Population-level variation in juvenile brown trout growth from different climatic regions of Norway to an experimental thermal gradient (vol 99, pg 1009, 2016)

Climate-change scenarios predict increasing temperatures and more precipitation at high latitudes. Ectothermic species are highly affected by these environmental variables and due to few dispersal opportunities many populations will need to adapt to these environmental changes. Understanding if, where, and how such adaptation processes occur is important for our understanding of the possible impacts of a changing climate. Individual growth, a key life-history trait influencing population-level parameters is directly affected by temperature especially in ectotherms. Thermal adaptations that optimize growth are therefore expected in such organisms. However, knowledge about how ectothermic animals modify growth rate in the face of climate change is poor at best for many species especially at the local population level. Here, we present a common-garden experiment exploring variations in growth reaction norms for three populations of Salmo trutta (a temperate freshwater fish) over three discrete temperatures. The populations originated from different climatic regions of Norway that vary in temperature and precipitation. Thermal growth reaction norms varied among populations, however we found no convincing evidence for either local thermal adaptations or countergradient adaptations. Rather, the population variation tended to correlate with a variable indicating east vs west climate region, that is strongly associated with a gradient in precipitation in Norway. This results suggests precipitation levels with corresponding flow regimes to have a stronger selection potential for early juvenile growth compared to temperature in these systems.