The Cold Hard Facts: Developing a Thermal Niche Definition for Juvenile Bull Trout

Bull Trout (Salvelinus confluentus) populations in some areas within the species’ range will decline as annual air temperatures increase and coldwater habitats shrink. However, future trends depend on local interactions between species ecology and local habitat thresholds. The Bull Trout thermal niche has most often been defined based on patterns in relatively warm streams at the southern extent of the species’ range. Similar to most other ectotherms, however, there may be cold environments that are beyond the optimal range of this species. Using temperature and biological datasets drawn from the northern and southern extent of the Bull Trout range in North America, we developed logistic regression curves to compare thermal niches occupied by juveniles at opposite ends of the range. Northern populations occupy a colder thermal niche than their southern counterparts; however, these data also suggest that some streams may be too cold to sustain populations. A latitudinal comparison to thermal niches occupied by allopatric Dolly Varden revealed another downward shift, further supporting the notion that lower thermal thresholds limit the northern distribution of Bull Trout. Future work will use data from northern populations to develop spatially explicit predictions of how populations may respond under different climate scenarios.