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

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.

Establishment of Bull Trout in a previously fishless subalpine lake by translocation

“…2001; Mochnacz et al. 2022), as the closest observations of Bull Trout between the Athabasca and Peace River drainages are separated by over 2000 rkm.…”

“…2001), but Bull Trout occupancy at northern latitudes is also controlled by overwintering temperatures (Mochnacz et al. 2022). Although not directly assessed in connected rivers, the presence of lentic habitat should provide a warmer winter thermal refuge from water in tributaries, which could drop below thermally optimal temperatures or freeze to the bottom.…”

“…Bull Trout from the Wildhay River were believed to be resident or fluvial (not adfluvial) based on the capture location (M. Blackburn, Alberta Environment and Protected Areas, personal communication). Although the linear distance between the source and Marie Lake was 36 km, the two locations are hydrologically separated by approximately 4000 river kilometers (rkm; Figure 1A), the majority of which would likely be a seasonal (summer) thermal barrier to Bull Trout (Selong et al 2001;Mochnacz et al 2022), as the closest observations of Bull Trout between the Athabasca and Peace River drainages are separated by over 2000 rkm.…”

“…Marie Lake was smaller and shallower than most lakes assessed by Galloway et al (2016) but possessed more kilometers of connected stream (~25 km more). Point measurements of daytime temperature taken at Marie Lake and connected tributaries were between 6°C and 15°C, which is optimal for Bull Trout growth (Selong et al 2001), but Bull Trout occupancy at northern latitudes is also controlled by overwintering temperatures (Mochnacz et al 2022). Although not directly assessed in connected rivers, the presence of lentic habitat should provide a warmer winter thermal refuge from water in tributaries, which could drop below thermally optimal temperatures or freeze to the bottom.…”