Caulerpa seaweed: Super-spreading scourge to be sucked from seabed in new trial

Since it was first discovered at Aotea’s Blind Bay two years ago, fears the highly-invasive exotic caulerpa seaweed may spell an environmental disaster have only grown with its recent detection in the Bay of Islands. Photo / NRC

Officials are moving to tackle a super-spreading pest scourge by sucking it straight off the seabed, in trials to begin at Aotea-Great Barrier Island this weekend.

Since it was first discovered at Aotea’s Blind Bay two years ago, ecologists’ fears the highly-invasive exotic caulerpa seaweed may spell an environmental disaster for New Zealand have only grown with its recent detection in the Bay of Islands.

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 - underscoring international experts’ calls for New Zealand to eliminate it as quickly as possible.

In a new Biosecurity New Zealand-managed trial expected to run several weeks, scientists are turning to suction dredging to strip the seaweed from Aotea’s Schooner Bay, Shoal Bay and Pūriri Bay.

The work, bringing together experts from Niwa, the Cawthron Institute and the firm Bay Underwater, will be carried out from three vessels moored in the harbour.

Divers will use a large vacuum device to suck up visible caulerpa from the seabed, and then store it securely on board a vessel so that any material can’t escape back into the water.

“We expect about 10 tonnes of exotic caulerpa to be removed from the area and then it will be disposed of on the island,” said Biosecurity New Zealand’s deputy director-general, Stuart Anderson.

As part of the trial, some parts of the treated areas will also have chlorine applied and contained with mats.

“The granular chlorine we’ll use is what is typically used in swimming pools and will be applied as small granules inserted under mats,” Anderson said.

These mats will be removed after four days.

“Internationally, chlorine has been effective for this use - killing the caulerpa quickly and breaking down in the natural environment within hours.”

The Cawthron Institute’s invasion ecology and management team leader, Dr Ian Davidson, saw the trial - coming after expert recommendations to the Ministry for Primary Industries earlier this year - as an “important step” in efforts to stop the pest.

Exotic caulerpa seaweed is known to grow by as much as 3cm in a day in warm clear conditions to form vast, dense mats on the seafloor that smother other marine life. Photo / Susan Botting

“We need to understand if we can have an impact on the trajectory of this invasive seaweed,” Davidson said.

“This is a positive step toward developing effective tools and methods to reduce its footprint or stop it from establishing in additional locations.”

Applying effective treatments and conducting removal efforts in the sea was a “tricky proposition”, he said.

“The environment can be tough to operate in and there aren’t off-the-shelf tools available like there are for pest control in homes, gardens, farms, and the landscape.”

Otago University marine scientist Professor Chris Hepburn similarly expected the trial to be challenging, but tools and strategies were nonetheless needed to deal with a threat he said could profoundly change some of our most important coastal ecosystems.

“Quick action to control and eliminate exotic marine species when they are first detected is a real hole in marine biosecurity in New Zealand,” Hepburn said.

“I hope this event and this research will help us develop plans and tools that promote action before more expansive and expensive operations are required.

“You don’t get a second chance at these things.”

Massey University’s Emeritus Professor Barry Scott welcomed the new trials, yet remained dismayed that, two years after the pest’s incursion, New Zealand hadn’t yet created a national management plan for it.

Apart from the establishment of Controlled Area Notices over three western harbours on Aotea, and a small-scale salt trial, no further “in-the-sea” effort has been made by Biosecurity New Zealand since its discovery to control its spread, Scott said.

“That is despite the warnings from expert marine biosecurity scientists in California that a rapid response is what is required, akin to that needed to deal with an oil slick,” he said.

“Communities are scrambling throughout the Gulf and upper east coast of the North Island to control this exotic seaweed, invariably in the absence of adequate resources.

“Although this is the most serious marine incursion of our time, the Government is still not responding with the urgency and resources needed to control it.”

Scott felt the response to date had highlighted “severe deficiencies” in marine surveillance and biosecurity preparedness.

“One cannot imagine such a slow response if we had an incursion of foot and mouth or other serious animal disease.”

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald reporting team in 2011 and has spent the last decade writing about everything from conservation and cosmology to climate change and Covid-19.