Bluff oysters at risk after lethal parasite discovered on Stewart Island farms

Thursday, 1 June 2017

The humble bluff oyster has come under threat after the discovery of a lethal parasite at two Stewart Island oyster farms.

The Ministry for Primary Industries confirmed the detection of Bonamia ostreae in two oyster farms on Stewart Island on Wednesday.

Bonamia ostreae has been in New Zealand since at least 2015 in the Marlborough Sounds and Nelson, but this is the first time it has been found in another area of New Zealand.

The parasite can be fatal for flat oysters.

Ministry biosecurity surveillance and Incursion manager Brendan Gould said Bonamia ostreae was a naturally occurring parasite and was difficult to contain and eradicate as it was spread easily through ocean currents and on vessels.

'We are concerned that Bonamia ostreae has been found in Stewart Island, and our priority is on determining the extent of the problem and limiting further spread.'

The ministry has been aware of the parasite in Stewart Island fisheries since May 24.

Targeted surveillance for Bonamia ostreae was carried out twice a year, or anytime we were alerted of an unusual flat oyster mortality event, Gould said.

The ministry has reissued a controlled area notice legally restricting movements of some shellfish species, including their spat, into and out of Nelson, Marlborough Sounds and Stewart Island. It also legally restricts movements of farm equipment and vessels out of Stewart Island to limit further spread.

'This particular strain of bonamia has been in New Zealand since at least 2015, however, strains have been here since the 1960's and are present in the marine environment.'

Extensive sampling to date has not found Bonamia ostreae in Bluff oysters in Foveaux Strait.

There is no food safety issue from eating oysters with Bonamia ostreae. 

Sanford has an oyster operation in Stewart Island that is a joint venture with Tio Operations Ltd, and has been affected by the situation with Bonamia ostraea. 

Sanford general manager corporate communications Fiona MacMillan said strict movement controls have been put in place and the company was not moving any shellfish anywhere.

'We are standing by for more instructions from [the ministry] and have asked for clarification in writing around next steps.'

Five staff were affected by the shutdown in shellfish movements and were being assigned to other duties in the meantime, MacMillan said.

'Our customers will not be affected right now, as we were not harvesting oysters at this point and mussel demand can be met by extra stock from our operations in other parts of the country.'

At this stage, it was not known what the financial impact would be, MacMillan said.

Sanford knew testing had started a couple of weeks ago, she said.

'Each test has to have two samples and we got confirmation of the results at around 3.30pm yesterday.'

Staff had no idea how the incursion occurred and, because the organism is waterborne and easily spread, there were many possibilities, MacMillan said.

'It is vanishingly unlikely it was introduced by a Sanford or Tio vessel as our boats only move around the area between the island and Bluff.' 

Barnes Wild Bluff Oysters manager Graeme Wright said the impact on the oyster industry could be huge, and he was concerned for his business.

'We just have to learn to understand and manage it.'

While the parasite was present on the two farms at Big Glory Bay, as far as he was aware there were no fatalities yet, Wright said.

Testing to date had shown the parasite had not been found in the wild population in Foveaux Strait and the industry would be encouraging further monitoring, he said.

The industry would be working closely with the ministry to ensure the best outcome was achieved for the fishery, Wright said.

In February 2015, the parasite was discovered in Marlborough Sounds and the farms there experienced a high mortality rate, he said.

Bonamia ostreae had devastated the European and North American stocks over a long time, Wright said.

Another strain, Bonamia exitiosa, had been present in the wild populations in Foveaux Strait since the early 1960s.

It wiped out more than 1.5 billion oysters - about 95 per cent of the total population - in 2001 and 2002.