Stronger effect of temperature on body growth in cool than in warm populations suggests lack of local adaptation

Body size is a key functional trait that has declined in many biological communities, partly due to changes in individual growth rates in response to climate warming. However, our understanding of growth responses in natural populations is limited by relatively short time series without large temperature contrasts and unknown levels of adaptation to local temperatures across populations within species. In this study, we collated back‐calculated length‐at‐age data for the fish Eurasian perch Perca fluviatilis from 10 populations along the Baltic Sea coast between 1953 and 2015 (142 023 length‐at‐age measurements). We fitted individual growth trajectories using the von Bertalanffy growth equation, and reconstructed local temperature time series using generalized linear mixed models fitted to three data sources. Leveraging a uniquely large temperature contrast due to climate change and artificial heating from nuclear power plants in two of the examined populations, we then estimated population‐specific and global (across populations) growth–temperature relationships using Bayesian mixed models, and evaluated whether populations are locally adapted to environmental temperatures. We found little evidence for local adaptation of body growth. Populations did not exhibit unique optimum growth temperatures nor unique growth rates at a common reference temperature. Instead, population‐specific curves mapped onto a global curve, resulting in body growth increasing with warming in cold populations but decreasing in one of the warmer populations. Understanding whether the effects of warming on growth are population‐specific is critical for generalizing predictions of climate impacts on body size, which affects multiple levels of biological organization from individuals to ecosystem functioning.