Conclusion
Our study documents that warming triggers a rapid numerical increase of cool-water fish at their northern range edge. The causal links between individual and population effects of warming considered in this study help to account for the speed and magnitude of the population responses. The magnitude of these responses is such that dominance is shifting from salmonids to percids, warning of an ongoing reorganization of high latitude fish communities. Evidently, water-temperature increase from climate change has already favoured cool-water fish at high latitudes, and future projected climate warming will accentuate this development, potentially at the further expense of cold-water salmonids. Climate adaptation strategies must therefore focus on limiting the ecological impact of warmer water fish in high latitude ecosystems. Consequently, management should focus on climate adaptation limiting future introductions and invasions of cool-water fish and mitigating harvesting pressure on cold-water fish.