Animal movement in space and time: the role of thermal barriers and individual variation in modulating predator–prey overlap in stratified lakes

Animal movements play a crucial role in ecosystem dynamics, regulating numerous functions from nutrient cycling to habitat coupling. Therefore, any factor modulating animal behavior (e.g. physiology and individual preferences in habitat choice) has the potential to influence ecosystem functioning. By using a small temperate lake as a model system, we investigated habitat selection in brook charr Salvelinus fontinalis to understand how thermal constraints determine its spatial overlap with zooplankton prey. More specifically, we tested that 1) fish will track zooplankton‐rich areas as long as the resource is in a suitable habitat (e.g. depth, temperature), 2) the magnitude of fish–zooplankton spatial overlap will depend on seasonal variations in the permeability of the thermal barrier, and 3) fish individual variability will modulate the selection of resources (i.e. zooplankton) and/or habitat (e.g. epilimnetic zone). Zooplankton was mapped weekly with an underwater vision profiler to interpolate their 3D distribution in the entire lake over 11 weeks, whereas acoustic tags tracked the movements of 21 fish during the same period at 10–14 s intervals. We found that fish mostly selected offshore benthic metalimnetic habitats during summer with some individuals selecting zooplankton‐rich areas. When the thermal barrier disappeared, at the end of the summer, most fish shifted from offshore to littoral areas with an increased selectivity of zooplankton‐rich areas, although some individuals never selected them. Our study demonstrated how habitat selection could seasonally fluctuate according to thermal habitat fragmentation, preventing a top predator from accessing specific habitats. By integrating individual differences in habitat or resource selection, our results help to improve our understanding of mechanisms of habitat selection by stenothermic predators with thermal constraints and also raise questions about the potential effects of climate change on temperate lakes that were only colonized by these fish.