally suitable natal habitat area and 27-99% of large (>10,000-ha) habitat patches, which suggests that population impacts may be disproportionate to the simple loss of habitat area. The predicted changes were not uniform across the species' range, and some populations appear to face higher risks than others. These results could provide a foundation for regional prioritization in conservation management, although more detailed models are needed to prioritize actions at local scales.