Coral bleaching threatens the diversity of reef fish

A new study has revealed that global warming is affecting not just coral organisms, but many fish species that inhabit coral reefs.

It has long been known that global warming is responsible for coral bleaching events across the globe.  Although ocean acidification is one of the primary threats to coral reefs, the increase in water temperature associated with climate change has too proved to have devastating effects on coral organisms.

Heat stress on the coral polyps that make up coral reefs results in the expulsion of the symbiotic algae that gives them their beautiful colours. This expulsion leaves the coral white in appearance, a process known as coral bleaching.

New international research led by PhD student Laura Richardson of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, has examined 16 reefs off Lizard Island, in the northern section of the Great Barrier Reef. The quantity and types of coral and fish species were surveyed before, during and after the 2016 mass bleaching event caused by a global heatwave.

The results revealed that coral bleaching events not only bleach these corals, but can also reduce the variety of fish occupying these highly-valued ecosystems.

We are learning that some corals are more sensitive to heat-stress than others, but reef fishes also vary in their response to these disturbances,” said lead author Ms Richardson.

Fish assemblages are significantly impacted by loss of coral cover as a result of bleaching events, and some fishes are more sensitive than others,” said co-author Prof Nick Graham of Lancaster University.

The loss of corals affected some types of fish more than others. Following the bleaching event, researchers recorded a sharp drop in the diversity of fish communities as the mix or species changed.

Fish that are highly dependent on branching corals, such as butterflyfish, declined the most.


Rainford’s Butterflyfish, Chaetodon rainfordi, at Lizard Island, Great Barrier Reef, Queensland. Source: Mark Sheperd / Lizard Island Field Guide, License: CC By Attribution

The results revealed increased taxonomic and functional similarity of previously distinct reef fish assemblages following mass coral bleaching, with changes characterized by subtle, but significant, shifts toward predominance of small‐bodied, algal‐farming habitat generalists.

Prior to the 2016 mass bleaching event, we observed significant variation in the number of fish species, total fish abundance and functional diversity among different fish communities. Six months after the bleaching event, however, this variation was almost entirely lost,” said co-author Dr Andrew Hoey of ARC Centre of Excellence for Coral Reef Studies at James Cook University.

Also known as ‘biotic homogenisation,’ this tendency towards individual and community similarity is increasingly considered one of the most pressing, but largely unrecognised, biodiversity crises faced globally.”


Some species decline and others survive 
severe bleaching events. Credit:
 ARC CoE for Coral Reef Studies

Laura Richardson


Header Photo credit:

Extensive bleaching of hard and soft corals at Moore Reef following sustained heat stress in March 2017 on Great Barrier Reef.

ARC CoE for Coral Reef Studies/ Ciemon Caballes

Article reference: 

Richardson, L. E., Graham, N.A.J., Pratchett, M.S., Eurich, J.G., and Hoey, A.S. Mass coral bleaching causes biotic homogenization of reef fish assemblagesGlobal Change Biology, 2018 DOI: 10.1111/gcb.14119

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