A fisherman in the Phillippines empties a bag of mussels at a local market - AP photo.

BANGKOK Overfishing and unchecked coastal development have resulted in the disappearance of 85 per cent of all oyster reefs, making the ecosystem one of the most severely affected marine habitats in the world, according to a new study.

The Nature Conservancy study released Thursday found that several reefs in China have seen drastic declines over the past 30 years, while those in Europe have almost entirely disappeared. Half of the shellfish populations in South America are under threat, while flat oysters have been virtually wiped out in Australia.

Native oyster reefs — essentially mountains of the bivalves cemented together — were once dominant features of many temperate estuaries around the world. Much as coral reefs are critical to marine habitats, the bivalve shellfish are vital to bays and estuaries, creating habitats for a variety of plants and animals, the study said.

Oyster reefs provide important benefits by filtering water, providing food and habitat for fish, crabs and birds, and serving as natural coastal buffers from boat wakes, sea level rise and storms, it said.

If you're sucking down a wild oyster, it most likely came from one of only five regions on the east coast of North America, and in most of these regions, oyster reefs are in poor condition, the study said.

'Centuries of intensive fisheries extraction exacerbated by more recent coastal degradation have put oyster reefs near or past the point of functional extinction worldwide,' according to the report.

Mary Seddon, chair of the International Union for Conservation of Nature's Species Survival Commission Mollusk Specialist Group, said the demise of the oyster goes back centuries, so the study's findings were not unexpected.

'Certainly, 85 per cent of areas that have oysters have had major degradation,' said Seddon, who did not participate in the study.

However, the study concluded these ecosystems could recover if measures are taken to reduce pollution and better manage the reefs. It also called for efforts to protect oyster reefs through international treaties and to reduce the introduction of shellfish species that spread disease.

It highlighted several regions that have made positive strides, including China's Laizhou Bay where river flows have been managed more sustainably, and a marine protected area was established in 2006.

Seddon said she believed creating a network of protected marine areas and maintaining water quality by limiting the amount of sediment flowing into estuaries were crucial to saving oysters.

'By putting protected areas in place and maintaining them, you are reducing the impact of further habitat degradation,' she said. 'You can't have exploitation.' — AP