Biosecurity official not warned of earlier oyster deaths

Shellfish farmers brought in a major marine science institute to investigate an unusual die-off of juvenile oysters last March - but neither the industry nor the scientists warned biosecurity officials.

Now Ministry of Agriculture and Forestry scientists have said an incurable herpes-like virus is behind huge new deaths of juvenile Pacific oysters over November and in early December at upper North Island marine farms.

The ostreid herpesvirus-1 (OsHV-1) is expected to cost the industry $15 million this year, potentially with extensive job losses, scientists say.

Half the aquaculture industry's farmed oysters due for harvest next year have died on marine farms from Parengarenga Harbour in Northland to Ohiwa in the eastern Bay of Plenty. On some farms, up to 80 percent of juvenile oysters have died, compared with 5 percent to 10 percent in a normal year.

MAF response manager Richard Norman told NZPA that in the course of the November and December herpes deaths, "MAF has been made aware that there was a smaller mortality event in autumn".
"MAF was not informed at the time of the autumn event and has no samples or data."

Asked whether MAF was aware of a private research institute testing dead or dying juvenile oysters, but telling farmers there were no substantive clues, and instead suggesting there might be environmental or husbandry problems, Dr Norman said "MAF is aware of the previous private sector research and the findings".

He declined to comment on whether those scientists had failed to carry out DNA testing for viruses during the autumn deaths, despite there having been several years of publicity over similar problems in France in the same oyster species, which was pinpointed to oyster herpes. A strain of the virus killed between 20 to 100 percent of breeding Pacific oysters in some French beds in 2008, 2009, and 2010.

"Die-offs do occur sporadically in aquaculture, and how these are viewed in retrospect can be very different to how they may appear at the time," Dr Norman said.

Cawthron Institute chief executive Gillian Wratt told NZPA that her scientists produced a report for the industry on the autumn deaths.

"At that stage, there were no definitive indicators of pathological causes," she said: "We did look for signs of pathological indicators, using OIE standards".

But Dr Norman told NZPA that the OIE - the World Organisation for Animal Health - did not list the OsHV-1 oyster herpes as a mollusc disease.

MAF pathologists used molecular tests and DNA sequencing to show the presence of ostreid herpesvirus-1 (OsHV-1) in samples from affected oyster farms, and Dr Norman said it was possible the die-off had been caused by a range of factors, triggered by unusually warm water temperatures.

Oyster Industry Association executive officer Tom Hollings told NZPA that the autumn die-off was a case of juvenile mortality.

"My understanding was that the research institute had a brief to tell us what was going on," he said.

Farmers encouraged those researchers to take whatever samples they needed.

"They did their thing and came back saying their histological analyses were that they couldn't find anything definitive," he said.

"That gave us no answers at all," Mr Hollings said.

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