New Zealand is testing underwater ultraviolet lights, sealed chlorine enclosures and large suction dredges against exotic caulerpa, an invasive seaweed that has carpeted parts of the seafloor in the country's north since 2021. The latest reminder of the stakes came in June, when divers found a new patch at the entrance to Whangārei Harbour, a spot officials say the weed most likely reached with human help.

Two species are involved, Caulerpa brachypus and Caulerpa parvifolia. According to Biosecurity New Zealand, the weed was first found in July 2021 at Aotea Great Barrier Island in the Hauraki Gulf and has since been confirmed at several more sites around the gulf, on the Coromandel coast and in Northland.

The fight matters well beyond New Zealand. Invasive caulerpa has turned up along the Southern California coast before, and the tools being tested now could shape how coastal managers respond the next time a fast-growing seaweed slips in on a hull or anchor.

A Weed That Travels with Boats

Officials have said for years that people, not currents, are the main way caulerpa moves. "Caulerpa can travel short distances naturally as small pieces adrift in the water, but it is spread primarily by people, snared in boating and fishing gear," Stuart Anderson, a deputy director-general at Biosecurity New Zealand, said in a 2023 media release.

The June find shows how that works. Northland Regional Council divers found a patch of about 1 square meter (11 square feet) at Smugglers Bay, Whangārei Heads, on June 4 during routine monitoring, roughly 1 kilometer from a marine reserve, 1News reported. Divers covered it with a seafloor mat and chlorine the next day, and the council said the weed beneath was killed when the mat came off June 12.

Controlled Area Notices ban anchoring and some types of fishing in parts of the affected areas. Independent scientific advisers told the government back in 2023 that full eradication was highly unlikely, so the goal has shifted toward slowing the spread and removing infestations where possible.

UV Lamps, Chlorine Tents and Underwater Vacuums

The toolkit has grown quickly since the government put $5 million into an accelerated program in February 2024 and another $10 million toward removal technology that August, according to Biosecurity New Zealand's project page. Those figures are in New Zealand dollars.

Ultraviolet light is one of the most promising options. In a 2024 field test off Rakino Island at depths of about 7 to 8 meters (23 to 26 feet), a single dose from a three-lamp UV unit killed more than 90 percent of Caulerpa parvifolia in small test plots, the University of Auckland reported. "The field tests were in line with our lab results," said Andrew Jeffs, a professor at the university's Institute of Marine Science. A larger UV-C light array built to work without scuba divers finished trials at Omakiwi Cove and Waiheke Island in August 2025.

Other tools attack the weed physically or chemically. A 75-square-meter (807-square-foot) inflatable enclosure called Rehabitat holds chlorine against the seabed under negative pressure, while divers using an underwater vacuum cleared caulerpa from 1.27 hectares (about 3.1 acres) at Iris Shoal off Kawau Island. Northland Regional Council is designing a submersible dredge meant to remove carpets in a single pass, and crews have also tried salt, biodegradable mats and ultrasonic noise.

A Surprise Retreat, and a Warning

Early this year, the weed did something no tool had managed. After Cyclone Tam struck in April 2025, caulerpa declined sharply in the Bay of Islands and near Aotea, with some sites that had 90 to 100 percent cover nearly cleared, RNZ reported in January. Biosecurity New Zealand's John Walsh said sediment from the storm likely settled on the weed and blocked its sunlight.

Nobody is declaring victory. "It may well grow back. We don't know," Walsh said, and local divers have already reported new growth near Ōmakiwi Cove and off Urupukapuka Island. The causes of the decline are not confirmed, and some scientists suspect seasonal cycles or water temperature also played a role.

The evidence on the new tools is also early. The UV results come from small plots, the chlorine enclosure is still being tested, and the large dredge remains in design, so none has yet been proven at the scale of the infestation. Biosecurity New Zealand says no control tools exist anywhere in the world at the scale Caulerpa occurs in its waters.

Lessons for U.S. Harbors and Boaters

The United States has fought this kind of battle before. A related species, Caulerpa taxifolia, was found in 2000 in Agua Hedionda Lagoon in Carlsbad near San Diego, with smaller infestations in Huntington Beach near Los Angeles. Crews covered patches with weighted tarps and pumped chlorine underneath, and the effort took six years and more than $7 million before the weed was declared eradicated in 2006, according to the University of California, Riverside. California now bans possessing, selling, or transporting that species.

The California response relied on catching infestations while they were small — a lesson New Zealand officials echo. "With early detection and rapid response, you have the best chances of getting it under control," Kaeden Leonard, Northland Regional Council's marine biosecurity manager, told 1News after the Whangārei find.

For boaters anywhere, the guidance is simple and inexpensive. Biosecurity New Zealand advises checking anchors, chains and fishing gear for seaweed before moving to a new spot, bagging any fragments and putting them in the trash on shore, and following local anchoring rules. In New Zealand, suspected sightings can be reported by photo and location through the ministry's online form or freephone line, and U.S. boaters who spot unfamiliar seaweed should contact their state fish and wildlife or water quality agency rather than pulling it up themselves.

The bottom line is that New Zealand is building a varied arsenal while the weed keeps moving, and a storm did more damage to it this year than any device so far. The next test is whether the UV, chlorine and dredge tools can scale up before caulerpa rebounds — and whether boaters keep it from hitchhiking to the next harbor.

What Readers Want to Know

What is exotic caulerpa?

It is a pair of fast-growing invasive seaweeds, Caulerpa brachypus and Caulerpa parvifolia, that can form dense carpets over the seafloor. New Zealand first found them in 2021 at Aotea Great Barrier Island.

How does caulerpa spread?

Mostly on boats. Biosecurity New Zealand says fragments snag on anchors, chains and fishing gear and are carried to new sites, while drifting pieces only move short distances naturally.

What tools is New Zealand testing?

Underwater UV-C lights, a sealed chlorine enclosure called Rehabitat, diver-operated vacuums, suction dredges, biodegradable mats and other methods. Most are still in trials.

Did the UV light work?

In a 2024 field test, a single UV dose killed more than 90 percent of Caulerpa parvifolia in small test plots. Larger trials ended in August 2025, and the tool has not yet been proven at full scale.

Has caulerpa reached the United States?

A related species, Caulerpa taxifolia, was found in Southern California in 2000 and declared eradicated in 2006 after a six-year, $7 million effort.

What should boaters do?

Check anchors and gear for seaweed before moving, bag any fragments for disposal on shore, follow local anchoring rules, and report suspected sightings to the relevant biosecurity or wildlife agency.