Exotic caulerpa washed up at Northland's Omākiwi Cove. People are being urged to keep an eye out for the marine invader this summer. Photo / Susan Botting
Exotic caulerpa washed up at Northland's Omākiwi Cove. People are being urged to keep an eye out for the marine invader this summer. Photo / Susan Botting
As efforts ramp up to control one of the worst marine invaders to reach our shores in years, Aucklanders are being urged not to make New Zealand’s caulerpa incursion worse this summer. Here’s what you need to know.
What is exotic caulerpa?
Caulerpa are a group of single-celled macroalgae set apart by their green photosynthetic fronds, which grow up to 10cm long in the shape of oar blades that rise from long runners or roots, known as stolons.
They can be found growing below the tideline between 2 and 30 metres on hard surfaces and in sandy areas.
New Zealand happens to be home to nine native species of this seaweed, but it’s two exotic species, naturally found in the Indo-Pacific region, that have been causing headaches since they were first detected in our waters three years ago.
Aotea Great Barrier Island's marine environment has been under threat from the highly-invasive exotic caulerpa seaweed since 2021. Here, it's pictured washed up at Okupu beach after Cyclone Gabrielle in February. Photo / Noel Nancekivell
Why is it such a nuisance?
The species is known to grow by as much as 3cm a day in warm, clear conditions to form vast, dense mats on the seafloor that smother other marine life.
In the Mediterranean, it took just six years to cause a 30 per cent fall in biodiversity and a 50 per cent drop in fish biomass. Now there are mounting concerns of caulerpa harming our own marine ecosystems.
“It is the very rapid and extensive growth that makes it such a major biosecurity threat for us,” Massey University’s Emeritus Professor Barry Scott says.
“If we don’t try to stop it spreading it will quickly expand from tens, to hundreds, to thousands of hectares, as has occurred in the Mediterranean.”
After being initially detected at Aotea Great Barrier Island’s Blind Bay in 2021, it has since spread to several other sites on the island, as well as Ahuahu Great Mercury Island, Waiheke Island and at Iris Shoal northwest of Kawau Island.
This map shows where exotic caulerpa seaweed has been detected in the Hauraki Gulf - so far. Source / Auckland Council
Further north, it’s also been confirmed at Rāwhiti Inlet in the Bay of Islands. But Aotea remains the worst affected area, with the seaweed having spread across hundreds of hectares of seabed.
What measures have been put in place?
Boaties are asked not to anchor or fish at Iris Shoal, Onetangi Bay and around Thompsons Point at Waiheke Island and between Ahikopua Point and Maunganui Point at Ahuahu Great Mercury Island.
Biosecurity New Zealand this year introduced Controlled Area Notices (Cans) at Aotea Great Barrier Island at Blind Bay, Whangaparapara Harbour, Tryphena Harbour and southern Port Fitzroy.
A Can is also in place at Rāwhiti Inlet, in an area bounded by Whau Point, the south-eastern tip of Te Ao Island, the eastern shoreline of Poroporo Island and the northern tip of Tokatokahau Point up to the high-tide area.
On top of these measures, which legally restrict some boating and fishing activities, mana whenua have imposed rāhui on Aotea Great Barrier Island and Waiheke Island with similar restrictions.
Caulerpa at Rāwhiti's Omākiwi Cove in the eastern Bay of Islands. Photo / NRC
While officials are trialling options to kill or knock back the seaweed, the scale of the incursion means that, at this stage, it’s unlikely New Zealand will be able to fully eradicate it.
How can I help stop caulerpa from spreading?
One of the main ways the pest spreads is by fragments hitching on to marine equipment like anchors, nets, dive and fishing gear, and crayfish pots.
It can also survive out of the water for a week or more in anchor lockers or bunched-up fishing nets, meaning it’s important boaties check their anchors, ropes and gear before leaving any anchorage site.
People who find any seaweed attached should remove it, bag or contain it securely so it can’t get back into the water, and take it ashore for disposal in a rubbish bin.
Those who suspect they’ve found exotic caulerpa should note the location, take a photo and report it to Biosecurity New Zealand on 0800 80 99 66, or online at report.mpi.govt.nz.
