Within the few years since its 2021 detection in Blind Bay on Aotea Great Barrier island, the exotic seaweed caulerpa has spread to cover 1500ha of seabed in the upper North Island. The most dramatic evidence of its invasive creep came after Cyclone Tam washed up tonnes of the tangled marine pest on a Bay of Islands beach in April.
Eradication is not impossible but it’s impractical, not least because the cost would be mind-boggling, says Niwa marine ecologist Dr Leigh Tait, whose work focuses on biosecurity and ocean ecosystem monitoring. But there are methods of control, some of which show promise and deliver temporary relief in areas of heavy infestation. And remotely operated vehicles equipped with AI detection tools are set to help keep the weed out of places it hasn’t yet reached.
Caulerpa moves fast and seems to thrive in most marine habitats. Soon after being identified in Blind Bay, it was also found in Aotea’s harbours at Tryphena and Whangaparapara. Then it spread to infest Ahuahu in the Mercury Island group and expanded across to the tip of the Coromandel Peninsula, into the Hauraki Gulf and north to the Bay of Islands. It can grow on top of most marine habitats, from rocky reefs to sandy seabeds, even on the fringes of kelp forests, Tait says.
Most caulerpa beds are found at a depth of 5-15m, but autonomous vehicles have detected it down to 40m. This limits what divers can do, Tait says, and is one reason why automated surveillance is important, especially for early detection. “From what we’ve seen of the invasion, the really thick, dense beds are relatively rare. In many places, it’s patchy and sparse.”
A New Zealand Institute of Economic Research analysis estimated caulerpa could cost the economy $9.4 billion due to its impact on tourism, fisheries, aquaculture and ecosystems. While there are concerns particularly about scallop beds, Tait says so far there is almost no evidence of massive ecological impacts.
The faintest of silver linings is that the two caulerpa species invading our waters are not as highly toxic as the “killer algae” variants found in the Northern Hemisphere. Kina will still graze caulerpa beds. “It’s more a herbivore deterrent than a really nasty toxin,” Tait says. But when eaten by grazers, caulerpa ramps up the production of these chemical deterrents, which allows it to spread further and faster.
Two methods of control have now been widely tested: suction dredging and benthic matting. The latter involves covering a patch of sea floor and injecting chlorine. It’s an indiscriminate method that kills almost everything under the cover. It’s also impractical for larger areas, but can be effective when a new incursion is detected, such as in recent surveillance dives to monitor Leigh harbour.
In some cases, suction dredging can affect the ecology of the sea-floor more than the pest.
A suction dredge is essentially an underwater lawnmower or vacuum cleaner. Dredging trials at Aotea suggest that success depends on how thick the caulerpa bed is. In some cases, suction dredging can affect the ecology of the sea-floor more than caulerpa and the “impact of removal could be worse than the pest”.
Caulerpa spreads by sending out runners which are sometimes buried under the sea-floor, making it difficult to remove every last piece. In bigger suction-dredging operations, Tait says fragments drift away, only to eventually colonise a new spot on the seabed.
That’s where early detection comes in. AI algorithms that can scrape real-time video collected by autonomous vehicles for any traces of the weed offer the best tool to limit caulerpa’s spread. Developed by Niwa, with the Ministry for Primary Industries Northland Regional Council and Ngāti Pāoa, this has already been tested in a surveillance campaign in Te Tai Tokerau and Waiheke. The challenge now is to standardise the algorithm and to make it open source so groups throughout New Zealand can use it to produce reliable results. “Early detection is key to doing something about caulerpa,” Tait says. “If it can be found at that early stage, then it can be removed from that area.”
