Mean daily river temperature of the Coppermine River, Nunavut, during the ice-free season 2018-2023

Shifts in migration timing in response to climate change have implications for fitness and survival of cold-adapted species. We used acoustic telemetry to investigate and compare environmental influences on freshwater return between sympatric anadromous Arctic Char (Salvelinus alpinus) and Dolly Varden (Salvelinus malma malma), and between overwintering locations (above or below an obstacle – a substantial cascade). Both Arctic Char and Dolly Varden that overwintered below the cascade returned to freshwater when sea surface temperature was warm and river temperature was cool, whereas Dolly Varden that overwintered above the cascade returned to freshwater at cooler sea surface temperatures and during warm and likely stressful river temperatures. Ascension of the cascade by Dolly Varden (no Arctic Char were observed to ascend the cascade) was likely assisted by high tide but unaffected by other environmental factors, including temperature. Fish demonstrated plasticity in response to inter-annual variation in environmental conditions, but may experience decoupling of environmental cues and suitable migratory conditions, which could impact persistence of these ecologically and culturally important species.