Climate change: Baltic Sea fish switch to lean diets when water temperatures rise

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The flounder, one of the species analyzed

Photo: H. Frei / imagebroker / IMAGO

When the water gets warmer, fish change their hunting strategy: from their preferred prey to more common but smaller food animals.

In times of climate change, this involuntary diet could contribute to the extinction of fish, according to a study published in the journal Nature Climate Change.

Like many other animals, fish require more food when temperatures rise because their metabolism increases.

Experts actually suspect that species adapt their search for food so that they absorb as much energy as possible.

"But our results suggest that fish - and other animals - could react to climate change in unexpected and inefficient ways," says Benoit Gauzens from the German Center for Integrative Biodiversity Research and the University of Jena, who is the first author of the study.

Findings also apply to other animal groups

The team analyzed data on the stomach contents of six economically important fish species, all caught in the Bay of Kiel, including flounder and cod.

The data was collected year-round from 1968 to 1978 and provides information about what prey the fish found in their area at different temperatures.

When temperatures rose, the fish switched from rarer food to more common prey.

Small crustaceans, brittle stars, worms and molluscs serve as quickly available energy sources.

The findings are important in the context of climate change, as a result of which the oceans are continuing to warm.

Model calculations for which the researchers used the data now show: In the long term, the fish are less able to cover their energy needs than with larger, higher-calorie prey.

The fish's adapted diet could therefore contribute to increased species extinction.

The fish starve because they don't absorb enough energy.

According to the researchers, the model could also be applied to other groups of animals: animals that adapt their hunting behavior in similar ways could be more vulnerable to climate change.

"Adapting food acquisition to local conditions is usually an important factor in maintaining a high level of biodiversity," says Gauzens in a statement.

"It is therefore surprising that this may not be the case in the context of higher temperatures."

The researchers now want to verify the findings from the model calculations through observations in natural ecosystems.

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