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Coral Bleaching Causes Fish To Make Poor Decisions

Coral bleaching.
Credit: Naja Bertolt Jensen on Unsplash.

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A new research study has found that coral bleaching compromises the ability of butterflyfish to identify competitor species. The work is published in the Proceedings of the Royal Society B.

Exploring the impact of coral bleaching

Across the world’s oceans, mass coral bleaching events are becoming increasingly common. Dr. Sally Keith, a senior lecturer in marine biology at Lancaster University, is the lead author of a new study that explores how such events could impact fish survival.


What is coral bleaching?

You may have encountered before and after shots of coral reefs, where the once bright and vibrant landscapes have evolved to a dull shade of white. The colors seen in coral reefs are caused by the presence of microscopic algae known as zooxanthellae. When coral is stressed – perhaps due to an increased water temperature – it expels its microscopic algae. This is coral bleaching.


The research team studied reefs across five Indo-Pacific regions, obtaining over 3,700 observations of butterflyfish to compare their behavior before and after coral bleaching. “Using a mass coral mortality event as a natural experiment and 3,770 field observations of butterflyfish encounters, we test how rapid resource depletion could disrupt recognition processes in butterflyfish,” the authors write.

Bleaching leads to poor, energy-expending decisions

Keith and colleagues discovered that butterflyfish’s ability to recognize competitor species and respond appropriately was adversely impacted by bleaching. Should mass coral bleaching continue to occur as global warming increases, it’s possible that the subsequent behavior changes could have implications for the species’ survival. “By recognizing a competitor, individual fish can make decisions about whether to escalate, or retreat from, a contest – conserving valuable energy and avoiding injuries,” Keith says.


She adds, “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 butterflyfish. It’s not yet clear whether these fish have the capacity to update their rule book fast enough to recalibrate their decisions.”


After coral bleaching, signaling between fish of different species was reduced, and encounters between such species escalated to chases in over 90% of case studies, an increase from 72% before bleaching. Fish chase one another for several reasons, such as territory defence, mating, competing for food and establishing their dominance. The team found that the distance of the chases also increased post-bleaching, requiring the fish to expend more energy.

Why is coral bleaching changing fish behavior?

Keith and colleagues hypothesize that coral bleaching is causing fish species to switch up their diets and territories in order to survive. Consequentially, long-established co-evolved relationships that enable fish species to co-exist are being affected.  


“By looking at how behaviour responds to real-life changes in the environment, and by seeing that those changes are the same regardless of location, we can start to predict how ecological communities might change into the future. These relatively small miscalculations in where to best invest energy could ultimately push them over the edge,” Keith explains.


Reference: Keith S, Boström-Einarsson L, Hartley I, et al. Rapid resource depletion on coral reefs disrupts competitor recognition processes among butterflyfish species. Proceedings of the Royal Society B. 2023. doi:10.1098/rspb.2022.2158.


This article is a rework of a press release issued by Lancaster University. Material has been edited for length and content.

Meet the Author
Molly Campbell
Molly Campbell
Senior Science Writer
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