Marine pathologists investigating the huge die-off of juvenile Pacific oysters hope to pin down the cause by Wednesday next week.
"Biosecurity New Zealand people who took about 250 samples last week are flat out analysing them and expect to have answers by Wednesday,'' Oyster Industry Association executive officer Tom Hollings said today.
Half the aquaculture industry's farmed oysters due for harvest next year have died on marine farms from Parengarenga Harbour in Northland toOhiwa in the eastern Bay of Plenty, and samples from juvenile oysters are being analysed.
On some farms, up to 80 percent of juvenile oysters have died, compared with 5 percent to 10 percent in a normal year.
"While some mortality events have been seen in New Zealand Pacific oysters in the past, and routinely there is low level mortality, this event has been big and sudden,'' said a spokeswoman for the Ministry of Agriculture and Fisheries, whose scientists are investigating.
But Mr Hollings said there had actually been an earlier die-off, last autumn, and at that time marine researchers at a private-sector institute had suggested the causes were environmental factors, such as exceptionally warm, dry and settled weather patterns.
"That gave us no answers at all,'' Mr Hollings said.
MAF has said not enough was yet known to link the New Zealand deaths with an oyster herpes virus which has killed between 20 to 100 percent of breeding Pacific oysters in some French beds in 2008, 2009, and 2010.
That Ostreid herpesvirus 1 (OsHV-1) has spread into British waters, and according to Kevin Denham of the British government's fish health inspectorate, it remains dormant until water temperatures exceed 16degC.
Ostreid herpes viruses are known to affect not only oysters but also clams, scallops, and other molluscs, according to French Research Institute for Exploitation of the Sea pathology lab director Tristan Renault, who has suggested in Europe that global warming ``could be an explanation of the appearance of this particular type of the virus''.
Animal health experts at the European Food Safety Authority (EFSA) are assessing the extent to which a combination of "infectious agents'' such as OsHV-1 and environmental factors are causing the die-off of Pacific oysters there, whether other shellfish species are involved and the risk of infection posed by the transfer of adult Pacific oysters from infected farms.
Asked whether the oyster herpes virus had been detected in the New Zealand shellfish, the MAF spokeswoman said: "Testing is at a very early stage and a wide range of factors are being considered.''
She noted that the 2008 outbreaks of late spring and summer in Ireland and France were blamed on a combination of adverse environmental factors, together with Vibrio bacteria, and a newly-described strain of the oyster herpes virus.
