Damage to coral reefs found to help growth of parrotfish New study finds parrotfish population thrives when corals bleach

Severe coral reef bleaching typically spells disaster for most of the fish species that live in them. But scientists have found an unexpected exception – the parrotfish.

These prosper so greatly in cases of extreme bleaching that their population surges to between two and eight times its original number.

At the same time, the average fish size increases by 20 per cent, according to a new study. Almost every other species of fish was in sharp decline in the bleached areas.

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Coral bleaching is caused by rising sea temperatures, causing it to expel the algae living on it and turning it white.

Nutritious algae

A layer of algae and bacteria then forms on the barren coral surface which, unlike most reef-dwelling creatures, the parrotfish finds to be highly nutritious.

“When bleaching reduces coral cover on the reefs, it creates large areas of newly barren surfaces,” said Brett Taylor, of the Australian Institute of Marine Science in Perth.

“This immediately gets colonised by the microalgae and bacteria, basically an internal and external layer of ‘scunge’, which provides nutritious, abundant food for parrotfish.”

Parrotfish, named because of their tightly packed teeth in a beak formation, use their nashers to scrape microorganisms off coral – and their presence in large numbers on damaged reefs probably helps repair it, the researchers found.

Increasing parrotfish numbers

They concluded that the coral and the parrotfish constitute a feedback loop, slowly bringing each other into balance.

When reefs are damaged, parrotfish numbers increase. This results in low levels of scunge, giving the coral the best chance to recover. As the reef then returns to health, parrotfish numbers decline again.

“We found reef ecosystems in two different oceans had the same response to global heat events which is indicative of the current magnitude of climate change effects. Parrotfish are a vital link in the reef ecosystem,” said Mark Meekan, also at the institute.

The research, published in the journal Global Change Biology, also included academics from the University of Lancaster, James Cook University in Australia and the University of Auckland in New Zealand.