Spatially‐explicit foraging by an apex predator linked to nearshore prey and their accessibility in lakes

Habitat coupling – where mobile predators forage over broad spatial scales and, in doing so, link food webs from semi‐discrete habitat patches – has emerged as a major structuring force in lake ecosystems. For the cold‐water apex predator lake trout Salvelinus namaycush , food‐web structure and morphometry‐driven accessibility to nearshore areas in summer strongly determine the degree of littoral–pelagic habitat coupling across lakes. Much of the evidence for habitat coupling, however, is based on stable isotopes of carbon to estimate littoral energy acquisition, whereas spatial data directly linking fish movements and foraging behaviour in lakes – on which this theory is based – are limited. Here we estimated nearshore prey abundance at sites of different thermal accessibility and collected stomach content data, which we combined with three‐dimensional acoustic telemetry positioning and acceleration data to directly measure the spatial location of summer foraging movements and habitat coupling by lake trout in lakes with and without an offshore prey fish. Both study lakes contained higher abundances of nearshore prey fish at the most thermally accessible (i.e. steep) sites monitored. Nearshore occupancy accounted for a small proportion of lake trout positions in both lakes (<5%), although prey fish were present in most (72%) diets sampled. High acceleration events indicative of foraging were concentrated in steep, thermally accessible nearshore areas in the lake where offshore forage fish were absent, but located further offshore in the lake with offshore prey fish. We directly demonstrate that habitat coupling by a wild, apex predator is driven by habitat and prey accessibility.