Diel variation in activity and feeding in sympatric brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus) throughout the ice-free season in a Norwegian lake

Most animals are active during distinct diel periods. Salmonid fishes may shift from being mostly diurnal to being nocturnal in autumn and winter. As visual foragers, diurnal variation in prey availability and predation risk may drive variation in their activity pattern. In an oligotrophic lake, we studied diel activity and feeding of brown trout (Salmo trutta) and Arctic char (Salvelinus alpinus) using gill nets in the epibenthic habitat from May through October. Brown trout demonstrate an overall crepuscular activity pattern, whereas Arctic char demonstrate a more complex diel activity pattern. The variation in activity reflected the variation in stomach content in both species, with highest stomach-content mass in individuals sampled during night. Diet overlap of brown trout and Arctic char was high in early spring and reduced thereafter. Our results characterize both brown trout and Arctic char as indiscriminate particulate feeders and neither species had a diel change in their feeding mode.