Members of the public are being asked to do their part this summer to stop the spread of exotic caulerpa, a seaweed that could have a huge ecological impact on seafloors around New Zealand.
The fast-growing seaweed has been found around Waiheke Island, Aotea Great Barrier Island, Ahuahu Great Mercury Island, Kawau Island and in Te Rāwhiti Inlet in the Bay of Islands.
"It grows terribly fast. It's been suggested 8cm a day in all directions," said Liz Brooks, Auckland Council's Pathways and Marine team manager. "It just smothers everything. It's a bit like kikuyu grass. It just covers everything it comes across – sand, rock, shellfish. It just covers everything."
Last month authorities began to fight the spread around Aotea Great Barrier Island. It's estimated around 800 hectares of caulerpa meadows surround the area. Biosecurity New Zealand, NIWA and Bay Underwater Services have started trialling an underwater suction dredge near Tryphena Harbour.
Divers used a large vacuum device to suck up the seaweed and move it into a secure container on the boat where it was then transported for disposal. The seafloor is then treated with chlorine and specially designed mats.
Stuart Anderson, deputy director-general of Biosecurity New Zealand, said early results are encouraging.
"The equipment is removing most caulerpa cleanly from the seabed and it appears there has been no obvious escape of fragments of the seaweed in the process," he said.
"The divers have been able to clear around a tonne of caulerpa from an area of approximately 60 square metres in an hour."
The fear is that many of our coastal environments are the perfect habitat for the seaweed – especially the Hauraki Gulf.
Brooks said: "It grows over 30 metres deep, and so we have about 188,000 hectares of suitable habitat it could grow in."
'Bag it securely'
Authorities are asking beachgoers and boaties to play their part this summer.
Dr Cath Duthie, Biosecurity New Zealand’s manager of readiness, urged people to be vigilant and keep an eye out for the exotic seaweed.
"When you're raising or lowering anchors, or indeed using any equipment that goes into the water – so diving equipment or swimming equipment. People pull up weed, any type of weed – it doesn't matter what it is. What they should do is bag it securely, and put it away where it can't get back into the ocean. Take it ashore and just put it in the rubbish," she advised.
If you can't bag the caulerpa securely, leave it where it is, Duthie suggested.
"We recommend that they just put it back where they found it because it's likely to come from a place where there is a lot of caulerpa anyway. Adding it back into that environment won't make much of a difference.
"What we really want to prevent is the movement of that caulerpa from one marine environment to the other one. That's the really important thing about these rules we're putting in place."
Although caulerpa is currently spreading in the upper North Island, Duthie said people on the mainland should also be on the lookout for signs of the seaweed.
"What we know about caulerpa from overseas experts is that the warmer waters of the upper North Island are ideal for growth. However, with every new invasive species you're never quite sure how it's going to react to a new environment. There's the possibility that there are microclimates further south that are suitable for its growth," she said.
"And, of course, with global climate change, all bets are off in the future.
"I think everybody really does need to keep an eye out, particularly those people who are using the water in the upper North Island."
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