The Great Shrinking Barrier Reef

Australia's Great Barrier Reef has seen a shocking reduction in the past 27 years.
The Reef is the most outstanding coral reef system in the world because of its great length, number of individual reefs and species diversity. But a new study released last week shows it's now half the size it was in 1985!
This depressing analysis (of more than 2,000 surveys from 214 reefs) shows a drop of 50.7%. Scientists were able to break down the causes into three: storm damage (48%), coral bleaching (10%), and 42% to the crown-of-thorns starfish.

Paradise lost?

John Gunn, Australian Institute of Marine Science CEO: "We can't stop the storms, and ocean warming (the primary cause of coral bleaching) is one of the critical impacts of the global climate change. However, we can act to reduce the impact of crown-of-thorns."

Villain of the piece...

The crown-of-thorns starfish is widely spread throughout the Pacific, and feeds specifically on coral polyps. It receives its name from venomous thorn-like spines covering its upper surface. Its numbers have dramatically increased in the last few decades, causing significant damage.
Several control measures are being used. Injecting sodium bisulphate into the starfish is the most efficient. This is deadly to crown-of-thorns, but does not harm the surrounding reef and oceanic ecosystems. However, to control high infestation areas, divers need kill rates of up to 120 per hour each. Another practice, of cutting them in half, yielded only a kill rate of 12 per hour per diver and the diver performing this test was spiked three times. Both methods are labour-intensive, as is the third option of simply burying the starfish under rocks. This can only be done where there's suitable material and without damaging the fragile coral.
Right now, the coral cover is declining at 3.38% per year. If all three causes (storms, bleaching, and starfish) were prevented, it would be growing at 2.85%. But if just the starfish could be contained, the coral cover would increase at 0.89% per year.
It's not a lot, and it's not fast, but it would be enough to offset the other damages.