Behavioural tactics across thermal gradients align with partial morphological divergence in brook charr

This study examines fine-scale thermoregulatory behavior in brook charr (Salvelinus fontinalis) using 3D acoustic telemetry in a dimictic lake. Individuals performed repeated diel forays between warm epilimnetic and cool hypolimnetic strata, with distinct temporal activity peaks around dawn and dusk. The frequency and intensity of epilimnetic excursions varied seasonally, closely tracking surface temperature dynamics. These results reveal flexible thermal foraging tactics that balance energetic gain and thermal constraints, highlighting behavioral plasticity as a mechanism of thermal adaptation in cold-water fish. Full data and scripts are available.