Trophic dynamics of anadromous brown trout and Arctic charr in NW Iceland and their correlation to salmon lice infection

Anadromous Arctic charr Salvelinus alpinus and sea trout Salmo trutta are commonly found in marine, estuarine and freshwater habitats across the Northern Hemisphere. The marine phase varies between populations but often involves summer foraging in nearshore waters while spawning and overwintering occur in freshwater. The dependence of anadromous salmonids on nearshore habitats makes them vulnerable to increased industrial activity, such as aquaculture developments, that may increase the risk of salmon lice Lepeophtheirus salmonis, infection. Lice infection in wild salmonids depends on farmed fish density but can also be correlated to environmental factors, including foraging ecology. In the current paper we examine niche use and trophic variation of co-occurring Arctic charr and sea trout in northwest Iceland, using stomach contents and stable isotope values of δ¹³C, δ¹⁵N and δ³⁴S. We then test if these factors correlated to salmon lice infection. The salmonid species differed in niche use, for example, in time of marine foraging, with Arctic charr having a longer marine phase than was expected, and in the level of piscivory between both species. Moreover, we show that both environmental factors and individual trophic use are related to salmon lice infection in sea trout. This information may be useful in evaluating the risk and spatial management of sea farming.