Habitat Use and Foraging Activity as Behavioural Indicators of Thermal Stress in Juvenile Brook Trout (Salvelinus fontinalis)

Across North America, Brook Trout (Salvelinus fontinalis) populations are declining in response to rising water temperatures, habitat degradation, and other environmental stressors. Behavioural responses such as changes in habitat use and foraging activity can provide early indicators of thermal stress, yet relatively few studies have examined both behaviours simultaneously in natural stream environments. We analyzed three years of publicly available Remote Underwater Video (RUV) footage collected by the Coalition for the West Credit River (CWCR), together with instream temperature records, to quantify juvenile Brook Trout habitat occupancy (MaxN) and foraging activity (s fish−1) within a coldwater reach of the West Credit River (Ontario, Canada). Occupancy within the monitored habitat increased when water temperatures exceeded commonly reported thermal thresholds for Brook Trout (>20 °C) and peaked during a prolonged summer heat event in 2025. In contrast, foraging activity declined under elevated temperatures and remained consistently low throughout the heat event. Together, these findings suggest that juvenile Brook Trout increasingly relied on the monitored habitat during periods of thermal stress while reducing foraging activity, indicating a behavioural trade-off between energy conservation and energy acquisition.