Double trouble from oyster pathogens

The spread of Bonamia ostreae from Marlborough Sounds to oyster farms in Big Glory Bay (Stewart Island) could spread to the valuable wild oyster population.

The public meeting was standing-room only. It was a gloomy evening in the small port community of Bluff. The mood was sombre.

After prying the oyster’s shell open, fisheries researcher Jeff Forman delicately extracts the oyster's heart which, at barely 1-2mm in diameter, contains the richest source of red blood cells necessary for bonamia testing.

The public were there to meet with local authorities, a Ministry for Primary Industries (MPI) representative and NIWA fisheries scientist and oyster expert Keith Michael. The topic for discussion was the spread of Bonamia ostreae from Marlborough Sounds to oyster farms in Big Glory Bay (Stewart Island). The 2015 discovery of B. ostreae in oysters farmed in the Marlborough Sounds was the first time this parasite had been found in the southern hemisphere. The intense interest arose because this parasite has been disastrous for oyster farmers and fisheries in the northern hemisphere.

While there was no risk to people from eating infected oysters, the parasite could kill the oysters in the farms, and the infection could spread to the valuable wild oyster population – where it was likely to have a devastating effect. Devastating because oysters have been fished from Foveaux Strait for more than 150 years, and the fishery is one of our oldest and most iconic.

Bluff Community Board Chairman Ray Fife said many attendees had worked with oysters all their lives. "We've depended on oysters for the wellbeing of Bluff."

He said these long-term interests meant stopping the spread of the infection was the priority. "We still want to have generations of fishermen that go out there and catch their oysters, for the next 100 years."

Substantial biosecurity risk

Keith Michael explained why the parasite poses a substantial biosecurity risk to the nationally important Bluff oyster fishery and to the Southland environment. It can cause 90% mortality in oyster populations that haven’t been exposed to the disease before.

"It is a particularly destructive parasite of flat oysters. It is a waterborne disease. It enters the oyster’s blood through the gills, mantle and gut, and multiplies rapidly. Intense infections cause blood cells and tissues to break down or rupture."

Molecular Biologist Judy Sutherland and oyster specialist Keith Michael prepare samples for testing. NIWA is making use of advances in molecular technology to develop a better understanding of shellfish diseases. In the near future NIWA's new droplet digital PCR (ddPCR) system will offer even more sensitive detection, and better quantification than current methods.

Since 2000, NIWA, the Bluff Oyster Management Company and MPI have carried out research on a closely related, endemic parasite (Bonamia exitiosa). Over the years, this parasite has caused substantial mortality in the Foveaux Strait oyster population and economic losses to the fishery – oyster mortality from this disease is between 10 and 50 times larger than the commercial catch.

NIWA has developed considerable capability and experience in B. exitiosa research, and NIWA and MPI science have worked closely to inform the MPI response to the Marlborough Sounds B. ostreae infection, and to carry out surveillance programmes to monitor any spread. It was an MPI designed surveillance programme, carried out by NIWA that detected the spread of B. ostreae to Stewart Island. Sampling in Big Glory Bay as part of this surveillance programme detected the presence of B. ostreae. This early detection—before any oyster mortality had been reported to MPI—at least provided an opportunity to contain the disease.

This appearance of B. ostreae, however, came just when things were looking up for the Foveaux Strait oyster fishery. The 2017 NIWA and Bluff Oyster Management Company stock assessment and bonamia surveys found that another disease cycle—during which wild oyster numbers halved—had ended. Disease mortality had declined to low levels over the last two years and recruitment had increased.

However, Keith Michael says that a combination of the two bonamia species (B. ostreae and B. exitiosa) was likely to pose an even greater risk to the Bluff oyster fishery.

The two Bonamia species look very similar in size and structure, so it has been a major challenge to distinguish between them. The most reliable way of telling them apart is by using molecular methods targeting species-specific DNA.

New method for the detection of bonamia

NIWA biotechnologists have designed a new method for the detection of bonamia in oysters from raw extracts.

indicate positive results for the presence of both Bonamia ostreae and oyster DNA.

This cost-effective approach can detect low-level infections, and identify this specific pathogen from other, closely related species. "We use a variation of this method to test for the presence of B. ostreae for MPI-run surveillance programmes," Michael says. "We screen large numbers of samples quickly, and provide the samples that test positive for B. ostreae to MPI for confirmatory testing".

NIWA's capability here goes well beyond detection. Seafood Innovations Limited, who provide Ministry of Business, Innovation & Employment research funding to the fishing industry, funded the development of a new method to quantify bonamia infections. As a result, NIWA is making use of advances in molecular technology to open new research areas to develop a better understanding of shellfish diseases. The new method will measure the intensity of infection to track the disease progress through oyster populations, and distinguish between non-fatal and fatal infections.

The community meeting in Bluff was followed by an MPI decision to remove all the farmed oysters from Stewart Island and the Marlborough Sounds. At the time of writing, farmed oysters and cages have been removed from Big Glory Bay, and the removal of lines growing oysters and mussels together is progressing well. The removal of farmed oysters from Marlborough Sounds is also progressing well.

Everyone is hopeful that removing the farmed oysters will help prevent the spread of the parasite. MPI has decided to move to three monthly surveillance for the presence of B. ostreae in Southland and remain as biennial for other areas. NIWA would work with the MPI biosecurity team to undertake these surveys.

[This feature appeared in Water & Atmosphere 19]