Climate change: When fish become warmer and warmer

The seas are hotter than ever. Fish in particular suffer from high temperatures. Because even their hunting behavior is changing. The result: they become smaller, and may even become extinct.

About a year ago, ocean temperature records were broken. This can be partly explained by the El Niño phenomenon. Researchers are still faced with the mystery of why temperatures are currently rising in many areas of the ocean.

Many fish in the sea are suffering from climate change. Higher temperatures mean less oxygen in the water. Their metabolism is only partially adapted to high temperatures. Another new observation is that fish appear to change their hunting behavior when exposed to heat stress.

Smaller prey, less energy

A new study published in the journal The nature of climate change It was deployed and examined in the Baltic Sea and the stomach contents of fish were determined. The result: When temperatures rise, fish are more likely to focus on smaller prey.

The researchers didn't expect this, says first author Benoit Goossens of the German Center for Integrative Biodiversity Research in Jena: “This was actually very surprising, because when the temperature rises, the energy requirements of fish tend to increase. So they need more food.” . To cover their energy needs.” Small prey items are less energetically rewarding than large prey items. Therefore, it means expending more effort while gaining less energy to switch to smaller prey.

According to a popular theory, researchers previously assumed that animals choose their food in such a way as to obtain as much energy as possible. Results from the Baltic Sea contradict this assumption. Under climate change, fish choose prey that does not provide them with sufficient energy. Why this is not yet clear.

More layers, smaller fish

There also appears to be another mechanism that affects predatory fish at higher temperatures. When the water at the surface of the ocean heats up, what's called stratification occurs. The colder, denser layers of water at depth rarely mix with the warmer, less dense layers at the surface. This also prevents nutrients from reaching the surface from deep within.

This means that fewer microscopic algae can grow. This means that the so-called zooplankton have less food. Shin-ichi Ito of the University of Tokyo and one of his colleagues examined what this means for fish at the bottom of the food chain in a study published in the scientific journal Fish and fisheries published. Their conclusion: Due to climate change, animals are becoming smaller and lighter.

Endangered

The Tokyo researchers examined this in relation to fish in Japanese waters, but the results are transferable. Layering and switching to less nutritious prey is especially problematic for large predatory fish. In the Baltic Sea, this could have serious consequences, as Benoit Goossens explains: “Large fish species will suffer much more from warming than previously expected and understood. This means that the biomass of the fish will initially decrease before they eventually die.” Outside.”

Many of the larger Baltic Sea species, including popular food fish, are threatened with extinction. And not just because they are being poached. But also because they have changed their hunting behavior due to climate change, says Benoit Gauzens.

Sustainable impacts

This is also the case in other seas, such as those around Japan. Shin-ichi Ito therefore calls for rapid measures: “Even if we stop carbon dioxide emissions, the temperature will continue to rise for at least 50 years. We must therefore deal more closely with the effects of global climate change on the marine ecosystem and in the marine environment.” “. The future offers more stable and sustainable fisheries management.”