Fish are shrinking not only due to intensive fishing but also because of evolutionary responses to climate change, according to research by scientists from Poland and Australia published in Science.
Professor Jan Kozłowski of Jagiellonian University, the study’s lead author, and his team developed a model that explains why fish are maturing at smaller sizes and predicts future effects on fisheries. The model shows that fish “decide” how to allocate energy between growth and reproduction.
‘Growth is an investment in the future. If fish die more often, for example, due to fishing or higher water temperatures, this investment is no longer profitable,’ Kozłowski said in an interview with the Polish Press Agency. He added that under such conditions, it is better for fish to reach maturity earlier and devote more energy to reproduction.
As a result, successive generations mature faster and remain smaller. ‘Any increase in mortality works in the same direction – it promotes smaller body size. This applies to both fishing and water warming, as well as other factors such as environmental pollution,’ the researcher explained.
The study demonstrates that evolution, previously considered a slow process, can occur rapidly. ‘Evolution can take place over a few or a dozen generations if environmental conditions change,’ Kozłowski said.
Previous fisheries models ignored evolutionary effects, relying on simplified assumptions about growth and egg production. ‘We introduced evolution into fisheries models and showed that it cannot be ignored,’ Kozłowski said. The new model, tested on data from more than 2,000 fish species, accurately reproduces growth and maturity patterns and highlights the reproductive importance of large females.
‘One 10 kg female will lay significantly more eggs than ten 1 kg females,’ he noted. Accounting for evolutionary processes alters fisheries forecasts: by 2100, catches of 43 key species could drop about 22 percent if size reduction is considered, compared with a 14 percent decline without the factor.
The study warns that overfishing and warming oceans are exacerbating the problem. FAO data for 2025 show that 35 percent of fisheries are overexploited. Baltic cod, for example, decreased in maximum length from 115 to 54 cm and in maturation size from 40 to 20 cm between 1996 and 2019.
‘If changes accelerate, the situation could become even worse,’ Kozłowski said. The decline in fish stocks threatens both the economy and global food security. ‘For hundreds of millions of people, fish are a primary source of protein. It is not a matter of luxury,’ he emphasised.
Scientists call for changes in fisheries management, including better protection of large individuals. ‘The best solution is the development of large marine reserves where fish can grow to large sizes and reproduce. Their offspring can then feed the areas where fishing is conducted,’ Kozłowski said.
He noted that the reversibility of these evolutionary changes depends on maintaining genetic diversity. ‘As long as genes exist that allow to return to the previous strategy, the process is reversible. Rapid evolutionary changes involve changes in gene frequency, not gene disappearance,’ he explained. He added that if certain gene variants are lost, returning to larger sizes will be very difficult. ‘The emergence of new genes is a rare phenomenon. Therefore, it is better to prevent than to try to reverse the effects of changes,’ Kozłowski said.
The researchers stressed the importance of reducing fishing pressure, protecting large fish, establishing marine protected areas, and slowing climate change. ‘If changes occur more slowly, nature has a better chance of coping with them,’ Kozłowski concluded. (PAP)
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