Drop in sea urchins and snails a boon to marine park
Corals in Hoi Ha Marine Park that had been damaged by a surge in the number of sea urchins that fed on them had recovered significantly, the Agriculture, Fisheries and Conservation Department said.
The park's coral coverage, which shrank to a low of 72 per cent in 2006, had increased last year to the pre-damage level of 77 per cent, after officials took steps to reduce the number of sea urchins and prop up the collapsed corals to encourage growth.
The park's coral beach was closed to the public in May 2007 for rehabilitation work to proceed undisturbed. This helped speed up the corals' recovery, said Edward Wong Cheuk-kee, a senior marine parks officer with the Agriculture, Fisheries and Conservation Department. The beach reopened in October.
In 2006, the dramatic growth of sea-urchin numbers led to the collapse of 330 coral colonies in a 100 square metre site, their foundations eaten and eroded. At its worst, the proportion of the dead or damaged coral coverage rose to 18 per cent, three times the normal proportion.
By last August, the proportion had dropped to an acceptable level of 6.1 per cent.
Tests on the sea urchins, a long-spined species called Diadema setosum, found coral skeleton pellets in their digestive systems, confirming they had fed on the corals.
Their numbers had dropped from a high of 584 for every 400 metres of the beach to 194 in August.
Since 2006, officials have removed about 21,000 sea urchins from the site and destroyed them. A breeding ground for the sea urchins close to the beach was also relocated to a site farther away.
About 30,000 small sea snails that also grazed on the corals, causing bleaching, have been removed as well.
Officials also used underwater cement to fill in the eroded corals and restore those that had collapsed.
The exponential growth of the sea urchins had been traced to the unexplained decline in the numbers of its predator, a crab species known as Thalamita prymna, which is hunted by humans for the plate.
Although overfishing had been blamed for the crab's decline, Mr Wong said it did not explain why the problem was apparent only at the coral beach, and not at other coral sites inside the park.
Officials had considered reintroducing these crabs at the beach, he said, but would put the option on hold now that the corals' health had improved. Their continued recovery would be monitored, he said.
To protect the corals, the beach might need to be closed periodically, he said.
Noting the benefits of closing the beach for nearly 11/2 years, he said: 'We have been puzzling if the damage by nature is more serious than by human activity. But after [seeing the results of] the closure, we believe the latter plays an important role too.
'This has raised the question of whether we should close the beach once every two to three years, since the corals in Hong Kong are very fragile.'