Oyster parasite Bonamia ostreae found in Foveaux Strait

Thursday, 25 March 2021

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.

Some southern oyster harvesters are “devastated” after Bonamia ostreae has been detected in Foveaux Strait, near Stewart Island, despite concerted efforts to protect the wild oysters.

Biosecurity New Zealand said on Thursday it would begin further testing after three wild oysters from a small area in the Strait were found infected with Bonamia ostreae.

Barnes Wild Bluff Oysters manager Graeme Wright said it was too early to tell how widespread the infection was or how it would impact commercial harvesting.

Foveaux Strait was a unique environment for oysters, which usually grew in estuaries and river, he said, and operators have been forever hopeful that the parasite would not reach the fishery.

* Wild Bluff oyster numbers on the rise

* Wild oyster infected with Bonamia ostraeae found in Big Glory Bay

Fowler Oysters manager Terina Stockwell said she and her team had been spiralling since hearing the news.

Map of surveillance areas and detection site of oyster parasite Bonamia ostreae in Foveaux Strait, near Stewart Island.

“We're devastated. We’re a small family-owned business, as it is,” she said, referring to the possible implications of an infection in the strait. “I don’t want to think about it.”

Biosecurity New Zealand scientists were travelling to Southland on Friday and Stockwell hoped to know more once their tests were complete.

Bluff oyster quota holders, who are in the middle of this year’s oyster season, will be able to continue to fish, and Biosecurity New Zealand will work with them on the best course of action - likely to be a small temporary and voluntary ‘no take’ zone around the area where the oysters were found until longer term options are explored.

The infected oysters were sampled as part of Biosecurity New Zealand’s routine surveillance for the oyster parasite and were from a sample site in the south-western area of the Strait.

No infection was found in the other five sample sites.

Bonamia ostreae was first found in New Zealand in 2015 in oysters from the Marlborough Sounds and then in 2017 in oysters in Big Glory Bay in Stewart Island.

It is not a food safety issue and fresh good quality Bluff oysters remain safe to eat.

Since first finding the parasite, Biosecurity New Zealand has funded an extensive programme to protect wild oyster populations, including those in Foveaux Strait, from the disease.

Biosecurity manager Cath Duthie said legal restrictions on moving oysters and other risk materials were enacted, and oyster farms in Stewart Island were removed in 2017 in a bid to reduce any risk of spread.

“It was, however, always considered possible that a find in wild oysters could occur. Foveaux Strait is a dynamic waterway with strong currents and the parasite can remain dormant.

“For this reason, we commissioned NIWA to do six-monthly surveillance sampling and testing of wild oysters from six different locations in the Strait to find any infections at the earliest possible time. We received a confirmed positive laboratory result for Bonamia ostreae from the three oysters today.

“Commercial Bluff oyster fishers have not reported any sign of the disease and we’ve not detected it in five years of six-monthly surveillance sampling in the area,” Duthie said.

“No commercial oyster fishing vessels have been working the affected area so far this season.

“The fishery has been affected for many years by a different type of Bonamia – Bonamia exitiosa. The oyster fishing industry has worked through that situation to maintain a sustainable fishery. We’ll work to support them to explore ways through this new challenge.”

In the coming weeks, Biosecurity New Zealand will work closely with the Bluff oyster fishers, iwi, other marine users and the wider public on the most appropriate and effective response to the situation.

“We’ve talked to fishing industry partners today and will be in Southland tomorrow to start talking to the local community. We want to involve local people, including the iwi of the area, in our response.

“Our immediate focus is to carry out more testing in Foveaux Strait to understand if and where else the parasite is, to enable the best future decisions,” Duthie said.