File photo. A marine biosecurity officer checks a caulerpa treatment trial in the Bay of Islands in 2023. Photo: Supplied/ Brett Sutton, Marine Environmental Field Services
A team of divers is in the water this week hoping to destroy the first patch of the invasive seaweed caulerpa found near mainland Auckland - but first they have to find it.
A roughly one square metre patch of exotic caulerpa was found in Leigh's Omaha Cove, north of Auckland, by divers carrying out routine surveillance on 7 June.
The fast-growing weed, often described as the world's worst invasive marine pest, is already well established at Aotea Great Barrier with other infestations around the Hauraki Gulf, the Bay of Islands and the Coromandel Peninsula.
However, when a different dive team went back to Omaha Cove, the patch of caulerpa seemed to have gone AWOL.
Liz Brooks, of Auckland Council, said divers returned to the water on 12 June to treat the patch, but could not locate the caulerpa despite having GPS coordinates of the initial find.
Divers resumed their search this week and, once the weed was located, would cover it with mats and dose it with chlorine.
That technique had proven effective for treating small areas of caulerpa in the past.
"Divers will also continue to look for any other patches of exotic caulerpa, in addition to checking for other marine biosecurity pest presence in these areas," Brooks said.
File photo. A warning sign about caulerpa on Great Barrier Island. Photo: Luka Forman
Ngāti Manuhiri Settlement Trust was closely involved and contributing to discussions about the appropriate response and swift treatment to remove the invasive weed.
Brooks said it was the first time exotic caulerpa had been found close to mainland Auckland.
"We hope to control it and remove the threat, but we know the challenges of dealing with unwanted organisms in the dynamic marine environment - and exotic caulerpa in particular."
It was a good reminder for all boaties to play their part in stopping the spread of caulerpa, she said.
"Stay away from known areas of exotic caulerpa so you don't cause it to spread elsewhere. If you do bring it up on anchor or hook, bag it and bin it, and if you find it on the beach, 'pin, pic, and phone'. That is, note the location, take a photo, and report on 0800 80 99 66 or online at report.mpi.govt.nz."
RNZ understands the patch was initially found in shallow water near boat moorings in Omaha Cove.
The cove also has a jetty used by commercial fishing boats.
Omaha Cove is a short distance south of Te Hāwere-a-Maki/Goat Island Marine Reserve and Auckland's University's Leigh Marine Laboratory.
The seaweed found on 7 June was confirmed as exotic caulerpa by Niwa experts.
It grows quickly and forms dense mats across the sea bed that smother other forms of marine life.
Its effects on New Zealand ecosystems are not yet well understood, but where it has become established overseas - in the Mediterranean, for example - it has wiped out shellfish beds and caused sharp declines in biodiversity and fish numbers.