I need to borrow a mask and snorkel and a pair of flippers, and Jade Webster from the Claris Store—a friendly, sweary diver and fisher with tattooed forearms and a cheeky smile—is happy to provide. She’s worried about my head and hands getting cold on this rainy May morning, so she throws in a neoprene hoodie and gloves as well.

A nor’wester is whipping up the waves in Okupu, the innermost part of Blind Bay. I shuffle backwards into the surf, then swim out through the clear water. At first, I see nothing but white sand and a sextet of young snapper. Then a small green tuft appears on the sea floor, resembling a tangled bundle of steel wool. Soon, I spot larger mats, folded on the sand like the freshly shorn fleeces of neon-green sheep: the invasive seaweed Caulerpa brachypus.

New Zealand has native caulerpa species, but this one doesn’t belong here—on Aotea or in Aotearoa at all. It probably arrived in Blind Bay on the anchor chain of a voyaging yacht a couple of years ago.

Overseas, caulerpas have a bad reputation. Another species, Caulerpa taxifolia, is such a problem in Europe and North America that it’s called “killer algae” and listed among the 100 worst invasive species on Earth by the International Union for Conservation of Nature.

Here on Great Barrier, the invasion is far worse than the scattered clumps I see during my snorkel. Caulerpa’s tendrils have spread across this bay and infiltrated two others on the western side of Aotea, the Tryphena and Whangaparapara harbours. (Webster thinks she first saw it while diving in Blind Bay at least five years back, but didn’t realise its significance.)

Now, it carpets areas the size of rugby fields. Caulerpa climbs over rocks and the sandy bottom alike, attaches itself to scallop shells, smothers sponges. It has the potential to transform vast areas of coastline from the tip of Northland to the Bay of Plenty—displacing native species, altering ecosystems that are already under pressure from overfishing and sedimentation, making it harder to gather kaimoana, and draining the mauri from the moana. Such a dystopian future is unthinkable for mana whenua on Aotea, and they say they’ll do anything to eradicate the invader. But is that possible? What else must be sacrificed, and what might be gained, in the fight to destroy it and prevent its spread?

[Chapter Break]

On Aotea, there’s so little traffic that every passing driver gives you a wave—or at least a laconic finger-lift. Kids bicycle hands-free and helmetless, teenagers hitchhike home from an evening soak in a thermal stream, and wild boar cross the road at night. Kākā are so numerous they strip fruit trees of their crops, surfboards are rented via honesty boxes, builders and millionaires jam together in the only pub, and word spreads quickly on the bush telegraph.

On this off-grid island, locals live off the land and the sea where they can, because jobs are scarce, petrol is $4 a litre, and a flat white costs $6—if the coffee machine is working and the generator hasn’t conked out. People give away fish to their neighbours and receive home-grown garden greens in return.

Jack Warden grew up at Okupu, his childhood defined by the bay’s wide sweep: snorkelling and fishing and diving for scallops. Now, he’s a 30-year-old plant-loving ecologist on the mainland, with a magpie’s keen eye for green things. In June 2021, he went home to Aotea for his father’s birthday, and the two men decided to spend a day fishing together in Blind Bay.

As they backed the boat across the beach with a tractor, the tide was as low as Warden had ever seen it. Rocks usually well under the water were breaking the surface, and growing on them, he noticed, was an unfamiliar seaweed. “It was weird enough that I photographed it and put it on iNaturalist.” (This came naturally to Warden, who has logged more than 6000 observations on the site, representing at least 1600 species.)

Scientists from NIWA saw the post and sent out a diver to collect samples. Genetic testing proved the intruder was mainly Caulerpa brachypus—with a close cousin, Caulerpa parvifolia, present as well. Neither of them had been seen in New Zealand waters before.

On the frontline of fighting the invasion are Ngāti Rehua Ngātiwai ki Aotea, who are based on the island, and Ngātiwai, whose rohe stretches along Northland’s east coast from the Bay of Islands to Great Barrier. When Jeff Cleave first found out about it—when he saw the underwater footage of the invasion, the smothering spikes extending as far as the eye could see—he felt “broken”, he tells me over coffee at Claris watering hole My Fat Puku. Think of it like the weed grass kikuyu, he says. “If you’ve ever tried to get rid of kikuyu, you understand how devastating it is if you get it in your garden.”

Cleave is mana whenua, and represents Ngāti Rehua Ngātiwai ki Aotea in the government’s response to the weed. He arrives at our meeting by motorbike, planning to go surfing and do some digger work afterwards. As we talk, he regularly pauses mid-flow to call to others, congratulating a pregnant cousin, greeting locals by name.

Caulerpa is just the latest in a long line of tauiwi kino—“bad visitors”—to arrive on the island from elsewhere, he says. Feral cats, rats, gorse, pampas. COVID-19. Devastating waves that keep breaking on the island from outside.

In blanketing the papa moana, the seabed, the invasive seaweed is displacing the animals and plants that should be there—the sponges, the dog cockles, the mussels, the scallops. “When you displace something, you disconnect it from its whakapapa,” Cleave says. That detachment affects the mauri of the ocean and of the people who depend on it.

Ngātiwai kaumātua Hone Martin later explains the invasive caulerpas to me as plants which don’t display whanaungatanga—a loving, balanced, familial relationship.

“You can tell they don’t whakapapa here, because they’re taking control of the environment they live in. They’re not family-oriented, they’re selfish.”

Caulerpa is new to New Zealand, so the full effects it might have on the ecosystem are still unknown. Invasive species sometimes go through a boom-and-bust cycle, so it’s possible it might die back by itself as the water cools over winter—except New Zealand is experiencing its longest-running marine heatwave, with scientists predicting a third warm La Niña in a row.

As mana whenua, as a child of Ngātiwai—which Cleave translates as the people of the sea—he feels deeply obligated to take a precautionary approach, and fight the invader. “I’m only here as caretaker. Where te ao Pākehā looks at land ownership, te ao Māori looks at land guardianship. We’re only here for a blink in time.”

[Chapter Break]

Two months after Warden discovered the caulerpa incursion, a ban against fishing and collecting kaimoana was put in place where the seaweed had been found: at Blind Bay and Tryphena Harbour. The Ministry for Primary Industries (MPI) and mana whenua acted in concert: laying down a rāhui and a controlled-area notice at once. When caulerpa was discovered in Whangaparapara Harbour, the restrictions were extended there, too.

Boats are forbidden from leaving the three bays without a permit, which requires a vessel’s anchor and chain to be cleaned of seaweed. A fragment of caulerpa can grow into a whole new plant, and anchors, ropes and chains can dislodge and transport these tiny hitchhikers to new bays.

Last summer, representatives of both MPI and mana whenua were out in force around the island every day, speaking with boaties and spreading the word about the weed and the restrictions. Ngāti Rehua Ngātiwai ki Aotea installed watchful carved pou whenua overlooking the three bays. Some locals acted as enforcers, too.

“You’ll get a phone call: ‘There’s some f— in the bay, some bastard’s in the bay anchoring’,” says Webster, who lives at Okupu. She has her own boat (“I love the wind in my hair, mate”) and will take it out to check whether visitors are obeying the restrictions. “I’m not a vigilante, but I will tell them to move.”

Over the past year, the response to caulerpa has been marked by an unusual degree of cooperation between MPI and mana whenua. Together, they established a governance group, including two representatives from each of the island’s marae, and the government signed a mana-enhancing agreement that aimed to foster a strong relationship.

It’s not perfect, but it’s working, says the governance group’s deputy chair, Martin Cleave. Mana whenua are often called on as advisors, but this time, they are decision-makers. “This is mana whenua-led, and that’s never happened [here] before.”

This Cleave is a sharply dressed Auckland television producer, the deputy chair of the Ngātiwai Trust Board and Jeff Cleave’s younger brother. There are seven kids in the family, four boys and two girls. They all grew up on the island, Martin says, their home lit by a kerosene lamp. “We never had anything. All we had was love—and fresh food. Eeling and fishing was our entertainment.”

Hunting and gathering is deeply embedded in the island’s culture, in both Māori and Pākehā families—one reason the closures have caused tension, says Jeff Cleave.

“It’s very difficult to tell somebody who’s dived for scallops in that spot all their life that they can’t any more. We’re asking them not to fish, and they caught their first fish here.”

But it’s not only people living near the closed bays that are affected, he says. People banned from fishing in the southwest have been crowding into the island’s north instead—where the marae are and where many mana whenua live—and crayfish, scallops and pāua in the northern bays are taking a hammering. Jeff Cleave went diving to find a crayfish for his father’s 73rd birthday and, finding only one, left it in the water.

[Chapter Break]

“It’s like a really beautifully manicured lawn, like on a golf course,” says Irene Middleton, one of the NIWA divers who has been checking on the caulerpa since last summer. Every time Middleton returns to Blind Bay there seems to be more of it: a golf course where there should be wilderness.

As well as checking for spread, Middleton and the other divers attempted to eliminate the weed from Tryphena and Whangaparapara. Unfortunately, you can’t spray herbicide under the water, or rip caulerpa out by hand like you would kikuyu—even a small disturbance is enough to dislodge fragments that risk spreading the weed. An expert panel decided the most promising option was a salt treatment, which had been used successfully overseas and in freshwater environments in New Zealand.

It’s not subtle. For every square metre of weed, you need 50 kilograms of coarse-grained salt crystals—600 kilograms for every three-by-four-metre test plot. Jeff Cleave went out on the NIWA boat to help with the application, upending the 25-kilogram bags into a hopper connected to a PVC pipe, while divers on the bottom angled the hail-like flow of salt onto the algae, then stapled a tarpaulin and hessian mats over the top.

A caulerpa plant is essentially one large cell, explains Middleton. When the salt concentration is too high, it breaks down the cell walls and kills the whole plant at once. By the following day, the salty meadows were already looking sickly, she says—translucent and flimsy, like boiled lettuce.

Visits over the following months showed that the treatment worked. In those areas, the caulerpa was dead. But so was almost everything else—small fish, crustaceans, and many of the tiny animals and microbes living in the mud. “It’s pretty scorched-earth,” says MPI’s caulerpa lead, John Walsh.

Although relationships between MPI and iwi aren’t always great, Walsh has built strong relationships with Ngātiwai leadership, says Martin Cleave. “You’ve got three heavy-hitter kaumātua going, ‘We’ve got your back, John’—this big white guy. They are super impressed.”

After three months, there were signs the biodiversity had recovered—but so had the caulerpa. In some places it was already growing over the top of the hessian mats. Even worse, each subsequent dive revealed it covered ever vaster areas, until by the end of summer the NIWA team had mapped 44 hectares of caulerpa in Blind Bay alone, spread over about a third of the cove.

[Chapter Break]

It’s last light when I arrive at the Claris Sports Club for the community hui about caulerpa. The dark knobs and ridges of Hirakimatā/Mount Hobson stretch across the western horizon, silhouetted against storm clouds. Inside, the wood-panelled room is packed with people. A carved boar’s head, a framed rugby jersey, and a pit saw adorn the enormous slab of timber hanging above the bar. Walsh and other representatives of MPI, the local board, the Auckland Council, and mana whenua—including both Jeff and Martin Cleave, and several high-ranking Ngātiwai kaumātua who’ve come over especially from the Bay of Islands—sit up the front around the pool table.

Kaumātua Hone Martin begins the meeting with a karakia, and the Māori attendees sitting in the front few rows join in with the hymn He Honore. Then Walsh fills everyone in on the latest with the weed. He has good news and bad news, he says, although most of it is pretty bad.

Recent dives identified caulerpa growing as deep as 38 metres under the surface, and revealed the infestations at Whangaparapara and Tryphena harbours were much larger than initially thought, meaning the salt treatment is unfeasible. Partly, that’s about cost and logistics—requiring tens of thousands of tonnes of salt be brought to the island on thousands of trucks by hundreds of barges—but also, entire bays would need to be dealt to at once or the fast-growing algae would simply reinvade the cleared areas.

“If nothing else, if you’re going to do that on 40 hectares of Blind Bay, you’re going to kill an awful lot of the natural environment,” Walsh tells the crowd. “Given the size of the infestation and the tools we have currently, there is no way we can eradicate this weed.”

During the summer, the NIWA dive team looked for caulerpa in 30 other places around Aotea, plus 50 high-risk locations around the Hauraki Gulf: sheltered shallow bays with sandy bottoms linked by boat traffic to Great Barrier, like Sandspit, Kawau, Leigh, Waiheke, and Rangitoto. They were all clear—until the very last dive. Off Ahuahu/Great Mercury Island in the Mercury Islands, they found the distinctive neon fronds. Caulerpa covered three hectares there, too.

The priority now is to prevent it from spreading elsewhere, says Walsh, so the governance group wants to extend the controlled-area notice and the rāhui for another two years. (“Shit,” the woman next to me says behind her mask.) But the group is considering allowing fishing from the shore in the affected bays, as it’s relatively low risk and will give locals back their connection with the sea.

Walsh opens up the floor for questions. “Is it edible?” asks a bearded guy in muddy red bands. (No, says Walsh, and it’s hard to compost, too.) “Can we gather it from the beach and put it on our gardens?” asks someone else. Because it’s classified as an invasive organism under the Biosecurity Act, it’s illegal to move it, Walsh says.

There are several questions about drift fishing—one man says it’s unfair that visiting yachts can get a permit to anchor but locals can’t drag a line behind a kayak. Others want to be able to dive for scallops. But the governance group decided these exemptions would be too complex, and too hard to enforce. “That opens up loopholes, and human nature is geared to find excuses to do things we’re not allowed to do,” says Jeff Cleave.

Sometimes, he says, taking a precautionary approach requires sacrifice. If caulerpa isn’t eradicated, it could be here for generations. In that context, waiting a year or two—while we figure out how to get rid of it—is nothing. “If people see it as their birthright to continue to extract for today, there’ll be nothing for tomorrow. We’re doing it for our great-great-grandchildren, who will never know us.”

[Chapter Break]

As this issue went to press, NIWA divers found small patches of the weed at four more locations on Aotea’s west coast. In August, the governance group agreed to extend the Controlled Area Zone from the southwest corner of the island to just south of Port Fitzroy. Anchoring will be banned except in emergencies, and fishing will only be allowed from shore.

This summer, the government plans to do more research on caulerpa to learn about its ecology, its effect on the ecosystem, and to look for any other tools that might help to eradicate it.

Hone Martin hopes it will also investigate potential solutions derived from mātauranga Māori—like using uwhi, mats woven from harakeke, a technique used by Te Arawa in Lake Rotomā, near Rotorua, which has had some success in suppressing freshwater weeds. In the meantime, he’s advising his boat-owning friends to leave Aotea alone for a while, to reduce the chance of bringing caulerpa to Northland.

Martin Cleave wishes there was a way to “get in there with vacuum cleaners and pipes”. Some way to get started now and tackle the weed head-on. “We’re Māori, we want to roll our sleeves up and put a jersey on and run it straight. We don’t want to wait. However, we still need to follow the advice, because we’re not scientists. What I don’t want to get into is that 12 months later, we’re still investigating and researching.”

Both Cleaves feel the intense intergenerational responsibility of kaitiakitanga. “We are the ones, my brother and I, that are responsible for what happens tomorrow,” says Martin. “If we don’t do something about it today, well, then we haven’t done our job.” Letting the caulerpa spread all around the island, until it chokes the bays in front of the two marae, is unconscionable, he says—mana whenua will not give up on elimination.

Right now, though, eradicating caulerpa from Aotea looks unlikely, a fact that Jeff acknowledges.

“I might not win with caulerpa,” he says, tearing up for a second. “But I won’t walk off the paddock. My grandchildren will not criticise the mahi.”

Anyway, he says, you don’t always have to beat the enemy to win. “A win might be walking together side by side for the first time.”

Out on the edge of the gulf, relatively isolated from the city, Aotea’s bad visitors usually come from the mainland. This time, Great Barrier is the front line, and the people of the island are making sacrifices to protect the rest of us.

“The pain that we feel on Aotea is the pain we don’t want the rest of Aotearoa to have,” says Jeff. “We have an environment that has been devastated by a foreign visitor, that is super aggressive, holds no prisoners, and displaces everything, to the detriment of the life force of our seabed. All we can do now is hold the fort.”