Only a year after a single Mediterranean fanworm was found in the Ōpua Marina a recent search and destroy mission found more than a thousand of them.

A dive crew of several people spent 20 straight days on the operation in August, in a joint-funded Northland Regional Council (NRC) and Biosecurity New Zealand effort.

The dives were the second in a two-stepped approach to try to remove a localised infestation of highly invasive fanworm (Sabella spallanzanii), NRC biosecurity manager Don McKenzie said. The first was in late April, yielding 267 fanworms, with this second search of a more targeted area resulting in 1013 worms.

A decision on whether to continue with a third step – likely to cost about $100,000 – would be made shortly.

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Despite the speed of the pest's spread, the NRC believes there is a chance of success in eradicating it from the Ōpua area, and it is vital to try, McKenzie said.

''We've still got 13 harbours in Northland that are fanworm-free. Fanworm are all through Whangārei Harbour and a few years ago they were in Tutukaka. Okay, Tutukaka's only a small harbour, but divers got in there and cleaned it out,'' McKenzie said.

"Our councillors felt strongly that the Bay of Island's importance across multiple fronts – including environmentally, economically and culturally – meant they couldn't pass up what may still prove to have been our only real opportunity to eradicate fanworm there.

"But with both the council and Biosecurity NZ already having collectively contributed several hundred thousand dollars to the removal efforts, essentially, we're reviewing the situation very carefully as we go to make sure it makes practical and financial sense to continue."

Data collected during the latest operation will help with that decision.

One of the Opua fanworms recovered earlier this year.
One of the Opua fanworms recovered earlier this year.

The maturity, numbers and spread reinforces earlier indications they may have been in the area for several years, after probably hitchhiking in on a visiting vessel with a dirty hull, McKenzie said.

Of the August haul, 913 individuals were on artificial structures, 85 on the seafloor and the rest on moorings and reefs.

A Ministry of Primary Industries survey of Ōpua Marina and Waikare Inlet over a few days also in August, primarily looking for new-to New Zealand marine species, found and destroyed 139 Mediterranean fanworm. No new species were detected.

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Authorities urge people in and around Ōpua to ensure they avoid spreading fanworm.

"It's vital boaties ensure their vessel and any associated equipment – moorings and their rope/chain and fishing nets – is clean and free from fouling which may contain marine pests like fanworm," McKenzie said.

"Under regional council rules, it's an offence to transport marine pests in Northland. Vessels entering Northland and moving between harbours must have no more than light fouling on their vessel.''

Mediterranean fanworm - an unwelcome, increasingly present inhabitant of Northland waters.
Mediterranean fanworm - an unwelcome, increasingly present inhabitant of Northland waters.

Mediterranean fanworm is the largest fanworm in New Zealand, with a body up to 20mm wide and 800mm long. It has a banded orange, purple or white crown of fan-like feeding tentacles extending out of a leathery, flexible tube.

It can quickly form dense colonies, forcing out native species. Sabella have been found growing on scallops in Whangarei Harbour and can potentially spread on to other shellfish, including farmed species. Their removal must be done by authorised personnel because they can spawn under stress.

"If you see a Mediterranean fanworm or suspect it on your hull, structure or substrate, please take a photo and call or email us with the location," McKenzie said.

To report, ph (0800) 002 004, email marinebiosecurity@nrc.govt.nz or call Biosecurity New Zealand's Pests and Diseases hotline, (0800) 809 966.

A fanworm on the bottom of a barge.
A fanworm on the bottom of a barge.