Fish that live in rivers, ditches, and streams are better able to withstand warming water than fish in the sea. This is the conclusion of research by ecologist Wilco Verberk of Radboud University. ’It is important not to group freshwater and marine fish together when predicting the consequences of climate-change-driven warming.’
For his study, Verberk compiled data from more than 300 studies covering over 500 fish species. His analyses show that freshwater fish are better able to adapt to changing water temperatures. ’That in itself is not very surprising,’ Verberk explains. ’Water temperatures in rivers and streams fluctuate much more over the seasons than those in the sea. From an evolutionary perspective, it therefore makes sense that freshwater fish are better equipped to cope with temperature changes.’
However, this greater heat tolerance cannot be explained solely by the larger temperature fluctuations in freshwater habitats. ’Freshwater fish perform better than you would expect based on temperature variability alone. This suggests they possess additional traits that give them an advantage over marine fish.’
Fisheries
Verberk has not yet identified what these additional traits are, but he does have some hypotheses. ’It may be related to oxygen metabolism,’ he says. ’In freshwater environments, not only temperature, but also oxygen availability is more variable than in saltwater. We know that fish that tolerate low oxygen levels are often also better able to cope with higher temperatures.’
Overall, the research suggests that marine fish may be more vulnerable to climate change than freshwater species. Moreover, temperature tolerance is only part of the story; exposure also plays a key role. ’Many people depend on fish as a source of protein,’ Verberk notes. ’Our research shows that laboratory measurements closely match the temperatures fish experience across their natural ranges. As a result, global warming is likely to shift the distribution of many species, posing a threat to food security.’