Competition among juvenile brown trout, grayling, and landlocked Atlantic salmon in flumes — predicting effects of interspecific interactions on salmon reintroduction success

Abstract

Worldwide declines in salmonid populations have generated major interest in conservation and restoration of wild populations and riverine habitats. Species reintroductions to previous habitats raise questions about their potential impact on these systems. In River Klarälven, landlocked Atlantic salmon (Salmo salar) have been extinct from upper reaches for over 50 years due to hydropower dams. Here we study competitive interactions among juvenile salmon, grayling (Thymallus thymallus), and brown trout (Salmo trutta) that occur in the upper reaches of the river. We examine foraging rates, aggression, and activity of juvenile fish in allopatry at three different densities and in sympatry with one or both potential competitors in laboratory flumes. Salmon captured prey less frequently in the presence of brown trout and grayling, whereas grayling and brown trout were unaffected by salmon, but affected each other. Grayling was the most aggressive and active species, whereas salmon the least. Consequently, reintroduction of salmon probably will have little impact on grayling and brown trout, whereas grayling and brown trout could affect the success of reintroducing salmon.