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Boaties in the Mahurangi region are being urged to be on the alert and take precautions after small patches of caulerpa, the fast-growing invasive seaweed whose discovery in waters further north has been alarming marine experts for two years, were found near Kawau Island
Biosecurity New Zealand readiness and response director John Walsh said on Friday (July 7) the agency would be meeting with Auckland Council and the Ngāti Manuhiri Settlement Trust within days to discuss the development and an urgent response.
“With their agreement, we plan to begin some control work as soon as possible as well as further surveillance in the area.”
Walsh urged boaties and others to play their part in preventing the spread.
“We’re asking people to be vigilant by keeping boats and other aquatic equipment clean to reduce the risk of introducing or spreading any exotic seaweeds,” he said. “Check your gear, especially anchors and chains. If you see any seaweed on your equipment, chuck it straight back in the waters it came from.”
Biosecurity NZ says anyone who thinks they’ve seen caulerpa to note the location, take a photo, and report it to the Pest and Disease Hotline on 0800 809966 or at report.mpi.govt.nz.
The small patches of the pest, 20-30 centimetres in size, were found by divers in the channel north-west of Kawau.
The find was “disappointing but not unexpected given the nature of the seaweed,” Walsh said. “Caulerpa can easily be spread to new locations as it’s commonly caught up as tiny pieces of seaweed on vessel anchors and fishing gear.”
According to Hauraki Gulf Forum executive director Alex Rogers, in locations overseas, caulerpa has matted the seafloor and reduced marine biodiversity by up to 50 percent.
“It’s devastating news that caulerpa has been found off Kawau,” Rogers says. “The forum strongly supports Biosecurity NZ and Ngāti Manuhiri taking immediate action to stamp it out.”
“Time is very much of the essence here: caulerpa is incredibly fast growing and smothers the seafloor, suffocating our native marine life. It also impacts the shoreline.”
Rogers recalled that during Cyclone Gabrielle an estimated 100 tonnes washed up on Blind Bay at Aotea Great Barrier Island, “coating the beach”.
“Imagine that at Omaha. While efforts to eradicate it kick into high gear, our message to boaties and fishers is to please be careful off the north-west of Kawau to avoid any further spread. Fragments of Caulerpa as little as 2mm can re-establish off a hook or an anchor at another location.”
“The forum has been consistent and clear on caulerpa for over a year now – this invasive pest is a serious threat to the gulf, and to our way of life, and must be eradicated.”
Caulerpa – the name comes from the ancient Greek and literally means ‘stem that creeps’ – was first found in New Zealand waters at Aotea Great Barrier Island in 2021. It has also appeared at Great Mercury Island and, earlier this year, in the Bay of Islands.
“Internationally, Caulerpa has not been successfully eradicated from areas the size of the finds in New Zealand, and climate change and warming oceans also present a tough biosecurity challenge for everyone.” Walsh said.
Biosecurity NZ was considering the use of chlorine pellets beneath nailed down tarpaulins, a response now being tested by the Northland Regional Council in the Bay of Islands. Suction dredging to remove the seaweed is also under consideration.
Aside from immediate work in the Kawau Island area, new in-water treatment trials will start shortly in the Bay of Islands, Aotea Great Barrier and Great Mercury islands, running through to the early summer.
“People in those areas want Caulerpa eliminated. This will be very challenging given the many hectares of Caulerpa involved, but these trials are geared towards that.”
“We are supporting the Ngāti Manuhiri Settlement Trust, mana whenua and mandated iwi authority, to bring experts from California to Aotearoa New Zealand early next month to provide first-hand insights about using suction dredge techniques to remove Caulerpa,” Walsh said. “And we’ve also set up another expert technical advisory group to revisit the latest information on this suction technique.”
