Source: NIWA – National Institute of Water and Atmospheric Research
NIWA’s Crispin Middleton is part of the team monitoring the caulerpa infestation. Photo: Irene Middleton, NIWA.
NIWA is part of a multi-agency biosecurity response to an invasive seaweed discovered at Aotea Great Barrier Island and subsequently at Ahuahu Great Mercury Island.
Two new species of non-indigenous algae were discovered in mid-2021, identified by NIWA researchers as Caulerpa brachypus and Caulerpa parvifolia.
New Zealand has some native species of Caulerpa but the two species identified in 2021 showed similar characteristics to the highly invasive Caulerpa taxifolia that ha been problematic overseas. Caulerpa taxifolia has been listed as a Notifiable Species under New Zealand’s Biosecurity Act since 2001.
The two new species of Caulerpa were found in a variety of habitats and depths near Aotea. They covered large areas of seafloor and appeared to spread quickly.
Biosecurity New Zealand, the biosecurity arm of the Ministry for Primary Industries (MPI), contracted NIWA to assist with the response to the Caulerpa incursion.
The response is being managed by two Caulerpa Governance Groups which comprise representatives from Mana Whenua for Aotea and Ahuahu, Aotea Great Barrier Island Local Board, Auckland Council, Waikato Regional Council and the Department of Conservation.
Marine ecologist Irene Middleton and Principal Technician Crispin Middleton are leading NIWA’s operational service for the response.
Irene and Crispin are part of NIWA’s Marine Biosecurity programme and are based at the Northland Aquaculture Centre.
The two are part of the dive team which has been monitoring Caulerpa and its response to treatment.
NIWA’s marine ecology technician team in Hamilton also has an important role in the response, processing sediment samples to understand responses to and recovery following local Caulerpa eradication tests.
Potential impact of Caulerpa
Most of what is known about the impact of exotic Caulerpa species is from overseas studies of the Caulerpa taxifolia and other invasive Caulerpa pecies (e.g., C. cylindracea, C. prolifera, C. scalpelliformis).
There is little research regarding the impacts of the Caulerpa species found in New Zealand, though they share growth and life-history characteristics with overseas species.
Like those species, they can form large monoculture mats, outcompeting native seaweeds and seagrasses, smothering shellfish beds and reducing the diversity of fauna.
At Aotea, the NIWA response team has seen exotic Caulerpa growing on scallops and dog cockle beds and the assumption is it will negatively impact these species, although no formal research has been completed yet.
It was initially thought the exotic species found would be restricted to depths shallower than 10m but the response team had found it at more than 40m in depth.
As these species are largely sub-tropical, there was an expectation they may die or slow at winter but that does not seem to be the case in New Zealand.
The other major difference in the exotic Caulerpa incursion in New Zealand is that it occurs in two relatively remote locations on the coast of the North Island.
What is being done?
NIWA has been engaged in a series of contracts to support the management of the two new Caulerpa species since June 2021.
The first was to collect samples and formally identify the species.
The response team then searched and discovered exotic Caulerpa populations around Aotea from Motairehe/Katherine Bay in the North to Rangitawhiri/Tryphena in the South.
Once there was a better understanding of the distribution of exotic Caulerpa at Aotea, MPI contracted NIWA to attempt to remove patches of the algae from two popular boat anchorages on Aotea – Whangaparapara and Tryphena – using coarse sea salt and benthic coverings.
The aim was to reduce the risks of the algae being spread by boat anchors and other activities from these areas.
But by the time the response team was engaged to treat the Caulerpa, the populations had expanded significantly, and the team ended up treating two experimental plots of Caulerpa to test the efficacy of salt treatments.
An additional contract was set up to delimit high risk locations around the wider Hauraki Gulf and Coromandel for exotic Caulerpa.
This work discovered a new population at Ahuahu Great Mercury Island, that was too large to be treated.
MPI put in place Controlled Area Notices in infected areas to stop people taking fish or shellfish from these areas and requiring anchors to be cleaned before departing from the areas, as the movement of fragments presents the highest risk of spread.
Local mana whenua supported a dual response and imposed rāhui over the same areas.
The area that exotic Caulerpa has been found growing on in New Zealand is too large for current known treatment based on advice from an independent Technical Advisory Group comprising international and local technical and scientific experts.
Challenge with Caulerpa
NIWA continues to develop treatment plan options with iwi partners. A tangata whenua representative is on board the vessel every time the team is at work diving at Aotea.
Irene said tangata whenua have provided technical advice to the response team during the operations and have ensured the team remained safe by advising on local tikanga.
Understanding and incorporating iwi and public values into the response has been a welcomed learning experience, she said.
Working in the marine environment is always challenging and particularly at an offshore island like Aotea, which is difficult to access.
“One of the major challenges with exotic Caulerpa was that by the time it was detected it was quite well established and more widespread than anticipated and our current control tools are best suited to small scale control of new infestations.”
“It spreads by fragments and any disturbance. Waves or storm events will dislodge fragments that can regrow in new locations.”
Salt was very effective at killing exotic Caulerpa but was labour intensive to apply.
Lab results from sediment samples showed salt impacted all species in the area treated, and therefore is not suited to broadscale removals.
“During the work we have trialed the Boxfish ROV (remotely operated vehicle) to detect Caulerpa at the limits of safe diving operations. Patches of the seaweed were found at depths greater than 40m at some sites.”
Another challenge was starting the response during New Zealand’s Covid-19 level 4 restrictions in 2021.
It required the team to obtain specific exemptions as essential workers, and clearance from the Ministry of Health, Maritime Police and local Harbour Masters to cross internal borders.
The team observed tikanga under alert level 4 by being welcomed to the island by local kaumātua in a physically distanced whakatau and remained in a work bubble on board the vessel for the entire survey.
How have other countries controlled Caulerpa?
Graeme Inglis is a Chief Science Advisor at NIWA and leads its Marine Biosecurity programme for the Coasts and Estuaries Science Centre.
He said there had been many attempts to control the Aquarium Caulerpa in other parts of the world following incursions in the Mediterranean, USA, South Australia and New South Wales, Australia.
It included the use of benthic covers or “shrouds”, mechanical removal, removal by hand, and the application of chemical biocides like chlorine and sea salt.
In one example in California, the Aquarium Caulerpa was eliminated from two canal estate systems, Agua Hedionda Lagoon and Huntington Harbour, using intensive surveillance and sequential treatment of detected patches with benthic covers and chlorine.
In New South Wales, small-scale trials that applied coarse sea salt showed that the alga was killed rapidly, and that the treatment had relatively minor effects on native biota (seagrass and infauna) and was relatively inexpensive.
Native species generally recovered six months after treatment.
The success of attempts at larger scale treatment (such as would be required in New Zealand) were more varied. One large-scale application of salt resulted in the apparent removal of almost 5,200 m2 of the algae.
In another, repeated salting of an infestation of ~3000 m2 reduced the density of the alga but did not completely remove it.
More research of the two invasive Caulerpa species found at Aotea is planned to help understand its ecology and develop options for its control.