From polar night to polar day: Distinct seasonal and diel activity patterns in two sympatric Arctic salmonids with divergent life history strategies.

Arctic fishes experience extreme seasonality in light and temperature, with consequences for energy expenditure, movement, and behavior. We examined seasonal and diel activity of sympatric adult anadromous Arctic char (Salvelinus alpinus) and lake trout (S. namaycush) using fine-scale acoustic telemetry positioning systems in two lakes. Arctic char activity was loosely synchronized with seasonal water temperature and day length, decreasing markedly over winter. In contrast, lake trout were 2.7 times more active than Arctic char during winter, with activity positively associated with body size. Both species exhibited diel activity patterns significantly influenced by photoperiod. Lake trout activity peaked strongly at dawn and to a lesser extent at dusk. Arctic char exhibited a minor increase in activity at dawn in one lake. During polar day, activity increased and became arrhythmic for both species. The distinct seasonal patterns exemplify divergent overwintering strategies among high-latitude fish: fasting anadromous Arctic char exhibit hallmarks of “winter dormancy” to prioritize energy conservation, while lake trout remain “winter active” and forage year-round. These contrasting strategies provide an important species-specific behavioural baseline that may help inform conservation and management of Arctic char and lake trout populations facing anthropogenic and environmental changes in Arctic ecosystems.