Change in relative abundance of Atlantic salmon and Arctic charr in Veidnes River, Northern Norway: a possible effect of climate change?

Temperature changes affect salmonids across a hierarchy of spatial and temporal scales, so that shifts in thermal river regimes may influence interspecific interactions in sympatric species. In Northern Norway, anadromous populations of Arctic charr ( Salvelinus alpinus ) and Atlantic salmon ( Salmo salar ) coexist, but in recent years landings of Arctic charr have decreased, while those of Atlantic salmon have been stable or even increased. Here we studied relative abundance, habitat use and growth rate of sympatric stream-living juveniles of both species in Veidnes River, where they are the only fish species present. In 1998/2000, juvenile Arctic charr dominated, especially in the upper and colder part of the river. In 2010, however, Atlantic salmon juveniles were now prominent in all habitat types, whereas nearly all Arctic charr were captured in slow-flowing water near the river bank. Summer air temperature has increased in the region during the last decade. Positive correlations between summer temperatures and back-calculated growth rates were documented in both species, but the growth response was significantly higher in Atlantic salmon. Accordingly, we suggest that juvenile Atlantic salmon may benefit from a warmer climate in northernmost Norway, at the expense of the more cold-water-adapted Arctic charr.