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Scientists studying remote reefs off Australia said Thursday sharks played a fundamental role in coral health, with overfishing of the marine predators increasing reef vulnerability to global warming and disasters.

A research team, led by Mark Meekan from the Australian Institute of Marine Sciences, studied the impact of sharks at the Rowley Shoals and Scott Reefs some 300 kilometres (185 miles) off Australia's north-west coast over a period of 10 years.

"Where shark numbers are reduced we see a fundamental change in the structure of food chains on reefs," said Meekan.

"We see increasing numbers of mid-level predators such as snappers, and a reduction in the numbers of herbivores such as parrot fishes.

"The parrot fishes are very important because they eat the algae that would otherwise overwhelm young corals on reefs recovering from natural disturbances."

The study compared the impacts of cyclones and bleaching events on the marine-protected Rowley Shoals, where fishing is banned, with the neighbouring Scott Reefs, where Indonesian fishermen are permitted to hunt sharks.

It found less coral and more algae on the fished reefs after a major disturbance, which Meekan said was a significant result as the pressures of global warming increased.

"With many of the changes from a warming climate already locked in, there may be little we can do to prevent increased frequency of disturbances on coral reefs in the near future," he said.

"However, this is not the case with the loss of reef sharks."

Meekan said the findings showed declining global reef shark populations due to overfishing was of "great concern" because it would leave the coral structures more vulnerable to bleaching events from warmer, more acidic oceans, and large cyclones.

But even small no-fishing zones in reef areas could provide valuable feeding sites for sharks, maintaining a delicate ecosystem balance ensuring algae-eating species could thrive, he added.

According to the team their study, published in the latest edition of peer-reviewed scientific journal PLOS ONE, had offered a "unique opportunity" to isolate and examine the impacts of sharks on an entire reef ecosystem's health in a way not attempted before.