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

Understanding how animals balance environmental constraints is essential for predicting species persistence under climate change. In thermally stratified lakes, cold water fishes such as brook charr (Salvelinus fontinalis) must navigate vertical gradients in temperature and oxygen to optimize foraging while avoiding physiological stress. We hypothesized that individuals would exhibit behavioural tactics that reflect a trade-off between accessing warm surface waters to exploit profitable prey and avoiding thermal stress, with greater constraints expected on epilimnetic use as surface temperatures rise. Using high-resolution acoustic telemetry, we quantified fine-scale patterns of thermal habitat use, vertical foray behaviour and diel timing in a wild brook charr population across the summer stratification period. We also assessed whether behavioural thermoregulation aligned with morphological differences, testing whether divergent behaviours reflect partial ecotypic divergence. As surface temperatures rose, brook charr reduced their use of the warm epilimnion, making fewer and shorter vertical forays. Hypolimnion use increased concurrently but was unrelated to limiting oxygen concentrations, indicating that deep water use was not physiologically constrained and may reflect an alternative foraging behaviour. Epilimnetic forays peaked at dusk and varied with moon phases, consistent with crepuscular visual foraging. Hypolimnetic use peaked at dawn and dusk but showed no response to moonlight. Diel patterns shifted seasonally: in warmer months, epilimnetic access was restricted to twilight hours, while cooler months saw broader surface use throughout the day. Principal component analysis of vertical movement and temperature exposure traits revealed two behavioural tactics: a 'warm' tactic, characterized by frequent epilimnetic forays and warmer average thermal exposure, and a 'cool' tactic, associated with greater hypolimnetic use and cooler average temperatures. These tactics corresponded with partial morphological divergence within the two sexes. Linear discriminant analysis showed that males following the warm tactic were morphologically distinct from cool ones, differing in traits related to feeding and swimming performance, while females showed weaker morphological differentiation. Together our findings reveal repeatable habitat use and consistent thermal tactics that reflect trade-offs between foraging and thermal stress. Vertical gradients in temperature and resource distribution may thus promote fine-scale individual specialization and phenotypic divergence in cold water species facing lake warming.