Ocean acidification disrupts the behavior of fish shoals

The rising concentration of carbon dioxide in the atmosphere is causing major changes in ocean surface temperatures and acidification, to the detriment of an increasing number of marine ecosystems.

A new study led by the University of Adelaide shows that the way fish interact in groups has been significantly modified by both ocean acidification and global warming. In particular, fish shoals become less cohesive and less capable of dodging predators when the water is more acidic.

“Fish show gregarious behavior and cluster in shoals, which helps them to acquire food and protect themselves against predators,” said study leader Ivan Nagelkerken, a professor at the University of Adelaide’s Environment Institute and Southern Seas Ecology Laboratories.

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

The scientists investigated how fish species interacted and behaved when water temperatures and acidification changed under controlled laboratory conditions. When coordinating together in a shoal after being spooked by predators, tropical and temperate fish species usually tend to move to the right. However, this bias significantly diminished in conditions when ocean acidification levels were higher. 

“Mixed shoals of tropical and temperate species became less cohesive under future climate conditions and showed slower escape responses from potential threats,” explained study co-author Angus Mitchell, a doctoral student at the University of Adelaide who performed the laboratory experiments.

These findings suggest that climate stressors such as acidification or warming have a direct effect on fish behavior and the ways in which different species of fish interact.

“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,” he concluded.

The study is published in the journal Global Change Biology.

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By Andrei Ionescu, Earth.com Staff Writer