Interacting effects of temperature and density on individual growth performance in a wild population of brown trout

Growth is a key life-history trait linked to population regulation in fishes and may be influenced by biotic and abiotic factors such as density and temperature. Exploring how growth performance is altered by such factors in the wild will aid our understanding of how climate change might influence fish populations. We explore the interactions between temperature and density on growth in a stream-resident brown trout (Salmo trutta) population by comparing observed individual growth rates with predicted rates, at maximum rations, as a function of natural variation in water temperature in a small, cold (average temp summer <11 degrees C) stream in south-east Norway. Variation in relative growth performance of resident brown trout was analysed using a linear mixed-model approach based on a 9-year-long time series of mark-recapture data that yielded 1043 individual growth rate estimates for the summer seasons. Observed growth rates never exceeded 60% of predicted growth. Density and temperature interacted in a non-additive and complex way as controlling agents of growth performance, where a general positive effect of temperature minimised an apparent negative effect of density. We also found an interaction between age and density, where young fish were more negatively affected by density than older fish. Individuals that were small for their age showed evidence of compensatory growth. As our system appears to be strongly resource limited and temperature seems to facilitate relative growth performance, we argue that the negative density effect is mitigated by increased food supply when temperature increases during the summer growth season. Further, the positive effect of temperature on growth appeared minimal at low densities, suggesting an unmeasured factor (e.g. food quality) was limiting some of the growth potential. Our results help elucidate potential effects of temperature changes on brown trout in a small and cold stream, where the positive influence of temperature is more pronounced at high fish densities.