Checks on more than 1000 boat hulls around Northland this summer are showing improved results in the fight against unwanted hitchhiking marine pests threatening the region's marine environment.

"The boats are way cleaner overall than they have ever been," Kathryn Lister, Northland Regional Council's biosecurity manager – marine, said.

"But people still need to pay attention to making sure [marine pest] niche areas on the boat's hull are better checked," Lister said.

Niche areas included the bottom of the boat's keel, propellers, bait tanks and other areas.

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"These areas require special attention when applying antifouling or during a boat's 'lift and wash' period. We strongly recommend double-checking them before a vessel goes back in the water after being hauled out," Lister said.

Boat hulls are the major culprit in spreading unwanted marine pests.

Northland-based Marine Environmental Field Services dive contractors are half way through checking about 2000 boat hulls in an eight-month Northland inspection that began in September and runs until May.

They have checked 1119 boats across Northland harbours and boating anchorages, up to 40 a day. Vessels checked include recreational boats, fishing vessels and barges. The boats come from Northland, nationally and overseas.

Inspections have been done throughout the Bay of Islands, Cavalli Islands plus Mangonui, Houhora and Whangaroa harbours. Divers have also inspected in Ngunguru Estuary, Tutukaka and Whangarei, Kaipara and Mangawhai harbours.

The checks looked for two things – the presence of a specified handful of marine pest nasties that like to grow on boat hulls and the overall level of slime and other general hull growth.

Lister said just 21 boats to date had been found to have the specific targeted unwanted hull marine pests being searched for.

This season's findings to date compared with 130 boats overall in the 2018/2019 season.

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Lister said this season's cleaner hulls overall reduced the risk of marine pest nasties establishing.

That was good news for Northland as boat hulls offered marine pests an easy opportunity to spread.

Pests being checked for under the Northland Regional Pests and Marine Pathway Management Plan 2017-2027 were Mediterranean fanworm (54 cases found), a highly invasive seaweed undaria (one case found), two types of sea squirts (Styela clava (164 cases found) and Pyura doppelgangera (no cases found), plus eudistoma (no cases found).

Divers will continue to inspect boats for another four months, checking 2000 all up by the end of the season.

The check is part of a growing fight against unwanted marine pests.

All boats coming into Northland or moving from one harbour to another must now have clean bottoms under the Northland Regional Pest and Marine Pathway Management Plan. This means they must have no more than "light fouling" on their hulls, defined as a slime layer and/or barnacles and just a few patches of macrofouling.

As well, all Northland marinas now require clean hulls' evidence before allowing boats to use their facility. Boats must provide evidence they had applied antifouling to their boat hull within the previous six months or hauled their boats out of the water for hull cleaning within the previous month.

Lister said continuing effort was being put into working with the marine sector towards strengthening boat hull cleaning management in the haulouts and hardstand areas where marine pests were water-blasted from hulls.

She said it was important for boaties applying antifouling to their hulls to do so in line with manufacturers' guidelines. Allowing enough time for the antifouling to dry hard before the boat went back into the water was also important in the fight against marine pests.