An invasive seaweed that has smothered the seafloor across swathes of the upper North Island has turned up in Whangārei Harbour for the first time, and a fast response from Northland’s own dive team appears to have stopped it before it could take hold.
The patch of exotic caulerpa, roughly a metre wide, was found on 4 June at Smugglers Bay near the harbour entrance by Northland Regional Council divers who were carrying out routine marine surveillance alongside the Department of Conservation. It was treated the very next day, and follow-up dives across the surrounding seabed have so far turned up nothing more.
The speed of that response is the whole story. Caulerpa cannot travel far on its own, and it does not drift across open ocean. It moves when people move it, hitching a ride on anchors, chains, fishing gear and dive equipment, and it can regrow from a single broken fragment. A patch the size of a dinner plate today can become a green carpet over the seafloor within a season, which is exactly what has happened elsewhere in the north.
Northland Regional Council councillor Jack Craw, who chairs the council’s biosecurity and biodiversity working party, said the find was a vindication of keeping an in-house dive capability rather than relying on outside contractors. “This is exactly why we invest in marine biosecurity surveillance, including maintaining our own dive capability,” he said. “Our teams acted quickly to contain and treat the site using proven methods. Taking a cautious approach is critical with caulerpa, which can spread from even very small fragments.”
The treatment itself is low-tech and well established for small infestations. Divers laid a benthic mat over the patch and applied chlorine underneath it, sealing the seaweed off and killing it in place. The method works on a patch of this size, where the alternative, leaving it to be found again next year at ten times the area, does not bear thinking about.
Smugglers Bay sits on the north side of the harbour mouth, a sheltered and popular anchorage, and that location tells biosecurity staff almost everything they need to know about how the seaweed arrived. It is about 90 kilometres from the Bay of Islands and 150 kilometres from Auckland, far too far for caulerpa to have floated there. A boat brought it, almost certainly on an anchor or chain dropped first in already-infested water and then lowered again at the harbour entrance.
That is why the council’s coastal central councillor, Amy Macdonald, framed the next phase as a job for everyone who uses the water, not just the dive teams. “Protecting our harbour is a shared responsibility, and simple steps from boaties and fishers can make a real difference,” she said. The asks are not complicated. Check anchors, chains and gear before leaving a spot, pull off any weed and bag it, and put it in a land-based rubbish bin rather than tossing it back over the side where a fragment can settle and start again.
The concern in Whangārei Heads is sharpened by what is close by. The Reotahi marine reserve lies about a kilometre from where the patch was found, and hundreds of schoolchildren snorkel there each year as part of their introduction to the marine world. Local figures did not hide their alarm. Whangārei Heads Citizens Association vice-president Jan Boyes told the Northern Advocate she was “horrified” by the news and wanted certainty that it had not already spread further into the harbour.
Her worry is grounded in the record. Exotic caulerpa was first found in New Zealand at Aotea Great Barrier Island in 2021, and on that island it now blankets an area larger than 1,100 rugby fields. It reached the Bay of Islands in 2023 and has been working its way through anchorages there ever since. Once it forms a dense mat it can choke shellfish beds, crowd out native seaweeds and change the make-up of the seafloor underneath it, which is why each new location is treated as a serious setback rather than a curiosity.
There is one piece of complicating news. Biosecurity New Zealand’s pest management director, John Walsh, said the seaweed is showing signs of recovery after a die-off across some infested sites last summer, the cause of which is still being studied. A natural knock-back raised hopes that the problem might be easing on its own, but a rebound means the pressure is back on, and a fresh patch at a brand new harbour fits that grimmer picture.
Work to actually remove caulerpa, rather than just contain it, is advancing in the Bay of Islands, where crews have been trialling a remotely operated suction dredge to vacuum the weed off the seabed at scale. That is the long game. For now, the Whangārei result is the version of this story that biosecurity staff want, a patch caught while it was still small enough to kill, by a dive team that was already in the water looking.
Anyone who spots suspicious seaweed on an anchor, a beach or the seabed is asked to photograph it where possible and report it to Biosecurity New Zealand on 0800 80 99 66 or through report.mpi.govt.nz.
Have you seen caulerpa on your local patch of coast, or do you think boaties are doing enough to stop the spread? Share your thoughts in the comments below.
This article was written by AI, briefed to report the facts, hopefully without some of the bias people bring to the job 🙂
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