Caulerpa covers rocks in Okipu bay, Great Barrier Island. Photo: Luka Forman
Scientists monitoring the spread of Caulerpa on Great Barrier Island say they're starting to see the invasive weed move away from the sandy sea floor.
It's the Caulerpa growing on the rocky reefs that's now proving the biggest challenge on the island, but it's hoped new technology could help fight the weed there too.
The exotic seaweed was first discovered in New Zealand in 2021, leading to concern about its rapid growth and potential to crowd out native marine species.
The government recently put over $10 million into new technology for the fight against the invasive weed, including dredging and pumping systems, UV light radiation, and chlorine blankets.
Glenn Edney is an ocean ecologist who has been leading regular dive surveys on Great Barrier Island since 2022, monitoring the health of the marine life including the spread of exotic Caulerpa.
In their recent surveys, they noticed a reduction in the total amount of Caulerpa in some sites, and a change in where the exotic weed was mainly growing.
"The Caulerpa has basically moved away from the soft-substrate sand areas, and that varies from place to place. But on the whole, it has kind of receded from those areas and is more concentrated on the reef areas."
There were also some signs of the Caulerpa and ecosystem adapting to one another.
Edney said during the recent survey they observed one fish species, the Two Spot Demoisel, using the weed as part of it's mating process.
"So, this is an adaptation by the Demoisels. The Caulerpa may be covering the normal nest sites such as the Coralline turf... seems to be very sucessful... we were doing a survey and I was able to film a female coming down and laying eggs, and the male fertilising the eggs."
This made him cautiously optimistic, he said, but it was far too soon to make any conclusions about the long-term effects of Caulerpa on the ecosystem.
Meanwhile in Northland, exotic Caulerpa has been found in several new spots in the Bay of Islands outside of the Controlled Area Notice, where anchoring is not allowed.
Caulerpa growing on rocks in the Bay of Islands. Photo: Supplied / Rana Rewha
Northland Regional Council Chair Geoff Crawford said it was disappointing the weed had spread from the controlled area, but not a huge surprise.
"It's not a big area. It's got currents running through and tides."
"It's not uncontrollable, they're only in random areas, so we're pretty confident that once we get our new technology that we're creating at the moment going we'll get that under control quite quickly."
The fight was being supported by a $10 million government grant for scaling up existing dredging and pumping methods, and developing ultra violet light treatments and a chlorine chamber to kill Caulerpa.
The dredging technology has already been proven to work on the sandy seabed, Crawford said.
"Now we're just looking at putting that technology onto a machine that'll be like an underwater tractor and that can go GPS, 24 hours a day."
Unlike on Great Barrier, the exotic seaweed is still mostly on the seabed in the Bay of Islands area, Crawford said.
Figuring out how to get rid of the weed on the rocky reefs would be important in the long term, he said.
Biosecurity New Zealand's director of Pest Management John Walsh hoped the dredging, UV and chlorine chamber treatments would compliment one another.
"We are very focussed on understanding what the wider impacts are so that we can have... a range of tools that can be used in different places under different conditions."
The new technology would be developed over the next 6 weeks, Crawford said, and they hoped to be harvesting Caulpera with it by the start of May.
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