Climatic limits to Atlantic salmon population fitness at continental scales

Anadromous fish populations are declining globally, partly due to acute pressure from rapid environmental change in their freshwater and marine habitats. A more mechanistic understanding of how climatic and land use changes impact their population-level fitness is needed to mitigate these declines. Here we develop a model that successfully captures the thermal niche of the declining Atlantic salmon. This allows us to predict the combined effects of changes in two dominant threats to this species: climate and resource availability. Specifically, the model predicts that a 50% reduction from metabolically optimal resource supply could constrict their thermal niche by ∼7°C. We also show that daily and seasonal temperature fluctuations have a relatively minor impact. A conservative increase of 1.5°C in global temperatures will cause fitness declines for populations in higher climatic regimes, across resource levels. Our results provide new and general insights into factors limiting the distribution of extant Atlantic salmon populations. They also highlight the relative importance of future threats from climatic warming, fluctuations, and changes in resource availability due to land use change.