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New research reveals that global warming affects fish, such as the damsel fish pictured here, who depend on corals.

Coral bleaching threatens the diversity of reef fish

The Great Barrier Reef (GBR) is revered for its kaleidoscope of colour.  But research shows that coral bleaching not only whitewashes corals, but can also reduce the variety of fish occupying these highly-valued ecosystems.

Laura Richardson, a marine biologist at the University of Exeter, found that the warming of the oceans which bleaches coral, reduces the number of species living on the reef. The fish  are affected by extreme heat stress themselves or by the loss of coral cover.

The Great Barrier Reef is home to over 1,500 species of fish including clownfish, parrotfish and lion fish. Many make their homes on the reef or seek cover in the coral.

But a survey of fish of 16 reefs off Lizard Island, in the northern section of the Great Barrier Reef, found a reduction in fish diversity after the 2016 mass bleaching of the coral, caused by a global heatwave.

The research found that in different coral reefs, the 2016 bleaching event caused reductions of some reef fish species, such as small-bodied damsel fish and cardinal fish which can be highly dependent on live branching coral for their survival on the reef.

The changes in fish communities over the bleaching period, caused a homogenisation, or loss of biodiversity, among the different reefs, causing them to appear more similar.

The study, conducted by researchers at the University of Exeter, Australian Research Council Centre of Excellence for Coral Reef Studies and Lancaster University, found that some fish, such as small-bodied species that are dependent on particular corals for shelter, were more sensitive to severe heat-stress and disturbance due to coral-bleaching  than others.

Following the bleaching event, researchers recorded a sharp drop in the diversity of fish communities as the mix or species changed. This was likely due to the mortality of some fishes and migration of others in search of undisturbed reefs.

Fish that are highly dependent on live branching corals for refuge declined the most, as live branching coral tissue was lost.

“The widespread impacts of heat stress on corals have been the subject of much discussion both within and outside the research community. 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, a researcher at the University of Exeter.  

Global climate change is warming the oceans, with extreme temperatures, or thermal anomalies causing tropical corals to bleach and often die.

During periods of extreme heat-stress, the corals expel the friendly micro-algae living in their cells that give them their colour, leaving behind stark white corals.
Reef fishes depend on corals for food and shelter, but some fish are more sensitive than others to heat-stress, and also to the loss of live coral habitat.

“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.

“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.”

The paper “Mass coral bleaching causes biotic homogenization of reef fish assemblages” is published in Global Change Biology.

Date: 6 April 2018

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