Biowar heats up on sea squirt threat to mussels

Biosecurity New Zealand is stepping up moves to contain a sea squirt described as a big threat to mussel farms.

Styela clava - a cylindrical pest up to 16cm long - was officially detected in New Zealand in August last year.

Checks have shown it is widespread in the Hauraki Gulf, with smaller populations in Lyttelton Harbour and Tutukaka in Northland.

Anecdotal evidence suggests it may have been in Auckland for up to 10 years. Biosecurity NZ senior marine adviser Brendan Gould described the squirt as "potentially quite a big threat" to mussel farms.

The pest competes for space and food, and can blanket mussel lines, suffocating shellfish.

The major areas for mussel farming in New Zealand are Marlborough, Coromandel and Stewart Island.

Mussel farming is also going on in Northland, Kaipara, Great Barrier Island and Banks Peninsula, and there is potential for further expansion around the country.

Gould said that in Canada's mussel industry the pest had added to processing costs because it had to be removed from lines and was dislodging mussels.

Canada had used a variety of containment techniques such as letting lines sink to the bottom, where juvenile squirts could be eaten by other marine life.

Trials with chemicals in Auckland have been promising for containing smaller areas of contamination but would not necessarily work in a bigger area, Gould said.

Keeping vessel hulls clean of fouling is a key way of stopping the spread of the squirt in New Zealand.

For the next three months, Biosecurity NZ will research small-scale containment measures at Lyttelton and Tutukaka. It will also check further areas for the squirt.

The results of the research will help the organisation develop a pest-related budget bid for next financial year, Gould said.