Mass coral bleaching events are making it harder for some species of reef fish to identify competitors, new research reveals.

Scientists studying reefs across five Indo-Pacific regions found that the ability of butterfly fish individuals to identify competitor species and respond appropriately was compromised after widespread loss of coral caused by bleaching.

This change means they make poorer decisions that leave them less able to avoid unnecessary fights, using up precious limited energy. The scientists behind the study believe these changes could have implications for species survival as further global warming increases the likelihood of coral loss.

“By recognizing a competitor, individual fish can make decisions about whether to escalate, or retreat from, a contest—conserving valuable energy and avoiding injuries,” said Sally Keith, a senior lecturer in marine biology at Lancaster University and lead author of the study.

“These rules of engagement evolved for a particular playing field, but that field is changing. Repeated disturbances, such as bleaching events, alter the abundance and identity of corals—the food source of butterfly fish. It’s not yet clear whether these fish have the capacity to update their rule book fast enough to recalibrate their decisions.”

“The impacts of global change on biodiversity are increasingly obvious,” said Nate Sanders, a professor in the University of Michigan Department of Ecology and Evolutionary Biology and a co-author of the study. “This work highlights the importance of studying the behavioral responses of individuals in light of global change.” 

The researchers took more than 3,700 observations of 38 species of butterfly fish on reefs before and after coral bleaching events and compared their behaviors.

After coral mortality caused by the bleaching event, signaling between fish of different species was less common, with encounters escalating to chases in more than 90% of cases—up from 72% before the event. Researchers also found the distance of these chases increased following bleaching, with fish expending more energy chasing away potential competitors than they would have done previously.

The researchers believe the environmental disturbances are affecting fish recognition and responses because the bleaching events, in which many corals die, are forcing fish species to change and diversify their diets and territories. Therefore, these large-scale environmental changes are disrupting long-established and co-evolved relationships that allow multiple fish species to coexist.

“We know that biodiversity is being lost—species are vanishing and populations are declining,” Sanders said. “Perhaps by focusing more on how the behavior of individuals responds to global change, we can start to predict how biodiversity might change in the future. And better yet, try to do something about it.” 

The study is published in Proceedings of the Royal Society B. In addition to Keith and Sanders, authors include Lisa Boström-Einarsson and Ian Hartley of Lancaster University, and Jean-Paul Hobbs of the University of Queensland. The study was funded by the Natural Environment Research Council, the Australian Research Council and the Villum Foundation.