The discovery of Mediterranean fanworm at Ōpua has prompted renewed calls from biosecurity experts for boaties to be extra vigilant for unwanted marine pest hitchhikers.

Northland Regional Council biosecurity manager — marine and strategy Sophia Clark said officials had been alerted to the discovery of a single fanworm (Sabella spallanzanii) by mooring contractors, growing on a mooring block opposite the Ōpua marina.

The council had deployed two teams of Northland-based diving contractors, who had recovered about a dozen more in three locations over a several-hundred square metre area.

Ms Clark said they had probably came from an unidentified infected vessel that, based on the size of some of the specimens, had potentially moored in the area some time ago.

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Both the marina and mooring owners in the area had been notified of the find.

"Divers will continue to search on a regular basis over the next couple of months and remove any fanworm they find, but so far searches of other moorings and vessels in the wider area have been fanworm-free."

"At this point we're hopeful the situation has been caught early enough that we can accurately determine the extent of any infestation and remove any fanworm from the area."

Ms Clark said timing was crucial when it came to attempts to contain fanworm, which were unwanted because they could form dense beds, forcing out other species (including natives) and interfering with other species' natural processes, like breeding.

"Eradication of fanworm in newly-infested areas is possible, but only if the population is caught early," she added.

"As an example, we're now only about a year away from being able to officially declare fanworm eradicated from Tutukaka Harbour following an incursion there several years ago."

Officials were grateful to the mooring contractors who detected the Ōpua fanworm and reported it to the council.

NRC had reiterated its earlier messages to boaties that it was vital to ensure their vessels and any associated equipment was clean and free from fouling that could contain marine pests like fanworm.

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"Under regional council rules, it's an offence to transport marine pests in Northland," Ms Clark said. Vessels entering Northland and moving between harbours must have no more than light fouling — defined as a slime layer and/or barnacles, and up to five per cent macrofouling.

She urged anyone who found a marine pest to notify the regional council by phoning (0800) 002-004 or emailing marinebiosecurity@nrc.govt.nz