The future of wild oysters appears bleak, but the cherished Bluff oyster won't be vanishing from menus in the foreseeable future, the industry insists.

Wild oysters are now "functionally extinct" in many places where they were once plentiful, Britain's Independent newspaper reported, quoting a report by marine biologists who examined 144 former oyster strongholds in 40 regions around the world.

More than 85 per cent of their reefs had been lost through overfishing. The overall condition of the various species is poor, according to the largest investigation ever into wild oyster stocks, published in the journal BioScience.

If nothing is done to protect remaining wild oysters, they could disappear within a generation, says Dr Michael Beck of the University of California.

The Bluff oyster harvested from the waters off the bottom of the South Island enjoys an environment quite different from those of many other wild oyster species around the world.

Graeme Wright, operations manager for the Bluff Oyster Management Co, said it was one of the most researched fisheries in New Zealand, and strict measures were in place to prevent overfishing.

While up to 15 million Bluff oysters could be legally harvested each season, for much of the past decade only half that had been taken by the industry, he said. About 9.4 million were expected to be harvested this coming season, beginning next month.

"The levels that we are currently harvesting, and have been harvesting since back in the 1990s are well sustainable," Mr Wright said.

"Obviously we are always learning, and you can never say never ... [but] we are confident that we are managing it as sustainably as we can."

The greatest threat is the parasite Bonamia, which has in the past killed up to 90 per cent of the mature oyster population, and closed the fishery.

Allen Frazer, of the Ministry of Fisheries inshore fishing team, said the future now looked "reasonably positive". While there had been mass "die-offs" from Bonamia in the past, numbers had always bounced back.