Coral reef fighting: Study says fish fighting has increased after mass coral bleaching



  • An international team of researchers studied behavioral changes among butterflyfish on a series of coral reefs in the Indo-Pacific before and after the 2016 global coral bleaching event.
  • They found that after the bleaching event, the fish began to behave more aggressively towards each other in their now degraded habitat.
  • The energetic cost of these combat encounters in which fish chase each other away could have implications for the long-term survival of coral reef fish species, the study authors conclude.
  • Given ocean warming rates and predictions for more frequent and intense coral bleaching in the long term, it is unclear whether reef fish will have the ability to adapt their behavior to their rapidly changing habitat.

Now as mass coral bleaching events grow in frequency and intensity, scientists are discovering how corals, which form the physical foundation of coral reefs, respond during times of heat stress. What is less known is how mass bleaching events affect other components of coral reef ecosystems, such as the range of incredibly colorful fish species.

The study , recently published in the Proceedings of the Royal Society B , suggests that coral death due to mass bleaching events is disrupting conflict avoidance behaviors in coral reef fish. Fish living on recently degraded coral reefs are finding it more difficult to respond appropriately to adversaries, researchers say. As a result, they enter into fights and chases more frequently, expending their precious and limited energy reserves and risking their long-term survival.

“By recognizing an opponent, individual fish can make decisions about escalating or retreating from a fight, conserving valuable energy and avoiding injury,” said Sally Keith, a marine biologist at Lancaster University in the United Kingdom and lead author of the study.



Keith and his colleagues studied butterflyfish, a colorful species of coral reef fish , at 17 sites in five Indo-Pacific regions, including Indonesia, Japan, the Philippines and Christmas Island. 

Coral cover declined by 18-65% after bleaching at the study sites, with a group of corals called Acropora particularly damaged. Butterflyfish are particular connoisseurs of Acropora, often creating territories around coral colonies belonging to the group. The loss of so much Acropora in 2016 dealt a severe blow to the butterflyfish, but also allowed researchers to study their response in detail.

Deprived of their preferred food within their territories, butterflyfish generally began to wander further afield to seek alternative food sources, the study says. This led them to encounter a greater number of opponents, usually different species of butterflyfish.

 Under healthy reef conditions, for example, territory-holding butterflyfish typically adopt a “signaling” posture in which they point their noses downward and raise their spiny dorsal fins to ward off intruders.

A bleached coral reef, including colonies of ramified Acropora. Photo by Ocean Agency/Ocean Image Bank.

The researchers say the increase in aggression may be due to the loss of signals in the coral reef environment after bleaching, meaning the fish have difficulty recognizing their opponents and lead to poor decision-making.

Michael Childress, an ecologist with expertise in evolutionary behavior at Clemson University in the US who was not involved in the research, said the study's findings are well supported by a substantial data set. However, he warns against directly tracing changes in fish aggressiveness to an inability to recognize adversaries due to the death of corals.

A more likely and simpler explanation, he says, might be that the dying corals drove fish out of their depleted territory to look for food elsewhere, bringing them into contact with new species and increasing the chances of aggressive encounters. “What really changes after mass coral mortality is the value of an area that is no longer endowed with valuable Acropora corals,” 

A gallon butterflyfish (Chaetodon trifascialis) defends its territory in Acropora. Photo by Sally Keith.

Regardless of the underlying cause, researchers say the energetic cost of increased conflict could have implications for the long-term survival of coral reef fish species. It could also alter long-established relationships that have evolved over millions of years and allow so many different species to coexist in the same reef habitat.

With global warming increasing the likelihood of further bleaching events and coral loss, the next step for researchers will be to find out whether butterflyfish and other species can adapt to their depleted habitats.

“These relatively small miscalculations about how best to invest energy could ultimately push [reef fish species] to the breaking point,” Keith said. “It is not yet clear whether these fish are able to update their behavior quickly enough to change their decisions.”