Auckland Mayor Wayne Brown is warning boaties to check their gear over the summer to avoid exacerbating the spread of exotic seaweed that poses a serious threat to the Hauraki Gulf ecosystem.
The recent detection of two species of invasive seaweed caulerpa, described by the Ministry of Primary Industries as “New Zealand’s most unwanted species”, has seen marine biosecurity precautions on Auckland’s seas.
Caulerpa can spread rapidly, forming large monopolised underwater fields competing with native species for space and potentially putting local ecosystems out of balance.
And the tenacious organism can colonise new territory if just a small piece of it is transported.
Not unlike kauri dieback, it’s an ecological risk colliding with the recreational activities of the Kiwi summer.
Over 1200 hectares of caulerpa is estimated to have grown across the gulf since its arrival was first noted in July of last year – mainly around Waiheke and Aotea Great Barrier.
The first appearance of the invasive plant was tracked back to a post on the amateur naturalist website iNaturalist.
How it got to New Zealand remains a mystery, although it is considered most likely to have arrived on a boat travelling from Australia or the Pacific Islands. The amount found suggests it has been in New Zealand for several years.
In July, nearly 20 hectares were discovered on Iris Shoal, west of Kauai Island.
Limited targeted surveillance since then has not uncovered any new populations, but Auckland Council staff told councillors the invasive plant is likely elsewhere in the region already.
A plan has been proposed to suction-dredge and hand-pull caulerpa from the Waiheke field, following trials at Kawau and Aotea.
But Auckland Council, in conjunction with MPI, has flagged that increasing the ability to survey larger areas within shorter time periods should be a priority.
The council recently acquired a towed underwater video camera to survey high-risk areas over the summer.
But as the weather improves and Aucklanders flock to the sea in their own vessels, it appears the mayor has chosen this as an issue to amplify.
“This incursion is a wake-up call to all boat owners to be extra careful about checking and cleaning anything that goes in the water, and making sure you’re not taking hitchhiking pests along with you as you move about.”
MPI is instructing boat users to report sightings of the plant, keep their boats and gear clean to avoid spread, and obey rāhui and legal restrictions on anchoring and fishing in certain areas.
New rules on areas around Ahuahu Great Mercury Island and Aotea Great Barrier came in at the beginning of this month. Much of the southwest coast of Aotea is an anchor-free zone this summer.
“Boatie to boatie, we need to do our bit to stop the spread of this pest. While we learn more about this seaweed, I ask the boating community to please be vigilant. Take every care not to spread it when you move your vessel,” said Brown.
Auckland Council pathways team manager Liz Brooks said the organisation was focused on driving education and awareness-raising efforts in the region to reduce spread.
“This includes deploying marine biosecurity champions, with financial support from Biosecurity New Zealand, at key locations to help educate as well as support beach and marine goers on what to look out for and follow the right advice.”
That advice includes boaties keeping a look-out for seaweed on anchors or hooks and then bagging it and binning it.
A report from MPI said caulerpa has the ability to reproduce asexually through clonal fragmentation, with even tiny fragments able to grow into new adults. It also has high growth rates and often possesses toxic compounds that protect it from grazers, especially in new environments.
In a workshop with councillors, staff from Auckland Council said they were expecting a busy boat season on the gulf following last summer’s bad weather.
In response, the council is launching a marketing campaign to marinas, boat clubs, island trusts and ferry passengers.
In the face of the threat, the organisation has also shifted more resources toward marine biosecurity.
Along with a new part-time outreach role, the council has recruited a third marine biosecurity officer to help with the load of managing existing marine biosecurity programmes while responding to the caulerpa invasion.
A technical advisory report from Biosecurity New Zealand earlier this year said our best bet for now is suppression and containment.
“The scale of the incursion is far beyond that at which successful eradication has ever been achieved in marine environments internationally,” it read. “Eradication within each infected area is also not possible with the current set of tools available.”
The report noted that the hard-hit areas around Aotea Great Barrier are “historical food baskets that have been harvested for thousands of years by Aotea mana whenua”.
“Should the long-term ecological integrity of these areas change, there is likely to be an adverse effect on mana whenua,” it read. “There is an expectation for some short-term negative impacts, however, the important assurances needed by mana whenua is that there is a positive return to the natural ecology in the long-term.”
The invasive seaweed poses a particular risk to kina and scallop beds in the gulf.
