Invasive seaweed caulerpa found in new location in Bay of Islands

Caulerpa on the Paroa Bay seabed. Photo / Conquer Caulerpa Charitable Trust

By Luka Forman - RNZ

The invasive seaweed caulerpa has been found in a new location in the Bay of Islands, sparking fears it could spread quickly through boating hubs around Paihia and Russell.

The exotic weed was first discovered in New Zealand on Great Barrier Island in 2021, and has raised concerns because it spreads quickly and smothers all marine life.

It has now been discovered by divers in Paroa Bay, only a few kilometres away from Russell in the Bay of Islands.

Caulerpa on the Paroa Bay seabed. Photo / Conquer Caulerpa Charitable Trust

The Government recently spent $10 million on new technology to help fight the weed, but a local charity said more was needed to help monitor and eradicate it.

Verdon Kelliher is chair of the Conquer Caulerpa charitable trust, which was set up to fight the invasive weed in Northland.

The divers they have contracted to monitor it recently discovered a new infestation in Paroa Bay.

“It starts at 30 metres out from the shore, and it’s pretty dense out to around 300 metres. It’s taken a pretty good foothold on the ocean floor, ranging up to around 15 centimetres,” Kelliher said.

If it spread to nearby bays around Russell and Paihia, that would be bad news, Kelliher said.

“That’s obviously a main thoroughfare going through there. You could imagine it could easily spread to multiple other places as the boats that are coming in and out of there go to lots of different destinations,” he said.

The trust recently raised $50,000 for survey work, but contract divers did not come cheap, Kelliher said.

“They cost us $5600 a day, so you can see how long they will last on the money we have and then we run dry.”

The new discovery of caulerpa underscored the importance of continued surveillance to find out where else the weed has spread to, he said.

He was also concerned about what would happen after the Government’s $10 million fund to trial new dredging, UV light and chlorine treatment technology ran out.

“What’s not known very well by anyone is there’s actually no money post those trials to actually use them. And there’s no guarantee that the Government is going to fund it in its budget for next year.”

Michelle Elboz (Ngati Kuta, Patukeha) is a trustee on the Conquer Caulerpa trust and said the spread of caulerpa threatened the Māori way of life in the Bay of Islands.

Caulerpa on the Paroa Bay seabed. Photo / Conquer Caulerpa Charitable Trust

“The waters out here are our kai basket. That’s where we get a lot of our food, the fishing, the shellfish, it’ll all be gone.”

The trust is hoping to raise $600,000 to keep divers in the water for six months - which Elboz admitted would be difficult, but necessary.

“The bottom line is - if we don’t, what happens? Once caulerpa is at 100% coverage there is very little alive underneath it.”

Northland Regional Council chair Geoff Crawford said if the community could raise significant funding, that would encourage the Government to keep up its end of the bargain.

“The Government’s keeping their cards close to their chest. Because once this technology is developed and it’s proven, the expectation from society that this will go ahead and eradicate it in the North Island is a very expensive bill.

“We could be looking at up to $100 million, so leveraging support from society would be a move in the right direction.”

The council has been working with local contractor Johnson Bros to develop the new dredging technology, and hopes to have it ready to go by May.