Wintertime growth in Atlantic salmon under changing climate: the importance of ice cover for individual growth dynamics

Abstract

Understanding the dynamic nature of individual growth in stream-dwelling salmonids may help forecast consequences of climate change on northern fish populations. Here, we performed an experimental capture–mark–recapture study in Atlantic salmon (Salmo salar) to quantify factors influencing wintertime growth variation among juveniles under different scenarios for ice-cover reduction. We applied multiple imputation to simulate missing size observations for unrecaptured fish and to account for individual-level variation in growth rates. The salmon parr exhibited substantial body length shrinkage in early winter, suppressed growth through midwinter, and increased growth rates in late winter and particularly in spring. Unexpectedly, the presence of ice cover had no direct effects on wintertime growth. Instead, our results implied increasing energetic costs with reducing ice cover; individuals exposed to absent or shortened ice-covered period gained mass at a lowered rate in spring, whereas the present, long ice-covered period was followed by rapid growth. This study emphasizes natural resilience of Atlantic salmon to wintertime environmental variation, which may help the species to cope with the reductions in ice-cover duration due to climate change.