When coral bleaching events cause coral to die, many reef species are forced to relocate, but not the parrotfish. New research suggests parrotfish thrive when coral dies.

Scientists at the Australian Institute of Marine Science surveyed fish populations living near a pair of severely bleached reefs, the Great Barrier Reef in the western Pacific and the Chagos Archipelago in the Indian Ocean. They found parrotfish numbers skyrocketed when elevated water temperatures killed off large sections of the reef.

Compared to parrotfish living near reef sections that remained unbleached, parrotfish populations found among bleached coral were two to eight times bigger. But population numbers weren't the only figures to increase. Individual parrotfish living among bleached coral were 20 percent larger than those living on healthy sections of the reef.

All other surveyed species suffered diminished numbers in the wake of coral bleaching. Parrotfish thrived because coral bleaching resulted in new food resources.

Scientists shared the results of their survey this week in the journal Global Change Biology.

"When bleaching reduces coral cover on the reefs, it creates large areas of newly barren surfaces," AIMS researcher Brett Taylor said in a news release. "This immediately gets colonized by the microalgae and cyanobacteria, basically an internal and external layer of 'scunge,' which provides nutritious, abundant food for parrotfish."

But while the demise of coral reefs is a boon to parrotfish, the latest research suggests the hardy fish help reefs recover from bleaching by cleaning off the scunge. When reef health rebounds, the parrotfish numbers decline once more.

This ebb and flow helps keep ecosystems in balance and lends the reef habitat an added level of resiliency to environmental stress.

"Parrotfish are a vital link in the reef ecosystem," said Mark Meekan, AIMS researcher and study co-author. "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."

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