Rāhui to be placed on Foveaux Strait after oyster parasite find

Friday, 26 March 2021

Wild oysters are still on the menu as the rāhui in the Foveaux Strait is unlikely to impact commercial operations.

A rāhui will be placed on the relatively small area of the Foveaux Strait where Bonamia ostreae was discovered in wild oysters for the first time.

Biosecurity New Zealand manager Dr Cath Duthie was in Southland on Friday, less than a day after the parasite was detected during routine surveillance.

Duthie met with Ngāi Tahu representatives and commercial oyster harvesters at the Environment Southland offices where there was unanimous agreement that a rāhui should be in place until the extent of the incursion could be determined, she said.

Official legal controls would be published in the coming days to ensure the space was not disturbed, Duthie said.

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Bonamia ostreae has only been in New Zealand since 2015 and while it poses no harm to humans, it kills oysters, potentially in large numbers.

The infection was found in three wild oysters during routine six-monthly testing that had been in place since the parasite was detected in Big Glory Bay in 2017. No infection was found in the other five sample sites.

Farmed oysters infected with Bonamia ostreae being unloaded in Bluff from the Foveaux Freighter under MPI supervision in 2017. The plastic wrapped farmed oyster were dumped.

Commercial operations would continue as normal in the Strait as the affected area was only a couple of nautical miles across in an area that had not been fished in the past five years, Duthie said.

The rāhui would cover the area about four or five nautical miles east of Saddle Point on Rakiura/Stewart Island, she said.

It helped that the area had a small oyster population and had not been dredged, which would increase the risk of spreading, Duthie said.

It would take a few weeks before further test results were in, after which Biosecurity New Zealand would be able to decided on the next steps in partnership with local operators, she said.

Duthie acknowledged that there was huge concern in the community, and said wild oysters from Foveaux Strait were a core part of Southland's identity.

But the unique species in their unique environment meant it was difficult to predict how Bonamia ostreae would impact the fishery.

Oysters typically grow in rivers and estuaries, not dynamic ocean currents.

Duthie did not want to speculate on what the find would mean for the oyster industry.

“We don't know much more than it's a small detection,” she said.

In June 2020, Ministry of Primary Industries (MPI) biosecurity readiness and response director John Walsh warned that the disease was unlikely to ever be eradicated from Stewart Island after being carried to the island from Marlborough Sounds.

Bonamia ostreae caused widespread devastation on the island in 2017 with MPI taking the decision to remove all oyster farms in Big Glory Bay to protect the wild oysters in the Strait.

The microscopic parasites could stay dormant and would be spread by ocean currents, so there was always a probability that it would reach the wild fisheries, Duthie said, even though everyone had hoped it wouldn’t happen.