The invasive Mediterranean fanworm has been found in Mangonui Harbour for the first time, prompting renewed calls for boaties to be extra vigilant about marine pests hitchhiking on their hulls.

Northland Regional Council biosecurity manager Don McKenzie said dive contractors discovered the infested boat last month during routine Northland-wide hull surveillance.

The find sparked a prompt response from the council and Biosecurity New Zealand with the 13.5m ketch ordered out of the water for immediate cleaning.

Diving contractors subsequently found and removed six individual fanworms from the seafloor directly under the vessel.

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Other sites and structures in the wider area were also searched and the owners of nearby moorings were being notified, he said.

"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."

Divers would continue searching the area but, apart from the initial finds, so far all other moorings, structures and vessels in the area had 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 remove any fanworm from the area."

Timing was crucial when it came to containing Mediterranean fanworm (Sabella spallanzanii), McKenzie said.

Mediterranean fanworm takes its name from the fan-like crown of feeding tentacles that extend from its tube. Photo / NRC
Mediterranean fanworm takes its name from the fan-like crown of feeding tentacles that extend from its tube. Photo / NRC

The invaders could form dense beds and force out other species, including natives, and interfere with their breeding. They smothered shellfish such as mussels and hindered oyster farmers.

McKenzie urged boaties to make sure their vessels and equipment were clean and free of fouling, especially when travelling from areas with established fanworm populations.

Under NRC rules it is an offence to transport marine pests in Northland.

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Vessels entering Northland or moving between harbours must have no more than light fouling, which is defined as a slime layer and/or barnacles with up to 5 per cent macrofouling.

Mediterranean fanworm is already well established in Auckland's Waitematā Harbour and is present in Whangārei Harbour and Marsden Cove. In 2018 more than 1000 fanworms were found at Ōpua.

The pest has been found but eradicated at Tutukaka and located on a vessel only at Whangaroa Harbour.

McKenzie said Northland had been battling the pest since 2012 and had so far kept it out of all but a few harbours. Regions which had been less proactive were feeling the effects of infestation, for example on oyster farms.

Most Northland boaties took care with their vessels with only a small minority putting others, and the environment, at risk.

■ Call the NRC as soon as possible on 0800 002 004 or email marinebiosecurity@nrc.govt.nz if you encounter marine pests. Go to www.nrc.govt.nz/pestcontrolhub and www.marinepests.co.nz for more information about marine pests and the council rules.