Heidi Swanson

Arctic char ( Salvelinus alpinus ) is a facultatively anadromous fish species that is critically important to many Inuit communities in the Canadian Arctic. Plasticity in life history has allowed the species to persist in a diversity of challenging Holarctic environments. Despite their ecological and cultural importance and their presence in aquatic ecosystems that are ice‐covered for much of the year, few under‐ice studies of Arctic char have been conducted. Most winter studies of adult Arctic char have focused on lakes, where they typically overwinter. Several populations of Arctic char, however, overwinter in large river systems, and subsistence fishers have reported that Arctic char overwinter in the lower reaches of the Coppermine River. The Coppermine River is a large Arctic river that flows into Coronation Gulf near Kugluktuk, Nunavut, Canada. The authors used acoustic telemetry to investigate the overwintering ecology of Arctic char in the region. Consistent with local knowledge, they detected Arctic char overwintering within the fluvial environment of the Coppermine River from 2018 to 2020. Unlike other fluvial environments known to be used by overwintering Arctic char, the lower reaches of the Coppermine River are completely ice‐covered throughout the winter, are of moderate depths (3.8–14.1 m) and have no known groundwater inputs. Acoustic telemetry observations indicated long‐distance movement (7–8 km) within the river in early winter (October) in response to dynamic ice formation. Under‐ice movement generally declined 2 weeks after river freeze‐up but continued throughout winter in the lower 5 km of the river, where there were fewer under‐ice disturbances. Migration into the marine environment before river ice break‐up (June), as well as winter (November–May) movements into and within the marine environment, was unexpectedly observed for some fish. Under‐ice use of the marine environment is unusual for Arctic char at the distances observed (up to 18 km) and has not previously been documented at the temperatures (fish body temperatures from −0.76 to 1.90°C) observed. Results allow further understanding of the diverse life‐history tactics employed by Arctic char and lay a foundation for future research into fluvial and other diverse overwintering tactics employed by the species.