The trophic niche of Arctic charr in large southern Scandinavian lakes is determined by fish community and lake morphometry

The influence of interspecific interactions and habitat characteristics on the ecological niche of species' populations is a controversial issue in ecology. We explored the niche use of Arctic charr (Salvelinus alpinus L.) in seven relatively large and deep lakes in southern Norway. We aimed to investigate how the presence of potentially competitive fish species, such as whitefish (Coregonus lavaretus L.), perch (Perca fluviatilis L.) and brown trout (Salmo trutta L.), and the lake morphometry affects the littoral reliance and niche breadth of Arctic charr. Stable isotope (delta C-13 and delta N-15) analyses showed that Arctic charr relied less on littoral food resources than brown trout and perch but had largely overlapping niche with whitefish. Arctic charr showed narrower niche use (i.e. less individual variation in delta C-13 and delta N-15) than the other fish species in all seven lakes, particularly in communities where both littoral and pelagic competitors were present. Our results demonstrate that Arctic charr rely much less on littoral resources (< 50%) in large and deep lakes in southern Scandinavia than in small, shallower subarctic lakes, and that interspecific interactions with both littoral and pelagic resource competitors can lead to niche compression of this generalist salmonid fish.