TIPPING POINT: Scientists have identified how many fish it takes to make a healthy or dying reef.

SCIENTISTS believe they have identified the tipping point of overfishing that could save the world's coral reefs.

They identified the stages or "thresholds" a coral reef eco-system goes through before collapse.

And they found how many fish it takes to make a healthy or dying reef.

"Hard coral cover is the last line of defence before a reef collapses," said Townsville-based Nick Graham, of the ARC Centre of Excellence for Coral Reef Studies.

"But it starts dying when the nuts and bolts go. You see patches of weeds replacing coral, you see more sea urchins devouring the coral, you see a general decline in the species richness on the reef, and you see less coral cover."

Dr Graham was part of an international team that surveyed 300 reefs in the Indian Ocean off the east coast of Africa.

They found that in well-protected areas there were typically 1000kg-1500kg of fish a hectare of coral reef.

As this is reduced below 1000kg, early warning signs such as increased seaweed growth and urchin activity began to show, Dr Graham said.

"It shows us multiple tipping points," he said.

"There is more than one line between life and death for a reef."

When fish stocks dropped below 300kg/ha, the reef was in real trouble, Dr Graham said.

The loss of hard corals, which had been thought of as a warning sign, was actually the last stage in the collapse of a reef, the study found.

The researchers found between 300kg-600kg of fish a hectare was the "maximum sustainable yield".

As debate rages over the impact of climate change on the Great Barrier Reef, Dr Graham said measuring the amount of fish was a tangible variable.

"It is easier to comprehend than some variable like the amount of phosphorous in the water," he said. "Fishermen and scientists have long wondered how many fish can be taken off a reef before it collapses. This sets a target.

"The consequences of overfishing can be severe to the ecosystem, and may take decades to recover, but hundreds of millions of people depend on reefs for food and livelihoods, so banning fishing altogether isn't a reality in many nations."

The report, "Critical thresholds and tangible targets for ecosystem-based management of coral reef fisheries", has been published in Proceedings of the National Academy of Sciences of the USA (PNAS).

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