Foveaux Strait oyster parasite linked to existing outbreaks

Genome sequencing shows that the strain of Bonamia ostreae found in wild Foveaux Strait oysters in March is closely related to those previously found in Marlborough and Big Glory Bay.

The parasite was first detected in New Zealand waters in the Marlborough Sounds in 2015, before being detected two years later in Big Glory Bay on Stewart Island, where thousands of farmed oysters were pulled out to protect the wild fishery in the strait.

The Ministry of Primary Industries announced on March 24 that it would begin further testing after three wild oysters from a small area in the strait were found infected with Bonamia ostreae during surveillance testing.

Biosecurity New Zealand said on Thursday that 972 samples from 39 sites had all tested negative for the parasitic disease.

However, Biosecurity New Zealand manager Dr Cath Duthie said it was still too early to interpret the results.

‘‘We are continuing to build our surveillance programme to understand more about what’s occurring in the strait,’’ she said.

‘‘While our genomic sequencing has shown the Foveaux Strait situation is most likely related to the Bonamia ostreae infections in Marlborough and Big Glory Bay, we still do not know how it got into the strait or the timing of that,’’ Duthie said.

The agency was building knowledge of what was happening, so it could make future decisions based on evidence, she said. Understanding where the infection originated would help MPI understand whether border and shipping processes needed to be reviewed.

Sanford general manager of aquaculture Ted Culley was pleased that MPI had moved quickly after discovering the infection and closed the affected area to harvesting. Testing results so far indicated that only a tiny area may be impacted, he said.

The company was among those that lost its farmed oysters in 2017.

Cully was not surprised to hear of the genetic relationship linking the infection to Marlborough and Big Glory Bay, and said it meant that this was not a new incursion, but rather a case of the parasite moving around within New Zealand.

‘‘It is unlikely that we will ever know how and when it was introduced into New Zealand and the order of spread. The challenge is to contain the spread in the wild fishery,’’ he said.

Barnes Wild Bluff Oysters manager Graeme Wright said the rough sea of the strait was a ‘‘saviour’’ because it meant less chance of the parasite latching on to more oysters.

He was still concerned about the impact on the fishery, given the devastation the disease had caused to overseas fisheries.

Depending on the extent of the infection, it could mean harvesters would need to test for both Bonamia exitiosa – which was endemic to New Zealand – and Bonamia ostrea in the future; which was costly and time-consuming, Wright said.

MPI is working with the National Institute of Water and Atmospheric Research to develop a surveillance programme for the detection of Bonamia ostreae in the Foveaux Strait fishery.

‘‘We need to ensure that it is scientifically robust to meaningfully inform any future activities,’’ Duthie said.