Global warming could lead to the decline of big fish in the oceans

The Peruvian anchovy is the heavyweight of the world’s marine fisheries. In Peruvian waters, this fish dominates the marine ecosystem. But by the end of the century, due to global warming, this species could give way to others, much smaller, according to a study published in review Science, Thursday January 6.

Global warming has two major consequences for marine ecosystems. On the one hand, the heat causes a decrease in the oxygen present in the water. On the other hand, it boosts the metabolism fish, which then need more oxygen. The bigger the fish, the more difficult it is for them to oxygenate the cells located at the heart of their body. In warmer oceans, therefore, the size of fish tends to decrease.

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Faced with this observation, a scientific team made up of German, Canadian, American, Spanish and French researchers proposed two hypotheses: either, faced with global warming, individuals of the same species would see their size decrease; or we would observe a change of community in favor of smaller species. “Until now, it was impossible to determine which of these two hypotheses was valid, explains Arnaud Bertrand, researcher at the Research Institute for Development (IRD) and co-author of the study, because fishing pressure also causes these two phenomena. It is therefore difficult to distinguish between the climate and the fishing. “

Proliferation of small fish

To try to validate one of the two hypotheses, the researchers compared the size of the fish and the communities present over two distinct periods, using the Humboldt Current in Peru as a model ecosystem. This current is currently the most productive in the world and represents about 15% of the global annual fish catch.

Fish vertebrae (anchovies, gobies, juvenile fish) found in marine sediment cores collected off Peru.

Thanks to the bones of fossil fish found in marine sediments, researchers first studied the species present during the last interglacial period, 130,000 to 116,000 years ago. At that time, temperatures and oxygen levels in Peru were similar to those predicted for the turn of the century. Then they analyzed the ecosystems of the Holocene, that is to say of the last 11,700 years. “Conclusion: during the last interglacial period, individuals of the same species were not smaller, explains Arnaud Bertrand. For example, anchovies were the same size as during the Holocene, 12 to 19 centimeters. ”

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Global warming could lead to the decline of big fish in the oceans

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