The reef-dwelling parrotfish doesn’t just thrive in adversity, it helps turn things around.

Marine scientists have discovered that populations boom in the wake of severe coral bleaching.

Studies carried out 8000 kilometres apart in Australia’s Great Barrier Reef and the Chagos Archipelago in the Indian Ocean found that parrotfish populations increased in number by as much as eight-fold in damaged areas of the reef, and that individual fish were about 20% larger than those in unbleached sections.

Almost every other species of fish was in sharp decline in the bleached areas.

The reason, it appears, is that the parrotfish (family Scaridae) – so named because its tightly packed teeth are in a “beak” – uses those teeth to scrape microorganisms off coral.

“When bleaching reduces coral cover on the reefs, it creates large areas of newly barren surfaces,” says Brett Taylor from Australian Institute of Marine Science (AIMS), who led a team of researchers from Australia, New Zealand and the UK.

“This immediately gets colonised by the microalgae and cyanobacteria, basically an internal and external layer of ‘scunge’, which provides nutritious, abundant food for parrotfish.” {%recommended 8275%}

As such, Taylor and colleagues conclude in a paper in the journal Global Change Biology, the coral and the fish form a feedback loop, slowly bringing each other into balance.

When reefs are damaged, parrotfish numbers swell. 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,” Taylor says.

The fact that plump parrotfish were found in large numbers on both reefs indicates the feedback loop is an inherent part of reef ecology and not caused by local factors.

“Parrotfish are a vital link in the reef ecosystem,” says co-author Mark Meekan from AIMS.

“As herbivores, their grazing shapes the structure of reefs through effects on coral growth and suppression of algae that would otherwise proliferate. Because of these important ecological roles, they have been described as ‘ecosystem engineers’ of reef systems.”