Freshwater fish populations that dwell nearer the poles are outperforming their equatorial counterparts, researchers have found.

Large-bodied migratory species such as Atlantic salmon are thriving as warming temperatures opens up new habitats at the poleward edge of their ranges.

The study, published today in PNAS, was based on a dataset of over 10,000 time series and included over 600 species of fishes.

Climate change has emerged as a key threat to biodiversity, leading to broad-scale shifts in distributions of marine and terrestrial species as they attempt to track thermally suitable habitat. Despite this, the climate responses of freshwater species are comparatively unexplored.

Co-author Professor Martin Genner explained: “A common biological response to climatic warming in marine and terrestrial habitats is that populations are increasing at the poleward side of species ranges, as new environments are generated, and populations are declining at the equatorward side of species ranges, as conditions become too warm.”

The team linked a multicontinental database of riverine fish population abundance time series collected from 1958 to 2019 to temperature data from the same period. Across the sampled localities, waters warmed by 0.21 °C per decade.

They found that observed population trends were consistent with expected patterns from climatic warming, and that the trends were more pronounced in time series covering the longer time periods of over 30 years.

The responses consistent with climate change were most evident in species with larger body sizes, higher trophic levels, river-sea migratory behaviour, and more widespread distributions. Species following this trend include brown trout and coarse fish species familiar to many anglers, such as European perch and northern pike. Moreover, positive abundance responses to warming were more likely at higher altitudes where conditions tend to be cooler.

Professor Genner said: “These findings indicate that projected future warming will likely lead to widespread shifts in riverine community structure, including abundance declines at the trailing edge of species distributions.”

Now the team aims to understand more about the importance of climate change in affecting freshwater fish population sizes relative to other stressors, such as pollution events, habitat modification and fisheries.

He concluded: “Our study has shown how warming waters are impacting freshwater river fishes, that are both rich in biodiversity and are traditionally important to cultures across the world.

“We hope that shining a light on how climate change can affect freshwater fishes will lead to positive actions to enable their conservation and sustainable use.

“We need to learn more about how the impacts of climate change can be lessened through conservation strategies, such as habitat restoration.”



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