CHARR II Change in relative abundance of Atlantic salmon and Arctic charr in Veidnes River, Northern Norway: a possible effect of climate change? Martin-A. Svenning . Kjetil Sandem . Morten Halvorsen . Øyvind Kanstad-Hanssen . Morten Falkega ˚rd . Reidar Borgstrøm Received: 10 August 2015 / Revised: 25 January 2016 / Accepted: 6 February 2016 Ó Springer International Publishing Switzerland 2016 Abstract 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. Keywords Thermal regime  Arctic rivers  Salmonids  Juvenile growth  Habitat use and competition Introduction Global air temperature, the dominant factor explaining long-term river and lake temperature trends, has increased significantly during the past 100 years, and is predicted to continue increasing (Isaak et al., 2010, 2011). Global warming is occurring faster in northern latitudes than elsewhere (Førland et al., 2009; Tingley & Huybers, 2013), and in Northern Norway air temperature has increased ca. 0.1°C per decade, with the smallest increase in the south-western region, and Guest editors: M. Power, R. Knudsen, C. Adams, M. J. Hansen, J. B. Dempson, M. Jobling & M. Ferguson / Advances in Charr Ecology and Evolution M.-A. Svenning (&)  K. Sandem  M. Falkega ˚rd Arctic Ecology Department (NINA-Tromsø), Fram Center, Norwegian Institute for Nature Research, PO Box 6606, Langnes, 9296 Tromsø, Norway e-mail: martin.svenning@nina.no K. Sandem  R. Borgstrøm Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, PO Box 5003, 1432 A ˚ s, Norway M. Halvorsen Museum Nord, 8445 Melbu, Norway Ø. Kanstad-Hanssen Ferskvannsbiologen AS (Ltd), 8410 Lødingen, Norway 123 Hydrobiologia DOI 10.1007/s10750-016-2690-1