Abstract The diet of Arctic char, Salvelinus alpinus (L.), and brown trout, Salmo trutta L., was studied in a large Norwegian reservoir over the first four decades (1954–1995) of regulation. Significant changes in diet occurred concurrent with changes in fish density and prey availability. Reduced biomass of fish due to the effect of hydropower development probably led to reduced inter- and intra-specific competition. Consumption of zoobenthos increased for both species during the first decade, fish then became an important food item concurrent with an increase in small-sized Arctic char in the littoral zone. The diet of Arctic char, but not brown trout, then changed radically as the introduced mysid, Mysis relicta Lovén, became an important food item.