Superspreader seaweed caulerpa to be dredged in Northland's Bay of Islands

A Northland council and local iwi are working on an innovative plan to attack an exotic seaweed infestation in the Bay of Islands with a large-scale dredging operation.

The invasive seaweed species Caulerpa brachypus and Caulerpa parvifolia were discovered mid-2021 at Aotea Great Barrier Island and Ahuahu Great Mercury Island.

The seaweeds can dramatically alter marine habitats by smothering shellfish beds and replacing native seagrasses.

They were first spotted in Northland at Te Rāwhiti, in the eastern Bay of Islands, on May 3, and later found in at least 10 other nearby sites.

In June, Biosecurity New Zealand put a Controlled Area Notice in place in the eastern Bay of Islands, helping stop the spread of the seaweed by making it illegal to fish or anchor vessels, in an area around 4km wide.

Northland Regional Council and NIWA dive teams have made a huge effort to gauge the infestation, checking a sea floor area of more than 22 hectares, council marine biosecurity manager Kaeden​ Leonard​ told a biosecurity and biodiversity working party on Tuesday.

The divers have treated the invasive seaweed infestation with plastic mats imbued with chlorine, while Biosecurity NZ and NIWA have also trialled salt application at small sites at Aotea.

Now, the council and hapū from Ngātiwai hope to treat large sites by using a large-scale suction dredging operation, which has been successful overseas, Leonard said.

Biosecurity Minister Damien O'Connor agreed to fund $1 million for the Te Rāwhiti incursion, with $800,000 to go directly on the dredge.

Initially, the Ministry for Primary Industries wanted this contract to be for research, which includes precise restrictions on how the suction is used, as well as strict controls to compare and analyse its effectiveness against the mats, Leonard explained.

This sort of research may be done at Aotea, but the council and iwi did not want treatment to be so rigid in Northland to ensure a greater chance of success.

They have now agreed with the ministry for an approach which can change according to empirical evidence, on sites of infestation greater than 1ha.

Leonard said the suction dredge would not be like a tradition sand dredge, which sucks up tonnes of sand – it is more sensitive and can be adjusted to avoid sucking up the likes of scallops.

Caulerpa is the most significant marine invader to arrive in Northland, he said.

“The current caulerpa proliferation will diminish access to kai moana and significantly impact ecological biodiversity, and our way of life.”

Further funding from Biosecurity NZ could be used to survey high-risk anchorage sites in Northland, Leonard said.