Researchers find familiar species pave the way for coral regrowth

In contrast to most other species, reef-dwelling parrotfish populations boomin the wake of severe coral bleaching.

The surprise finding came when researchers led by Perth-based Dr Brett Taylorof the Australian Institute of Marine Science (AIMS) looked at fishpopulations in severely bleached areas of two reefs – the Great Barrier Reefin the western Pacific and the Chagos Archipelago in the Indian Ocean.

The sites are 8000 kilometres apart.

Bleaching is coral’s stress reaction to prolonged exposure to higher seasurface temperatures.

“Warming oceans place enormous pressure on reefs and if the temperaturesremain high for too long the coral will die. The more frequently this occursthere is less time for coral reefs to recover,” Dr Taylor said.

In the damaged areas of the reefs, the study found that parrotfish populationsincreased in number by between two and eight times, and individual fish wereabout 20% larger than those in unbleached sections.

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

Parrotfish, named because of their tightly packed teeth in a beak formation,use their teeth to scrape microorganisms off coral – and their presence inlarge numbers on damaged reefs very likely helps the process of repair, Taylorand his colleagues suggest.

“When bleaching reduces coral cover on the reefs, it creates large areas ofnewly barren surfaces,” Taylor said.

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

The researchers concluded that the coral and the parrotfish constitute afeedback loop, slowly bringing each other into balance. When reefs aredamaged, parrotfish numbers swell. This results in low levels of scunge,giving the coral the best chance to recover. As the reef then returns tohealth, parrotfish numbers decline again.

“We found reef ecosystems in two different oceans had the same response toglobal heat events which is indicative of the current magnitude of climatechange effects,” he said.

The fact that plump parrotfish were found in large numbers on both reefsindicates the feedback loop is an inherent part of reef ecology and not causedby local factors. “Parrotfish are a vital link in the reef ecosystem,” saysAIMS co-author Dr Mark Meekan.

“As herbivores, their grazing shapes the structure of reefs through effects oncoral 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.”

As well as AIMS, scientists working on the project came from James CookUniversity in Australia, the University of Auckland in New Zealand, and theUniversity of Lancaster in the UK.

The research is published in the journal Global Change Biology.

Image: Credit: Kendall Clements

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