Strong Spatial Gradients in Habitat Suitability for Salmonids in a Proglacial River

Climate change is causing the rapid retreat of glaciers, leading to the formation of new lakes and rivers as well as the evolution of downstream floodplains that could provide new habitat for species such as Pacific salmon ( Oncorhynchus spp.). However, these habitats downstream of glaciers may initially have hyper‐cold temperatures, elevated turbidity and high rates of sediment movement that could preclude successful spawning and rearing by salmonids. There is currently great uncertainty about the environmental conditions needed for proglacial habitats to become suitable for salmonids and support potential local range expansion. Here we examined the environmental thresholds facilitating potential range expansion of important fishes into proglacial rivers by quantifying fish‐habitat associations in the Taaltsux̱éi Héen (Tulsequah River), a lake‐headed sub‐catchment of the transboundary T'aakú Héeni (Taku River), British Columbia, Canada.

To quantify associations between habitat and salmonid occurrence, we sampled 137 sites for fish communities and water quality along the Taaltsux̱éi Héen over 3 years (2022–2024). The study focused on three species of salmonids: Dolly Varden, juvenile sockeye salmon and juvenile coho salmon.

We discovered strong spatial patterns of water warming and clearing as distance downstream from the glacier increased. Off‐channel habitats in the complex floodplain provided salmonids with thermal refuges from the colder mainstem habitats. Dolly Varden and juvenile salmon occurrence were less likely in the coldest habitats and more likely in intermediate levels of turbidity, but environmental thresholds differed across species. Specifically, juvenile sockeye and coho salmon had > 20% probability of occurrence at 3.9°C and 5.4°C, respectively, whereas Dolly Varden were observed across the entire range of temperatures (0.9°C–14.6°C).

Collectively, our results reveal strong spatial gradients in habitat suitability that predict the probability of salmon presence in the Taaltsux̱éi Héen. As glaciers retreat and the ecological succession of downstream ecosystems advances, glacial rivers and lakes will represent opportunities for the local range expansion of important fishes such as Pacific salmon.

Some proglacial habitats are threatened by industrial pressures such as mining and the biological productivity of these systems is often underappreciated. Therefore, effective proactive conservation will entail not just protecting current habitat, but also future habitats.