Caulerpa brachypus seaweed mats cover a patch of seafloor in Auckland
Caulerpa brachypus in Auckland. Photo: Shaun Lee (CC-BY)

Officials to trial removing invasive seaweed Caulerpa – Expert Reaction

MPI is trialing suction dredging methods to remove the invasive seaweed Caulerpa from the seafloor at Aotea Great Barrier Island.

The operations are due to begin on 18 September and run for two weeks. After the removal some areas will also have chlorine applied and contained with mats, to test if the suction alone works, or if further treatment is needed, and how effective this is. MPI will be holding a community meeting to discuss the trials at 6pm today.

The SMC asked experts to comment. 

Professor Chris Hepburn, Director, Aquaculture and Fisheries Programme, University of Otago, comments:

“This trial is important and well overdue. We need tools to deal with Caulerpa – a species with the potential to profoundly change some of our most important coastal ecosystems. The proposed trial will be challenging but we need tools and strategies to control Caulerpa.

“Quick action to control and eliminate exotic marine species when they are first detected is a real hole in marine biosecurity in New Zealand. I hope this event and this research will help us develop plans and tools that promote action before more expansive and expensive operations are required.

“You don’t get a second chance at these things.”

No conflict of interest.

Emeritus Professor Barry Scott, Molecular Genetics, Massey University, comments:

“The announcement of a suction dredge trial at Tryphena, Aotea Great Barrier Island, to remove exotic caulerpa is great news for a remote island community that felt abandoned after the initial find of this highly invasive seaweed in Okupe Harbour in July of 2021. Apart from the establishment of Controlled Area Notices (CANs) over three western harbours on Aotea, and a small scale salt trial, no further ‘in the sea’ effort has been made by Biosecurity New Zealand since then to control the spread of this exotic seaweed. That is despite the warnings from expert marine biosecurity scientists in California that a rapid response is what is required, akin to that needed to deal with an oil slick. The decision to proceed with two suction dredge trials, one in Te Rāwhiti in the Bay of Islands and this one at Tryphena, picks up on a set of key recommendations put forward to Biosecurity NZ by the Exotic Caulerpa Suction Dredge Technical Advisory Group in July of this year.

“Given exotic caulerpa can grow by up to 3 cm a day from fragments as small as 1 cm, in warm clear waters, it very quickly forms dense mats on the sea floor that smother all other marine life. Research in the Mediterranean has shown that over a period of 6 years exotic caulerpa caused a 30% reduction in biodiversity and a 50% reduction in fish biomass. The prolific growth of this seaweed in NZ waters was brought into stark relief when over 50 tonnes of it was washed up on Okupe beach after cyclone Gabrielle in February of this year. Suction dredging is one of two tested methods – the other being the use of tarpaulins, which block the light – to eliminate caulerpa from the seabed. Although suction dredging is a very efficient method to remove biomass, equally critical are the manual follow ups by divers over several months/years to remove every last fragment of the seaweed to ensure elimination. Even though diver controlled suction dredging can avoid some sea organisms such as scallops and cockles, it will impact on the marine environment. However, those impacts have to be weighed up against the potentially more serious impacts of uncontrolled spread of this seaweed. This trial will determine the efficiency and efficacy of this methodology in removing caulerpa at a remote site and provide important baseline information on the environmental impacts.

“While I am pleased to see this trial getting underway, I am dismayed that even after two years of this incursion there is still no National Management Plan in place. Communities are scrambling throughout the Gulf and upper east coast of the North Island to control this exotic seaweed, invariably in the absence of adequate resources. Although this is the most serious marine incursion of our time, the government is still not responding with the urgency and resources needed to control it. The response so far has highlighted severe deficiencies in marine surveillance and biosecurity preparedness. One cannot imagine such a slow response if we had an incursion of ‘foot and mouth’ or other serious animal disease.

“So what is the way forward? There are four “must dos” to stop the spread, and the Californians on the recent Te Wero Nui roadshow are adamant we should be doing all of these at once:

  • Locate it by methodical surveillance using remote operated vehicles and divers
  • Exclude boats from anchoring and fishing at known sites, and educate marine users
  • Remove it by hand, use of tarps and suction dredging, depending on the scale of infestation
  • Monitor the sites long term to treat and prevent further regrowth.”

Conflict of interest statement: Barry Scott is a trustee on Aotea Great Barrier Environmental Trust and recent co-chair of Caulerpa Suction Dredge Technical Advisory Group.

Dr Christopher Cornwall, Lecturer in Marine Biology, School of Biological Sciences, Victoria University of Wellington, comments:

“It is imperative that action is taken to stop the spread of Caulerpa brachypus and Caulerpa parvifolia. Other Caulerpa species overseas have caused extensive damage to ecosystems, taking over entire reef systems and replacing native species.

“For example, in the Mediterranean I have previously worked on other Caulerpa species, which tend to do better in anthropogenically modified environments. However, it should be noted that this species appears to spread and smother other marine organisms much more easily and thus has the capacity to cause similar or greater levels of local ecosystem modification that those species.

“Hopefully these trials will be the start of an intensive program of eradication, as action must be taken quickly to prevent further spread and hopefully bring this species under control. However, it must be noted that this species can live in depths below those that are safely accessed using SCUBA gear and eradication will be difficult. Thus, we need further peer-reviewed work to assess the possible spread of this species and how it might grow and function in New Zealand in our conditions and to what extent it will spread.”

Conflict of interest statement: “I’m funded by the Royal Society Te Apārangi and the Tertiary Education Commission as well as Victoria University of Wellington.”

Dr Ian Davidson, Team Leader – Invasion Ecology and Management, Cawthron Institute, comments:

“The Aotea suction dredge trial is an important step in marine invasion management for exotic Caulerpa. We need to understand if we can have an impact on the trajectory of this invasive seaweed. This is a positive step toward developing effective tools and methods to reduce its footprint or stop it from establishing in additional locations. Using the best information available about the site, we have designed a robust approach to answer key questions about efficacy, efficiency, logistics, and non-target impacts. These are all key elements that will inform a broader plan to tackle this invasion.

“Applying effective treatments and conducting removal efforts in the sea is a tricky proposition. The environment can be tough to operate in and there aren’t off-the-shelf tools available like there are for pest control in homes, gardens, farms, and the landscape. This Aotea trial is happening at an offshore location to assess some logistical challenges of operating there and to meet the challenge where it is occurring.

“The primary tool being trialled is suction dredging, while chlorine under containment will also be examined. We will find out how well the method removes exotic Caulerpa without leaving fragments or sub-surface components behind. We will find out how long it takes to treat certain areas of the seafloor. We will also find out how the method performs across different types of seafloor – like sandy versus bedrock habitats – and whether there’s an effect on native species or habitats by doing the work. We expect some non-target impacts like sediment disturbance and removal of small animals and seaweed, but additional monitoring after the trial will collect information on recovery of native biota. There are also procedures in place to minimise non-target impacts on larger shellfish and crabs. In previous examples of this type of work, we’ve learned that it’s sometimes necessary to take a short term hit for a longer-term gain of removal. That potential ecosystem resilience is a valuable asset if removals are effective. We won’t answer every question we have for this particular problem, but there’s a good trial ahead that will answer some key questions.

“The team has been moving apace to get various components in place for a logistically challenging field operation. Everyone involved is pulling in the same direction with good collaboration and discussions across all stakeholders to have a strong trial and strong impact.”

Conflict of interest statement: Ian Davidson is part of MPI’s ‘Aotea Operational Trial Team’ for exotic Caulerpa response.