A caulerpa suction dredge in action at Ōmakiwi Cove in the Bay of Islands. Photo: Supplied / Rana Rewha
A fast-growing seaweed described as the world's worst marine pest has spread to three new locations around the Bay of Islands.
Exotic caulerpa - which has smothered other forms of marine life when it has become established in other parts of the world - was first found at Aotea Great Barrier Island in 2021, then at Omakiwi Cove in Northland in 2023.
The seaweed pest has since been found at several other sites around the Hauraki Gulf, but until now had not spread beyond the original location in the Bay of Islands.
Northland Regional Council chairman Geoff Crawford said exotic caulerpa had recently been confirmed at Paradise Bay, on the western side of Urupukapuka Island; at Army/Waiwhapuku Bay, off Moturua Island; and at the southern end of Motukiekie Island.
He said the pest was discovered by a boatie who pulled it up on an anchor at Paradise Bay earlier this month.
Newly-discovered dense caulerpa meadow on sandy shoal at Motukiekie Island, Bay of Islands. Photo: Supplied / LDR / NRC
Since then, the council had sent divers around the island and worked with Biosecurity New Zealand to determine the extent of the seaweed's spread.
Previous dives found the area to be free of caulerpa as recently as April last year.
Yellow lines show the areas of newly-discovered exotic caulerpa off Urupukapuka, Motukiekie and Moturua Islands. An existing Biosecurity New Zealand 'Controlled Area Notice', where anchoring is prohibited, is indicated by yellow shading. Photo: Supplied / NRC
The unwelcome news came as Biosecurity Minister Andrew Hoggard announced a further $6.2 million to help the council continue developing a large-scale mechanical suction technique which aimed to remove the pest plant from the seabed.
The money would come out of last year's $10m allocation for battling the seaweed pest.
Crawford said it was not yet clear how exotic caulerpa had spread beyond Omakiwi Cove, but likely explanations included hitching a ride with an unsuspecting boatie or natural dispersal from another site.
"While this latest development is very concerning, our focus is on ensuring we continue to develop the tools that can fight exotic caulerpa. Without these we haven't any effective response."
Northland Regional Council chairman Geoff Crawford. Photo: Susan Botting / Local Democracy Reporter Northland
He said ongoing government investment in new technology was critical, and gave the council a chance to remove exotic caulerpa at scale and prevent further spread.
The funding would allow the council to continue working with Ōpua-based marine contractor Johnson Bros and mana whenua partners Ngāti Kuta and Patukeha hapū to remove exotic caulerpa in Omakiwi Cove.
Crawford said Johnson Bros founder Andrew Johnson had developed a world-first, large-scale suction dredge, which essentially vacuumed the seaweed from the sea floor using a barge-mounted digger with a custom-built dredge head.
Andrew Johnson, of Russell firm Johnson Brothers, with an early version of the caulerpa suction dredge he designed and built. Photo: RNZ / Peter de Graaf
The latest funding would allow the development of a new tool called a "submersible dredge planer" (SDP), which would operate remotely on the seafloor and aimed to remove exotic caulerpa in a single pass.
The new system would include a remotely operated SDP, dredge head, pump, GPS position system, dredge spoil processing plant, and disposal system.
Crawford said the advantage over the current barge system was that it would be able to move more quickly and accurately across the sea floor under its own power. It was also less likely to be affected by poor weather.
The concept was still in the development phase, but the new tool was expected to be tested at Omakiwi from September.
Crawford said the council appreciated the efforts of Ngāti Kuta and Patukeha hapū, who had worked closely with authorities since kaitiaki Rana Rewha first discovered the invasive pest in the Bay of Islands.
It was too soon to say how the find at Urupukapuka Island, a popular boating destination about 7km from Paihia, would be managed.
The council was liaising with Biosecurity New Zealand on the appropriate measures, Crawford said.
In the meantime, boaties and fishers were urged to be extra cautious.
Exotic caulerpa could get snagged on anchors and dive and fishing gear, accidentally moving it to new locations.
Simple steps to avoid that included checking the vessel's anchor and anchor chain, or any equipment used in the water, before leaving a location.
Any seaweed found should be removed, bagged or contained securely so it could not get back into the water, then taken ashore for disposal in a rubbish bin.
If it could not be securely contained it should be returned to the water where it came from.
Crawford said suspected sightings of exotic caulerpa, including any washed up on beaches, should be reported to Biosecurity New Zealand.
That could be done by taking a photo, noting the location, then calling 0800 809 966 or completing an online form at report.mpi.govt.nz.
Go to www.biosecurity.govt.nz/caulerpa for more information.
