The Northland Regional Council hasn't abandoned its goal of eradicating the Mediterranean fanworm at Ōpua, but is now reviewing the situation to ensure it makes practical and financial sense to continue after divers removed more than 1000 of the creatures from the water in the latest search and removal operation.

Biosecurity manager Don McKenzie said favourable conditions had enabled contract divers to work on 20 successive days in August, continuing the eradication effort that began after the discovery of a single specimen in winter last year.

The August dives, partly funded by Biosecurity NZ, were the latest in a series as part of a stepped approach to try to remove the pests, divers carrying out a removal operation then standing down while the results and associated data are examined by biosecurity experts and the best management approach is decided on.

"Our councillors felt strongly that the Bay'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," Mr McKenzie said.

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"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."

The first response, completed in late April, had yielded 267 fanworm, the second, which traversed a more targeted area, producing 1013 over 20 days, 90 per cent of them found on artificial structures, 8 per cent on the sea floor, and the remaining handful had been removed from moorings and reefs.

They typically ranged from 30mm to more than 400mm long, averaging about 120mm.

Mr McKenzie said the maturity, numbers of fanworm and their spread had reinforced earlier indications they may have been in the area, probably having hitchhiked in on a visiting vessel with a dirty hull, for several years.

The results of the latest operation had already been reported to key stakeholders, and regional councillors had also been kept abreast of developments, including an update at a recent monthly meeting in Whangārei. A decision on whether to continue the operation via a third step, likely to cost about $100,000, was expected shortly.

In the meantime authorities were continuing to urge people in and around Ōpua to ensure they did what they can to avoid spreading fanworm.

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

The species is the biggest fanworm found in New Zealand, with a body up to 20mm wide and 800mm long. A prominent crown of fan-like feeding tentacles, up to 150mm wide, extend out of the tube, while the crown is often banded orange, purple or white. Tubes are leathery, flexible and muddy-looking, and they are generally found on hard, sub-tidal structures, but can also be buried up to 10cm deep in soft substrates.

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They can quickly form dense colonies, forcing out native species and interfering with their ability to feed and breed. They have also been found growing on scallops in Whangārei Harbour, and have the potential to spread on to other shellfish, including farmed species.

Under regional council rules it is an offence to transport marine pests in Northland. Vessels entering the region and moving between harbours must have no more than light fouling, defined as a layer of slime and/or barnacles, and up to five per cent 'macrofouling, large, distinct multi-cellular organisms visible to the human eye, such as barnacles, tubeworms, or fronds of algae.

Permissible macrofouling excludes pest species, including Mediterranean fanworm, clubbed tunicate and the Asian kelp undaria.

With the summer months looming, Mr McKenzie urged everyone who encountered marine pests to notify the regional council as soon as possible (phone 0800 002-004, email marinebiosecurity@nrc.govt.nz, or to call Biosecurity NZ's pests and diseases hotline, 0800 809-966.

"It's important that you don't try and remove any fanworm you may discover yourself, as this must be done by authorised personnel, because fanworm need to be removed intact and 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," Mr McKenzie said.