Didemnum vexillum is a spongey textured, light mustard coloured marine organism originally brought to New Zealand on the hull of a steel logging barge from the Philippines. It thrives on underwater surfaces like wharf piles, boat bottoms, mussel lines and salmon cages. When a mussel line is smothered by the pest, the shellfish are eventually dragged off the ropes and onto the seafloor.

By 2005, the pest was spreading through the Marlborough Sounds and an industry-led Didemnum Working Group was set up to mastermind a clean-up. Aaron Pannell, from the Marlborough Mussel Company led the project which used plastic wrap and sleeves to cover underwater structures and mussel lines to stop Didemnum feeding and breeding.

“But the problem with plastic wrapping is the collateral damage on mussels – everything underneath the plastic is killed - so we began a research and development effort to find a simple, effective and safe way of controlling Didemnum,” he says. The industry was particularly interested in a method to eliminate Didemnum from mussel seed stock so that the pest was not spread when mussels were transferred between growing areas.

Both Aquaculture New Zealand and the Foundation for Research, Science and Technology contributed funding totalling NZ$42,000 for research to address the issue. Senior Scientist Dr Barrie Forrest, who heads the Marine Biosecurity Group at Cawthron, trialled a range of possible treatments, building on knowledge from previous research into biosecurity management tools that had been funded by the Foundation and the Ministry of Fisheries.

“Treatments included dipping mussel seed stock in supermarket-strength concentrations of vinegar and also bleach. Both were effective against Didemnum but the risks were high – if the seed stock was treated too long or the mix was too strong, it killed the mussels,” says Dr Forrest.

He had earlier explored the possibility of using freshwater and went back to the laboratory to test its effectiveness, with excellent results. “We found that Greenshell mussels have a surprisingly high tolerance to freshwater – you can leave them in it for as long as three days with very little effect on their survival and we have successfully killed Didemnum by immersing infected seed stock in water for as little as one hour.”

The solution is said to be ideal for treating seed stock which is thinned by being stripped off mussel lines at regular intervals and can be immersed in fresh water before being re-attached.

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