The discovery of Mediterranean fanworm (Sabella spallanzanii) on a Mangonui-based boat has prompted renewed calls from Northland Regional Council for boaties to be extra vigilant for unwanted marine pest hitchhikers, especially when travelling to areas known to have the pest.

Biosecurity manager Don McKenzie said divers discovered the infested boat moored in Mangonui Harbour last month, as part of a regular Northland-wide hull surveillance programme, prompting a demand by the council, and Biosecurity New Zealand, that the 13.5m ketch be taken out of the water for immediate cleaning.

Divers had subsequently found and removed six fanworms from the seafloor directly under the vessel.

Mr McKenzie said mooring owners in the area were being notified of the find.

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"At this point we still don't think there's an established local population of this pest, but we'll be continuing active surveillance in the lead-up to winter so we can re-evaluate things if necessary," he said.

Divers would conduct regular searches, but follow-up searches of other moorings, structures and vessels in the wider area had all been fanworm-free.

"At the moment we're hopeful things have been caught early enough that we can accurately determine the extent of any infestation and successfully remove any fanworms from the area," he added.

Mr McKenzie said timing was crucial when it came to attempts to contain fanworm, which could form dense beds, forcing out other species.