Caulerpa, a virulent exotic seaweed that forms dense mono-cultural mats across the seafloor, is marching along parts of New Zealand’s coast, prompting fears it could become the worst ever threat of an invasive species to the local marine environment.
Caulerpa can grow up to 3cm a day, in warm, clear, shallow waters, creating a green carpet across the seafloor and enveloping everything in its path. It competes with native seaweeds and sea grasses, smothers shellfish and can shrink the diversity of marine life.
“The frightening thing about it, is its speed of growth … you just see mats of the stuff right across the seabed, over rocks, over scallops, and shells,” said Barry Scott, a molecular geneticist, professor Emeritus at Massey University, and deputy chair of the Aotea Great Barrier Environment Trust.
“It also impacts on all the things that grow below the sediment [including] all the small critters that fish feed on,” Scott said.
New Zealand has nine species of native caulerpa that are kept in check by local ecosystems. But two new species of the non-indigenous algae, caulerpa brachypus, and caulerpa parvifolia, have made their way to the country, likely carried into its waters on boats.
The exotic species were first detected around the coast of Aotea/Great Barrier Island in Auckland’s Hauraki Gulf in 2021. They have since been sighted near other islands in the gulf and in areas farther north.
The ministry for primary industries believes the algae has been in New Zealand waters for a few years and it now covers roughly 821 hectares across five locations, at varying densities.
Invasive caulerpa has led to problems in other parts of the world, including an outbreak of another species – caulerpa taxifola – in the mediterranean during the 1980s, which spread across tens of thousands of hectares of seabed, according to the Global Invasive Species database. The same species was eradicated from southern California in 2002 by sealing the plants under tarpaulins and then trapping chlorine, used as a pesticide, underneath.
That method leads to collateral damage to other species, Scott said, but added that “what people forget, is the damage that will be done by doing nothing”.
A second US outbreak, in Newport near Los Angeles in 2021, was eradicated with the help of suction dredging, followed up with rigorous GPS tracking and removal of new plants.
Scott believes exotic caulerpa could become the worst threat to the seafloor New Zealand has seen but efforts to control it are becoming fraught.
In August, the local iwi (tribe) on Aotea, Ngāti Rehua Ngatiwai ki Aotea, said they were heading to the Waitangi Tribunal – the commission that inquires into claims brought by Māori relating to crown actions – alleging the crown was mismanaging efforts to control the algae.
Communities are becoming “absolutely furious” about its spread, Scott said, adding that New Zealand should be responding as thoroughly as California did.
“Caulerpa requires a response akin to an oil slick,” he said.
‘A difficult and complex pest’
The ministry has poured cold water on the suggestion it is not doing enough. Caulerpa was easier to eradicate in California because it was found in lagoons, as opposed to the open ocean where it is spreading in New Zealand, and the patches were smaller, it said.
“The independent science advice from the outset is that the scale of the exotic caulerpa incursion in New Zealand is well beyond any known successful eradication attempts elsewhere in the world,” said Stuart Anderson, Biosecurity New Zealand’s deputy director-general.
Since its discovery in Aotea’s waters, there has been “a significant response to this pest” alongside mana whenua (Māori with customary authority over a territory), local authorities and the department of conservation, Anderson said.
Roughly NZ$5m has been spent, or allocated to fund efforts to understand the pest, its behaviour and distribution, to trial treatments and prevent its spread, he added.
The ministry has tried smothering the algae with coarse salt, which it said worked in the short term but that it indiscriminately killed all other marine life in the treatment area and the caulerpa came back. Meanwhile, hand removal of plants was extremely labour intensive and slow.
Suction dredging was used in three spots, which led to 17 tonnes of caulerpa being cleared. Monitoring is now under way to determine how effective it will be.
“We visited a fortnight ago and found there is still caulerpa in some of the areas.” Anderson said. “It’s not clear if this is a re-infestation or patches not caught in the initial treatment.”
A trial of a different type of suction dredge technique is due to start soon in Northland. Meanwhile, some anchoring spots around the affected areas have been off-limits to boaties, for fear their vessels could spread the algae farther afield.
Anderson said that while some species of caulerpa have become invasive overseas, it was too early to speculate on what effect widespread distribution could have in New Zealand, but added that the ministry is committed to controlling the pest.
“This is a difficult and complex pest that can travel on currents, so there are no easy solutions.”
But Scott is worried the ministry’s progress on trials and monitoring is too slow and not enough funding is being allocated to the problem, which could make eradication very difficult.
“We want intensive surveillance [and] community groups to be empowered,” Scott said. “This thing is not going to stop … the growth rate alone scares the pants off me.”
