The way that fish interact in groups is being upset by ocean acidification and global warming.

“Fish show gregarious behaviour and cluster in shoals which helps them to acquire food and for protection against predators,” says Professor Ivan Nagelkerken from the University of Adelaide.

In a new study, researchers evaluated how species interact and behave amid changing water temperature and acidification, in a laboratory setting.

The rising concentration of carbon dioxide in the atmosphere is driving up ocean surface temperatures and causing ocean acidification. Although warming and acidification are different phenomena, they interact to the detriment of marine ecosystems.

“Many gregarious tropical species are shifting poleward under current ocean warming and interacting in new ways with fish in more temperate areas,” Dr Nagelkerken says. 

“We found that tropical and temperate fish species tend to move to the right when coordinating together in a shoal especially when spooked by a predator, but this bias significantly diminished under ocean acidification,” says University of Adelaide PhD student Angus Mitchell.

“Mixed shoals of tropical and temperate species became less cohesive under future climate conditions and showed slower escape responses from potential threats.”

The experts say their findings highlight the direct effect of climate stressors on fish behaviour and the interplay with the indirect effects of new species interactions. 

“Strong shoal cohesion and coordinated movement affect the survival of a species: whether to acquire food or evade predators,” said Professor Nagelkerken.

“If the ability for fish to work together is detrimentally affected it could determine the survival of particular species in the oceans of the future. Tropical species may initially fare poorly when moving into new temperate areas.”

The full study is accessible in the journal Global Change Biology.