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Freshwater fish populations near the poles are outnumbering equatorial populations due to climate change. Warming creates new habitats at the edges of poleward ranges, allowing large-bodied migratory species such as Atlantic salmon to thrive. In a recent study, researchers from the University of Bristol analyzed a dataset that included more than 10 time series and more than 000 species of freshwater fish.
How do fish respond to climate change?
The research sheds light on an important aspect of how animals are responding to climate change, which has caused large-scale changes in the distribution of marine and terrestrial species. However, the responses of freshwater species remain less well understood.
"Understanding how population sizes of freshwater species have responded to climate warming over recent decades is important for predicting how they are likely to be affected by future conditions," the study authors write.
Climate-induced changes in fish habitats
"A common biological response to climate warming in marine and terrestrial habitats is that populations increase on the poleward side of species' ranges as new habitats are formed, and populations decrease on the equatorial side of species' ranges as conditions become too warm," explained the co-author of the study. Professor Martin Genner.
Using a multicontinental database, the researchers linked data on river fish populations from 1958 to 2019 with temperature records. The results showed that water temperatures increased by an average of 0,21°C (0,38°F) per decade across all sampled locations. This warming is consistent with demographic trends predicted by climate models, especially over longer time series spanning more than 30 years.
Species that benefit from warming water
The study found that fish species with specific traits – larger body sizes, higher trophic levels, river-to-sea migratory behavior and widespread distribution – showed population growth in response to climate warming. Examples are brook trout, European bass and northern pike. These positive trends were more pronounced in cooler, high-altitude environments where rising temperatures create favorable conditions.
"These results indicate that projected future warming is likely to lead to large-scale changes in river community structure, including declines at the back end of species' distributions," Professor Genner said.
Looking beyond climate change
Although climate change is a significant factor, researchers seek to assess its relative importance compared to other stressors such as pollution, habitat alteration and fishing. Understanding these interactions may be key to developing effective conservation strategies.
"Our research has shown how warming waters are affecting freshwater river fish, which are both rich in biodiversity and traditionally important to cultures around the world," Professor Genner said. "We hope that shedding light on how climate change may affect freshwater fish will lead to positive actions for their conservation and sustainable use."
Professor Genner added that we need to learn more about how the impacts of climate change can be reduced through conservation strategies such as habitat restoration.
Protecting ecosystems from climate change
This study highlights the urgency of addressing the impacts of climate change on freshwater fish and marine ecosystems. By identifying species and regions most vulnerable to warming, policymakers and conservationists can prioritize actions to conserve biodiversity and preserve the cultural and ecological role of freshwater fish. Habitat restoration and sustainable management practices can play a critical role in mitigating the effects of global warming on these vital ecosystems.
Polar freshwater fish
Polar freshwater fish are uniquely adapted to survive in some of the harshest aquatic environments on Earth. Species such as arctic char, whitefish and grayling thrive in cold waters, often below 4°C, where oxygen levels remain high and competition is relatively low compared to temperate zones. These fish exhibit special physiological properties, such as antifreeze proteins in their blood, that prevent ice crystals from forming and damaging their tissues.
In addition to physiological adaptations, many polar freshwater fish exhibit behavioral features that enhance survival. For example, Arctic char can seasonally switch between freshwater and marine habitats, optimizing food availability and reproductive success. This migratory behavior also makes them key indicators of ecosystem health, as their movements are closely related to water temperature and ice cover.
In addition, polar regions often host fewer species compared to more temperate zones, but these ecosystems are vital for global biodiversity. The limited number of species creates a fragile food web where climate change in fish populations can have cascading effects on predators such as birds and mammals, as well as on human communities that rely on these fish for subsistence and cultural practices. The study is published in the journal Proceedings of the National Academy of Sciences.
