Oyster experts are battling to find out what’s killing as much as 80 per cent of young oysters on upper North Island farms.

Farmers first noticed the alarming mortality rates in mid-November – Pacific oysters usually lose no more than five to ten per cent of a crop.

MAF Biosecurity and Aquaculture New Zealand are working with the Oyster Industry Association to find out what’s causing the deaths.

Executive Officer Tom Hollings says the investigation is still in its early stages.

“We’ve had MAF Biosecurity on it as soon as we could in November."

"They’ve taken 250-odd samples, and if they’re flat out analysing them, they might have a result by the middle of next week."

Tom Hollings says the loss of young oysters now could seriously affect the 2011 harvest.

"It’s largely confined to the juveniles, but there are some adults who have been affected too, probably as part of the same problem.

"They all die quickly, in the water, and I hasten to add there’s no food safety issues because you turn up to harvest them, and they’re all gone, you see these gaping, empty shells.”