Future of oyster farming remains uncertain

    An oyster farm in Stewart Island's Big Glory Bay ahead of the oyster cull operation last week.

    AS the operation to cull all farmed flat oysters from Stewart Island’s Big Glory Bay nears the end of its second week, the question of whether the industry will be able to recover remains uncertain.

    The removal process to prevent the spread of the lethal parasite Bonamia ostreae began last Monday. Bonamia ostreae is fatal to flat oysters, but poses no risk to humans.

    Ministry for Primary Industries (MPI) field headquarters manager Andrew Sander said the first phase of the operation, which was the removal of about 700 cages of farmed oysters, had been completed ahead of schedule.

    of the operation would involve lifting the last cages and then removing the ropes, an alternative method for farming oysters.

    How long the process would take to complete was not known at this stage.

    National Institute of Water and Atmospheric Research (NIWA) fishery scientist Keith Michael said what happened after the farmed oysters were removed would be determined by MPI.

    Overseas experience had shown once oyster-growing areas were infected with Bonamia ostreae, repopulating the area with farmed stock maintained a continued risk of spreading the disease because the disease could persist for long periods in the absence of an oyster host, he said.

    In the northern hemisphere, all oysters and oyster structures were removed from sites affected by Bonamia ostreae and repopulated with parasite-free hatchery stock six years later. Within eight months, researchers found disease mortality, he said.

    Dying oysters substantially increased the probability of spread. Continued farming of oysters in Big Glory Bay under such a scenario would have put the Bluff oyster fishery at risk, he said.

    Oyster options discussed

    MR Michael was concerned about misinformation being presented about disease-tolerant stock. Infected but tolerant stock posed the same risks as infected stock, he said.

    The full life cycle of the parasite was not known, but it was known Bonamia ostreae cells could live in the seawater and in seabed sediments, and be held by carrier organisms such as mussels, but how long the cells survived was unknown, he said.

    One difference between the Big Glory Bay situation and Marlborough Sounds was MPI had detected infection early enough so that no abnormal mortalities had been reported from Stewart Island oyster farms, he said.

    The rationale for the Stewart Island response was to remove the oysters before there was mortality, which was when the parasite’s infected particles were released. If the response was successful, the disease could be contained, but there would need to be further testing to provide evidence of potential eradication, he said.

    “If Bonamia ostreae infection is eradicated from Big Glory Bay, it will be the first eradication from an infected site in the world.”

    Sanford operated two oyster farms at Stewart Island and was the second-largest quota holder of the Foveaux Strait wild oyster fishery.

    Sanford corporate communications general manager Fiona MacMillan said whether the company would want to farm oysters at Stewart Island again in the future would come down to what MPI and science said was possible around when, how and what variety of oysters might be able to be farmed there.

    MPI held public meetings at Stewart Island and Bluff last week to outline the intervention and answer locals’ concerns.

    At the Bluff meeting, MPI readiness and response director Geoff Gwyn said there would not be any oyster farming in Big Glory Bay in the foreseeable future.

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