Could the oyster disease bonamia ostreae be completely eradicated from the deep south?

Saturday, 20 July 2019

Hariata Hema, left, with her cinnamon oysters, and Mary-Helen Lum, 91, both of Wellington, get stuck in to their favourite delicacy at the Bluff Oyster and Food Festival in May.

Good news for oyster lovers: a famed spot for collecting the Bluff delicacies is thought to be the first place in the world to successfully eradicate the bonamia ostreae disease.

Big Glory Bay in Stewart Island has been given a tentative all clear after an outbreak threatened to derail the Bluff oyster season, and push prices up.

Ministry of Fisheries inshore fisheries manager Allen Frazer said there were no signs of bonamia ostreae in Big Glory Bay from the last surveillance.

If continued testing showed no signs of bonamia ostreae in Big Glory Bay, it would be the first example of bonamia eradication in the world, Frazer said.

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MPI say close monitoring will be necessary to ensure the disease doesn't make a comeback.

A Ministry for Primary Industries spokesperson said the latest round of testing conducted in March and April showed all oysters tested for bonamia ostreae showed up negative in Southland waters tested that include Big Glory Bay and Foveaux Strait.

'The results to date suggest that the decision to remove flat oyster farms in Big Glory Bay may have successfully contained bonamia ostreae to the upper South Island.'

Mussel farms in Big Glory Bay, Stewart Island, where the oyster parasite Bonamia ostreae was found two years ago.

Bonamia ostreae, commonly found in Europe, was first detected in New Zealand waters at Marlborough Sounds in 2015 and subsequently discovered in Stewart Island's oyster farms at Big Glory Bay two years later.

While efforts were made to limit the spread of the disease outside Marlborough and Nelson, a decision was made by Biosecurity New Zealand to remove all the farmed oysters in Big Glory Bay in 2017 that proved to be successful.

It's possible that the disease can remain in an environment at low levels for many years so it will be difficult to know how long monitoring at Big Glory Bay will have to continue to confirm the parasite is no longer present.

Barnes Wild Oysters general manager Graeme Wright said the future was looking optimistic with an encouragingly high number of juvenile oysters currently in the Foveaux Strait.

Barnes Oysters general manager Graeme Wright said oysters could be entering a recovery phase with large numbers of juvenile oysters and a low density of bonamia exitiosa

It was known that bonamia acted in cycles and went through phases of periods of no spawning and high levels of the disease before a period of recovery, he said.

It was good news that the latest surveillance showed no signs of bonamia ostreae because what he had researched about waters in Europe that had both forms of bonamia were not positive for oyster farming.

It was not his position to state if flat oyster farming would ever return to Big Glory Bay but MPI would continue to monitor the risk going forward, he said.

Oyster fisheries would prefer to take a precautionary approach, he said.

NIWA fishery scientist Keith Michael said while Big Glory Bay was looking more likely as the first example of bonamia containment in the world, eradication would be difficult to prove.

A lot of research had been done in Europe on bonamia ostreae but no research had been able to confirm the life cycle of the parasite, Michael said.

There had been examples in Europe where the hosts for the parasite had been removed but the parasite still remained thereafter, he said.

Bonamia ostreae in New Zealand had been able to be contained, which was a big achievement because surveillance systems were able to detect the disease and remove the oysters before the disease could spread outside of Big Glory Bay, he said.

In 2017 the Ministry for Primary Industries ordered the removal of oysters from 10 Marlborough Sounds oyster farms.

Apex Marine Farm director Bruce Hearn of Blenheim said they had been dealt a cruel blow by the discovery of bonamia ostreae in Marlborough Sounds.

'I've been farming oysters for 35 years and it's all gone down the drain.'

When the parasite was discovered in Marlborough in 2015 there were changing governments and Hearn believed no efforts were made to eradicate oysters soon enough.

While there was 80 to 100 per cent mortality rate when the the disease was discovered it would have been possible to live with the parasite in the same way fisheries had been living with exitiosa, he said.

When the disease hit, oysters had no resilience to the disease but changing techniques they could get the survival rate of oysters to 80 percent, he said.

An MPI spokesperson said a containment zone for bonamia ostreae in Marlborough Sounds was formed to prevent the spread of the disease which meant flat oysters could not be taken out of the area.

'Infected farms were removed in 2017 to reduce the likelihood of bonamia ostreae spreading from the Marlborough Sounds to other locations in New Zealand.'

Tasman Bay and Cloudy Bay which border the containment zone have shown no detection of bonamia ostreae, the spokesperson said.