Killer starfish are eating the Great Barrier Reef by smothering and digesting its flesh

Scientists are blaming the problem on fertiliser from nearby sugar cane farms

Fish are seen swimming around coral formations on the southern-most coral cay of the Great Barrier Reef. (Mark Kolbe/Getty Images)

A species of carnivorous starfish devouring Australia’s Great Barrier Reef is to blame for coral depletion, according to researchers.

Scientists from Australia's Institute of Marine Science (AIMS) have mapped the destruction, and told the BBC that the crown of thorn starfish (COTs) are the biggest of three main contributors.

The team is now looking to reduce the impact of the starfish, which smothers coral before digesting its flesh.

“Coral cover is half of what it was 27 years ago, coral cover is going down at an alarming rate,” Dr Katharina Fabricius, coral reef ecologist and AIMS principal research scientist, told the BBC World Service programme Discovery.

She explained that the two other sources of coral decline are storms, and bleaching, which occurs when corals become stressed by environmental factors.

“42 per cent is attributed to crown of thorns starfish - and just 10 per cent due to bleaching. This compares with 70 per cent due to bleaching for reefs elsewhere in the world, such as in the Caribbean,” said Dr Fabricius.

“Crown of thorns are amazing creatures, they can grow to larger than a dinner plate they have multiple arms they're covered in spines,” Craig Humphrey, manager of the AIMS SEASIM project, the world's largest marine environment simulator, told the BBC.

While COTs have damaged coral for centuries, increasingly frequent outbreaks in recent years have been blamed on the excessive use of agricultural fertiliser, particularly in Queensland where sugar cane is farmed.

"The conjecture is the agricultural practice has reduced the time between COT outbreaks," said Mr Humphrey.

"At a point north of Townsville, you'll get an initial outbreak then you'll see waves of COTs spreading down the reef, and they'll consume massive amounts of coral - they do massive damage," he said.

John Brodie, team leader of the Catchment to Reef Processes Research Group at James Cook University, described how while only a small proportion of COT larvae survived to adulthood in the past, agricultural nutrients have changed that trend.

Mr Brodie has been working with Queensland's sugar cane farmers to try and reduce the amount of fertiliser entering the sea.

So far there has been a 2 to 3 per cent decrease after he suggested farmers use less fertiliser to save money and help protect the recreational fisheries that they use.