Growing Alarm As Great Barrier Reef Shrinks

Australia's Great Barrier Reef has shrunk by more than half over the past 27 years with starfish partly to blame.

And scientists believe that coral cover on the reef - the world's largest and heritage-listed - could halve again by 2022 if trends continued.

Intense tropical cyclones - 34 in total since 1985 - were responsible for much of the damage (48%), with outbreaks of the coral-feeding crown-of-thorns starfish linked to 42%, a study has found.

Two severe coral bleaching events in 1998 and 2002 due to ocean warming also had "major detrimental impacts" on the central and northern parts of the 345,000 sq kms of reef, it said.

"This loss of over half of initial cover is of great concern, signifying habitat loss for the tens of thousands of species associated with tropical coral reefs," the study said.

Author Hugh Sweatman said the findings, drawn from the world's biggest ever reef monitoring project involving 2,258 surveys over 27 years, showed that coral could recover from such trauma.

"But recovery takes between 10 and 20 years. At present, the intervals between the disturbances are generally too short for full recovery and that's causing the long-term losses," Mr Sweatman said.

John Gunn, head of the Australian Institute of Marine Sciences, said researchers could focus their short-term efforts on the large, poisonous and spiny starfish, which feasts on coral polyps and can devastate reef cover.

"We can't stop the storms but perhaps we can stop the starfish," said Mr Gunn. "If we can, then the reef will have more opportunity to adapt to the challenges of rising sea temperatures and ocean acidification."

Improving water quality was key to controlling starfish outbreaks, with increased agricultural run-off such as fertiliser along the reef coast causing algal blooms that starfish larvae feed on, scientists said.

Unesco has warned it is considering listing the reef as a heritage site in danger earlier this year due to the unprecedented gas and coal mining boom in northern Australia and increasing coastal development.