Parasite which decimated southern oyster industry may never be eradicated

Tuesday, 16 June 2020

A farm in Big Glory Bay, Stewart Island, where the oyster parasite Bonamia ostreae was found.

The oyster parasite Bonamia ostreae, which decimated the southern fishery, is unlikely to ever be eradicated from Stewart Island according to the Ministry of Primary Industries.

That was despite surveillance testing in February finding no instances of the disease in Big Glory Bay or the Foveaux Strait.

Ministry biosecurity readiness and response director John Walsh said Bonamia ostreae could not be considered eradicated from Big Glory Bay and surveillance would need to continue.

An infected oyster was discovered in the bay in October 2019.

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In 2017, an outbreak of the disease led to Biosecurity New Zealand removing all the farmed oysters in Big Glory Bay.

'The removal of affected farms from the area has reduced the risk to wild flat oyster fisheries. But full eradication, which is based on absolute confidence the organism is no longer present, is unlikely,' Walsh said.

The parasite was a microscopic organism that could persist in the marine environment for a long time, he said.

'Overseas experience suggests eradication is very difficult to achieve,' Walsh said.

'Therefore, the measures enacted in 2017 [removal of oyster farms in Big Glory Bay] were principally to contain Bonamia ostreae to known distributions in the Marlborough Sounds and Big Glory Bay and to reduce the risk of the disease spreading to the wild oyster fisheries.

'Those objectives have been achieved. Based on international experience, it is MPI’s view that Bonamia ostreae would be very difficult to eradicate and is likely to continue to be present in Big Glory Bay at low levels.'

However, last year a Ministry of Fisheries spokesman told Stuff that continued negative tests would mean eradication.

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

Walsh highlighted that Frazer specifically said 'continued testing would need to show no signs of Bonamia'.

'Also, he made no mention of how long that continued testing might need to carry on for', as testing would continue for the foreseeable future, Walsh said.

According to information compiled by NIWA fisheries scientist Keith Michael, the parasite can cause a mortality rate in oysters of more than 90 per cent.

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.

It multiplies rapidly in the oysters blood cells, causing the cells and tissues to break down or rupture, releasing infective matter, Michael said.

Michael said eradication would be difficult to prove.