An ecothermal paradox: bull trout populations diverge in response to thermal landscapes across a broad latitudinal gradient

Abstract

Abstract Maintaining natural thermal regimes in montane stream networks is critical for many species, but as climate warms, thermal regimes will undoubtedly change. Mitigating impacts of changing thermal regimes on freshwater biodiversity requires knowledge of which elements of the thermal regime are limiting factors for aquatic biota. We used full-year stream temperature records sampled across a broad latitudinal gradient to describe the diversity of the thermal landscapes that bull trout (Salvelinus confluentus) occupy and identify potential divergences from thermal regimes where this species has been studied previously. Populations of bull trout occupied stenothermic, cold thermal niches in streams that exhibited low to moderate thermal sensitivity throughout the species’ range. However, winter thermal regimes in the central and northernmost streams were colder and more stable than in the southernmost streams, reflecting differences in sensitivity to air temperature variation and contributions of perennial groundwater to baseflow. In the southernmost streams, bull trout distributions appeared to be regulated by warm summer temperatures, whereas in northern streams, unsuitably cold temperatures may be more limiting. Our results also suggest that local differences in the extent of complete freezing during winter among northern streams may further limit the distributions of suitable habitats. Contrasts in limiting factors at bull trout range extents would suggest differential responses to climate warming wherein northern populations extend their range while southern populations contract, and an overall change in species status that is less dire than previously anticipated.