Coral cover in the Great Barrier Reef has been reduced to half of what it was in the 1980s, an alarming new study has found.
According to a paper from researchers at the Australian Institute for Marine Science, total coral cover in the region dropped from 28 per cent in 1985 to 13.8 per cent in 2012.
In effect, that means the total coral coverage has dropped by 50.7 per cent.
The figures are based on analysis of 2258 surveys of 214 individual reefs over the past 27 years.
Researchers say cyclones are responsible for 48 per cent of the loss, while crown of thorns starfish accounted for 42 per cent and coral bleaching the remaining 10 per cent.
Importantly, the study finds coral coverage would have grown during the same period were it not for the starfish.
Pesticide and fertiliser run-offs have likely increased the frequency of crown-of-thorns starfish outbreaks.
The study says reducing the numbers of the starfish could improve the reef's outlook, but only if climatic conditions are stabilised.
WWF Australia spokesman Nick Heath said the report showed the government needed to take immediate action to prevent chemical runoff reaching the reef.
"This latest research demonstrates that more decisive action to cut chemical fertiliser is urgently needed to prevent unprecedented and ongoing outbreaks of crown of thorns starfish," he said.
The Queensland and federal governments are currently preparing a response to a UNESCO report which criticised the management of the reef and said coastal development in the area posed "serious concerns over its long-term conservation".
UNESCO warns the reef could be listed as a World Heritage site in danger if "threatening" developments are allowed to proceed.