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

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 relative abundance and
importance, and possibly displace cold-water adapted species.  We
aim to document if and how population-level effects of warming are
mediated by individual-level responses to increased temperatures, shifts
in community structure and composition in high-latitude ecosystems. 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. 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 recruitment, faster juvenile growth and ensuing earlier
maturation, all boosted by climate warming. 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.