Baltic Sea fish put themselves on a diet

Baltic Sea fish put themselves on a diet

Among other things, the researchers focused on flounders in the Baltic Sea. © Sakis Lazarides/iStock

In order to cover their increased food requirements when water temperatures rise, some fish apparently tend to adopt a strategy that is unfavorable to them, according to a study: According to a study, flounders and co. in the Baltic Sea are increasingly grabbing the next best food animals – small ones – instead of their usual “fat” prey and more common organisms. But even many of these tiny creatures ultimately provide less energy than a few large prey animals, which can lead to malnutrition. The researchers say that climate change-related changes in hunting behavior could pose a further threat to fish stocks.

How do rising water temperatures due to climate change affect the aquatic environment? The potentially complex consequences are currently the focus of intensive research. As for fish, it is believed that their metabolism is stimulated by increased temperatures, causing them to use more energy. This in turn must be compensated for by more food intake. Since warmer waters can lead to increased food production, the fish could be able to do this without affecting stocks.

How do fish adapt?

The research team led by Benoit Gauzens from the German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig has now examined in more detail to what extent this is actually true, using Baltic Sea fish as an example. The scientists evaluated archive data from six economically important fish species that were collected in different areas of the Bay of Kiel over a period of ten years. The information included analysis results of the stomach contents of flounder, cod, etc. as well as data on the food animals available locally and the respective water temperatures.

As the team reports, their evaluations showed that despite their different nutritional strategies, there was an overarching trend among the fish species examined. Although the supply has remained the same, as temperatures have increased, they have shifted their prey preference from larger but less common aquatic animals to more readily available food: they have snatched up more common organisms, such as smaller crustaceans, brittle stars, worms and molluscs. The problem with this change in hunting behavior, however, is that the bottom line is that there is less nutritional value, according to the study.

Problematic trend towards less “fat” prey

This means that the changed hunting behavior means that the fish are less able to cover their energy needs in the long term than by eating larger, high-calorie prey. “It is actually assumed that species adapt their search for food so that they absorb as much energy as possible,” says Gauzens. “But our results suggest that fish – and possibly other animals – could respond to climate change in unexpected and inefficient ways,” said the scientist. Ultimately, the discrepancy between the fish’s energy requirements and their actual food intake can lead to critical malnutrition, according to the team’s calculations.

Based on their results, the researchers also modeled how the problematic changes in the way certain species acquire food could in turn affect others and the entire ecosystem. The results indicate that changes in hunting behavior as temperatures rise can lead to widespread species losses and threatening cascade effects in the food web.

“Fish species in the Baltic Sea and elsewhere are exposed to a variety of human influences, for example overfishing or pollution,” says co-author Gregor Kalinkat from the Leibniz Institute for Freshwater Ecology and Inland Fisheries in Berlin. “The impact of more inefficient hunting behavior as warming warms could be another previously overlooked factor that prevents fish stocks from recovering, even if fishing quotas are significantly reduced,” the scientist concludes. In order to understand the significance of this aspect in more detail, the team is now planning further investigations.

Source: German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, specialist article: Nature Climate Change, doi: 10.1038/s41558-024-01946-y

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