Increased importance of cool-water fish at high latitudes emerges from individual level responses to warming

1. High latitude ecosystems are experiencing the most rapid warming on earth, expected to trigger a diverse array of ecological responses. Climate warming affects the ecophysiology of fish, and fish close to the cold end of their thermal distribution are expected to increase somatic growth from increased temperatures and a prolonged growth season, which in turn affects maturation schedules, reproduction and survival, boosting population growth. Accordingly, fish species living in ecosystems close to their northern range edge should increase in numerical importance and possibly displace cold-water adapted species. 2. We aim to document if and how population level effects of warming mediated by individual level responses to increased temperatures, shift community structure and composition in high latitude ecosystems. 3. We studied 11 cool-water adapted freshwater fish populations in communities dominated by cold-water adapted species to investigate changes in the relative importance of cool-water fish during the last 30 years of rapid warming in high latitude lakes. In addition, we studied the individual level responses to warming to clarify the potential mechanisms underlying the population effects. 4. Our long-term series‘ (1991-2020) reveal a marked increase in numerical importance of the cool-water fish species, perch, in ten out of eleven populations, and in most fish communities the cool-water species is now dominant. Moreover, we show that climate warming affects population level processes via direct and indirect temperature effects on the individuals. Specifically, the increase in abundance arises from increased survival of 0+ individuals, faster juvenile growth and ensuing earlier maturation, all boosted by climate warming. 5. The speed and magnitude of the response to warming in these high latitude fish communities strongly suggest that cold-water fish will be displaced by fish adapted to warmer water. Consequently, management should focus on climate adaptation limiting future introductions and invasions of cool-water fish and mitigating harvesting pressure on cold-water fish.