Background

There are various tools available to remove or treat exotic caulerpa in the ocean but there are limitations to each method depending on the substrate on which the caulerpa is found, the density of growth, the depth where it is found, and the extent of cover across the seafloor.  Importantly, most elimination/eradication operations will always require a primary and at least one secondary treatment.

While very small patches of caulerpa can be removed by hand weeding, for larger areas the most effective removal method is suction dredging, which minimizes fragmentation and dispersal of the caulerpa during removal. The two main types of suction dredging are diver-directed suction dredging and remotely (barge) controlled mechanical dredging. The former is of much lower impact on the seabed than the latter because it is diver controlled. In general, dredging is most effective on open sandy or muddy seabeds where there are few physical barriers to the dredge. A current limitation of remotely controlled mechanical dredging is the depth at which the dredge can operate, which is around 10 metres. Where the caulerpa is anchored to rocks, shellhash or growing on rocky reefs, dredging is a much more difficult method to deploy. It is also not cost effective if the exotic caulerpa is sparse and widely dispersed across the seafloor, as is the situation on the Iris Shoal at Kawau Island and around the ‘Man of War’ passage at Port Fitzroy. Various innovations around dredging have been recently developed including: the Mini Fragmentation Stopper Dredge developed by Commercial Dive Specialists (Brook McCrae) for removing small patches of exotic caulerpa; the use of trommels by Johnston Brothers to improve the filtering and harvest of caulerpa by mechanical dredging; and more recently and still under development, the submersible dredge planer, also by Johnston Brothers. The latter is one of three projects funded from the $10.2 m Tools Projects announced by Minister for Biosecurity, Andrew Hoggard, in February 2025 (Note: original announcement was in August 2024). The submersible dredge planer project will receive $6 m from this Tools Development Fund.

The most effective method for treating exotic caulerpa in situ is the use of benthic mats either with or without the addition of chlorine. Benthic mats block out the sunlight that is required for the caulerpa to grow thereby causing it to die. With benthic mats alone, either synthetic PVC or woollen whāriki mats, the mats need to be in place for at least two weeks, but often longer, to be effective in killing the caulerpa. Trials to date suggest benthic mats are effective in killing caulerpa if it is not too dense, as tends to be the case where C. parvifolia is found. The dense carpets of C. brachypus are much more difficult to treat with benthic mats alone. The addition of chlorine, provided in various chemical forms, is by far the most effective method for killing exotic caulerpa in situ. The use of synthetic benthic mats together with chlorine treatment was the primary method used for eradication of the aquarium strain of C. taxifolia from Agua Hedionda lagoon and Huntington Harbor in Southern California, an operation that ran from 2000 to 2006.

The use of chlorine as a treatment for exotic caulerpa in Aotearoa

A primer in chlorine products

It is the strong oxidizing properties of chlorine and chlorine products that make them very effective in killing bacteria, fungi, algae and viruses. Most commonly used household bleach/disinfectant products contain chlorine in the form of sodium hypochlorite. The chlorine products used to treat drinking water supplies are liquefied chlorine gas, sodium hypochlorite and calcium hypochlorite. Dose is all important in striking that balance between effectiveness and toxicity. New Zealand law requires that there be a minimum of 0.2 parts per million of chlorine in town water supplies. In practice the chlorine level is maintained within a range of 0.2 – 1 ppm. A different form of chlorine is used for treating swimming pools (usually sodium dichlorocyanurate / DichlorTM or trichlorocyanurate / TrichlorTM) to ensure it does not breakdown in the strong sunlight. The level of chlorine recommended for level for swimming pools is typically 1-3 parts per million (ppm). For the avoidance of doubt these are all different forms of chlorine to the sodium chloride found in seawater.

Chlorine treatment of exotic caulerpa in New Zealand

Chlorine treatment of exotic caulerpa has been used in both removal operations and in treatment trials at three sites around the NE coast of New Zealand.

Following detection of exotic caulerpa in Omaki Cove at Te Rāwhiti in May 2023, Northland Regional Council in partnership with Te Rāwhiti hapu, Ngāti Kutu and Patukeha, rapidly mobilised to carry out extensive surveillance around the Te Rāwhiti channel to delimit the extent of the incursion. They then acted to limit the spread by treating patches of caulerpa around the periphery with synthetic benthic mats and chlorine. The first benthic mats were laid in June 2023 by Marine Environmental & Field Services, and chlorine pellets (in the form of hypochlorite in PVA Berley Bags) sprinkled underneath. Mats were left for a few days then removed. I have no information on how these sites have recovered since treated but given they were relatively small and isolated patches and were not surrounded by a carpet of exotic caulerpa, I would posit that native marine flora and fauna would have likely re-colonised these sites.

Besides the benthic mat operation on the periphery, two remotely operated mechanical dredging trials (Phase I: Feb to April 2024; and Phase II: April to June 2024) were carried out in Omakiwi Cove in 2024, and extensive surveillance in wider Northland completed. Despite this impressive effort three new sites of exotic caulerpa outside the CAN were recently detected (February 2025) at Paradise Bay on the western side of Urapukapuka Island, the southern end of Motukiekie Island, and at Army Bayon Motorua Island. More recently (end of March) divers found a large infestation of exotic caulerpa in Paroa Bay, very close to Russell.

As part of the diver directed suction dredge trial carried out by Bay Underwater Services in Tryphena Harbour in September 2023, a trial on the effectiveness of chlorine treatment (in the form of sodium dichloroisocyanurate; DichlorTM) was also carried out. However, for some inexplicable reason the mats were laid over sites that had already been dredged, so not surprising the benthic mat treatment had no or only minor additional effects on the caulerpa when compared to sites that were just dredged. Three follow up studies of the sites in October, November and December of 2023 showed that dredging had impacted on the structure of the seafloor sediments and on the biological communities living on and in the seafloor, but there was still considerable species diversity within the seabed, post treatment. It is difficult from the design of the trial to directly assess the impact of the chlorine treatment.

The effectiveness of chlorine treatment in treating a patch of newly identified exotic caulerpa was highlighted by the find at Omaha Cove / Leigh Harbour in June 2024. The surveillance in this area by Auckland Council was in response to concerns that commercial crayfishers from Leigh, who had obtained permits from Fisheries NZ to lay pots through the western harbours of Aotea Great Barrier, within the CAN, and at sites of known heavy infestation of exotic caulerpa, might have spread the seaweed to Leigh. While there was a little hiccup in the response in that a commercial dive team were unable to relocate this small patch of C. parvifolia, University of Auckland divers were able to find it and rapidly treat it within 10 days (17th June 2024) of the original find (7th June 2024). The patch was very small (1/4 m2) and in a shallow (3.5 m) area of the well protected bay. The patch was treated with chlorine pellets, covered with a tarp, then left overnight before removal. Visual examination of the caulerpa showed it had been effectively killed but as an extra precaution the divers suctioned off the dead caulerpa and removed it from the site. Despite being dead the caulerpa was still difficult to remove from the rocks, even with knives. The coralline algae covering the rocks that were treated appeared to be unaffected by the treatment. A presentation of this operation together with a video of the ‘in sea’ treatment was shown at the 27th June Exotic Caulerpa Operators Forum with the Leigh section starting at 38 min.

Floating Chlorine Chamber

A second project being funded from the new exotic caulerpa tools fund is the development and trialling of a floating chlorine treatment chamber by Commercial Dive Specialists (CDS). They will receive $1.9 m in funding from this new tools allocation. This new technology has been developed in response to the inability to effectively use benthic mats or dredging methods to remove or treat exotic caulerpa growing on large boulders, rock shelves and reefs and in areas subject to strong currents. While detail of the design of the floating chlorine chamber and how it will operate, as well as some of the specific questions they want to answer from a trial at Aotea, that cannot be addressed at Omakiwi Cove, were provided in files from MPI and Commercial Dive Specialists, much of the detail was still unclear.

To get greater clarity around the operation I spoke to Brook McCrae from CDS. The main reasons CDS want to trial their floating chlorine chamber at Aotea Great Barrier are:

  • The exotic caulerpa mats/carpets found at Aotea are (were?) up to 1 m or more in thickness compared to 400 mm or less at Omakiwi Cove. The trials undertaken at Omakiwi to date have shown the floating chamber is very effective at killing the caulerpa there and at much lower doses of chlorine than originally anticipated.

  • Aotea provides an extensive range of different seabed terrain from open sandy/muddy/shellhash flats through to rocky boulders, reefs and shelves. CDS would want to first start trialling the system on open sandy seabed terrain as found in the western harbours of Okupe and to a lesser extent in Whangaparapara and Tryphena. 

  • Aotea is physically remote thereby providing a good site to test the self-reliance of the operations

  • And finally a genuine concern from CDS about how exotic caulerpa has impacted on Aotea and an eagerness to help through trialling this method. 

CDS have consents to use either DichlorTM or TrichlorTM as the chlorine source for their chamber, but it is the former they have mostly used at Omakiwi. The pellets of Dichlor are dissolved in barrels on the boat to generate a very concentrated chlorine mixture. Seawater is pumped from the chamber up to the boat where it is ‘loaded’ up with chlorine through a peristaltic pump system to control the concentration, then fed back into the chamber. Highly sensitive meters are placed within the chamber to monitor the chlorine levels being used. All operations are captured on video. The chamber is then very slowly manually or mechanically winched across the seabed surface to treat the exotic caulerpa. They have chambers in a range of sizes and configurations depending on the terrain they want to test on. Work to date suggests a period of about 30 min exposure is required to kill the exotic caulerpa but as you would expect there are a number of variables at play here such as the thickness of the caulerpa, dose of chlorine (concentration x exposure time) etc. Hence the need for trials. Preliminary data suggest that crustaceans such as crabs are able to survive treatments of less than one hour but further data is needed to determine the effects on molluscs and other marine animals as well as other seaweeds.

The Aotea Dilemma

The dilemma facing the Aotea community is exotic caulerpa has now spread across an estimated 800 ha, making it near impossible now to consider eradication. In 2023 we (Aotea Caulerpa Action Group) had the goal of ‘stopping the spread’ if at all possible by keeping the anchor zones within the CAN in Ports Fitzroy and Abercrombie free of exotic caulerpa. Also the bays further north, along the east coast and around Rakitū. Spread by vessels and ocean current makes it difficult to achieve that goal, especially in the absence of sufficient resources to support an on-island response team that is able to carry out extensive surveillance and treat or remove when new small patches are detected.

In assessing whether we support this trial or not, we need to assess the risks, costs and benefits. Such a trial will provide considerable new knowledge and information on the effectiveness of this method to control exotic caulerpa and what impact it will have on the marine ecosystem. While Ian Davidson from the Cawthron has been contracted to write the scientific report on the CDS Floating Chlorine Chamber trial, it is unclear how much in-sea monitoring will be carried out during the trial, and what post-treatment monitoring will be carried out on how the marine ecosystem recovers. An important recovery consideration is where the trial is carried out. If the trial is carried out in the middle of a bay covered in exotic caulerpa, then inevitably exotic caulerpa will readily re-colonise cleared sites. If an area of exotic caulerpa is cleared which is surrounded by a rich native marine flora and fauna then there is a much higher probability that those species will fill the ‘vacant’ space. This is a scenario we are all familiar with following treatment of terrestrial weed species. 

Knowledge Gaps

 The big knowledge gaps are:

  • How do marine ecosystems recover following treatments such as dredging and chlorine treatment? While there is good information on native eelgrass recovery in Newport Bay following the suction dredge operation carried out in 2021 I have been unable to find information on the recovery of native marine flora and fauna in the two Southern Californian lagoons that were treated with chlorine under tarpaulins. Also, I have been unable to find any details on marine seabed recovery at Te Rāwhiti following the use there of chlorine under tarpaulins. A discussion with Northland Regional Council and Te Rāwhiti hapu, Ngāti Kutu and Patukeha, is recommended.

  • What is the long term impact of exotic caulerpa on marine ecosystems and the economic and community impacts? Given the incursion into Aotearoa is recent some of these answers will only become apparent with time as we are dealing with considerable uncertainty. But even for overseas incursions, and in particular the spread of the aquarium strain of C. taxifolia, which started as a small 1 m2 patch below the Oceanography Museum of Monaco (in 1984) and subsequently spread to a seafloor area of 13,000 ha across six Mediterranean countries by 2001, there is a dearth of scientific information. What we do know is captured in The Anatomy of an Invasion. Exotic Caulerpa at Aotea Great Barrier Island, and references therein, as well as in the Martin Jenkins Fighting Invasive Caulerpa: An Indicative Business Case, which is due to be publicly released this week.

Besides the NIWA, Cawthron and community-led studies that are summarized in the above reports, MPI did announce in February this year that they were going to fund Auckland University, in partnership with Ngāti Manuhire, to conduct a long-term impact study on the ecology and impacts of exotic caulerpa on marine ecosystems and cultural impacts of exotic caulerpa incursions. The project will assess the environmental drivers that support the establishment and growth of exotic caulerpa, the impacts of it on seafloor biodiversity and ecosystem function, and cultural impacts. There will be approximately monthly trips to focus sites (Mokohinau Islands, western Coromandel and Kawau Island) throughout April and May, with a report on this research expected to be completed by 1 June 2025. 

MPI is also funding artificial intelligence surveillance research, which is a continuation of the study started under the Accelerated Fund announced in 2024, and a NIWA study of why there has been a reduction in the biomass of exotic caulerpa at several sites around Aotea and elsewhere in the Hauraki Gulf.

UV-C Light as a Treatment Tool

The third project that is receiving funding from the Tools Development Fund is the use of short wave length ultra-violet (UV-C) light irradiation as a tool to kill exotic caulerpa. UV-C is a powerful radiation that damages the genetic material (DNA and RNA) in cells thereby preventing the cells from growing, and eventually killing the organism. Trialling is required to determine the optimum dose (radiation with time) and how effective the tool can be utilized over different seabed terrain and on different densities of exotic caulerpa.

The first trials of this tool were carried out at Rakino Island in 2024 and are described in Tools for controlling exotic caulerpa in Aotearoa New Zealand. Advanced Aquarium Technologies is the company that is developing this tool and has received $2 m from the Tools Fund for further development of this technology. The trial is currently taking place in Omakiwi Cove, at Te Rāwhiti.

Proposed Chlorine Chamber Trial for Aotea

Barry Scott - April 2025