| Summary: | Previous studies suggest that global warming threatens populations of Arctic char (Salvelinus alpinus L.) and that interspecific competition from brown trout (Salmo trutta L.) can have negative effects when the species occur in sympatry. The aim of this study was to investigate if populations of Arctic char have declined in the last decades in lakes where they in the past have coexisted with brown trout. I examined this by revisiting five lakes previously sampled in the mountain region of Västerbotten (Sweden) during the 1960´s. This study repeated earlier sampling methods and examined the population size, biomasses and individual size and weight of all fish. Results showed increased abundances of brown trout and a trend towards declining Arctic char populations in the five lakes. Both Arctic char and brown trout currently had higher mean lengths than previous sampling years. This study also examined the diet and growth rate of both species. The diet analysis showed that the species to some extent overlapped in diet, while they also showed clear differences, where Arctic charr e.g. consumed more pelagic zooplankton. The growth rate was higher for Arctic char and differed between lakes. The shift towards more brown trout dominated lakes could depend on both that the two species are differently affected by a warmer climate, and that fishing habits have changed since the 1960´s. A warmer climate likely changed the species interaction in favor of trout and could lead to more competition and predation on Arctic char.
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