Full Breakdown
Decline of Invasive Seaweed Caulerpa: A Temporary Retreat or a Permanent Solution?
1/30/2026, 12:21:42 PM
Overview of the Caulerpa Situation
The invasive seaweed Caulerpa has shown significant signs of decline in certain areas of New Zealand, particularly following the disturbances caused by Cyclone Tam in April 2022. Biosecurity New Zealand's pest management director, John Walsh, confirmed that the dieback of Caulerpa was first observed off Aotea Great Barrier, where previously dense carpets of the seaweed, covering 90-100% of the seabed, have virtually disappeared. This reduction is attributed to sedimentation from the cyclone, which starved the seaweed of sunlight, although other factors may also be involved.
Ongoing Management Efforts
In response to the Caulerpa invasion, several management strategies have been implemented. Controlled Area Notices remain in effect, prohibiting fishing and anchoring in affected regions while divers monitor the Hauraki Gulf and Bay of Islands. Community initiatives, such as those led by the Conquer Caulerpa Trust, are actively working to eradicate patches of Caulerpa using methods like mats and chlorine tablets. Additionally, a barge-mounted suction dredge, designed to remove large areas of the weed, is being developed in collaboration with the Ministry for Primary Industries (MPI) and the Northland Regional Council.
Perspectives on the Decline
Local kaitiaki Rana Rewha expressed caution regarding the apparent decline of Caulerpa, noting that new growth has been observed in some areas, indicating that the threat may not be over. He highlighted the importance of monitoring, as past experiences have shown how quickly Caulerpa can proliferate. Professor Barry Scott, a member of Aotea Great Barrier’s Tai Tu Moana steering group, suggested that seasonal variations and the reproductive cycle of Caulerpa could also explain its decline, although these theories remain unproven.
Divergent Approaches to Management
The management strategies for Caulerpa have varied between regions. In the Bay of Islands, a technology-driven approach has been adopted, focusing on large-scale removal methods. Conversely, Scott advocates for a more protective strategy at Aotea, emphasizing the importance of maintaining healthy ecosystems to resist invasive species like Caulerpa. He noted that areas with thriving ecosystems are less susceptible to the weed, suggesting that ecological health is a critical factor in managing invasive species.
Future Considerations
Despite the current decline of Caulerpa, experts warn that it is premature to assume the problem has been resolved. Walsh indicated that while there are signs of regrowth, the potential for Caulerpa to return remains high. Continuous monitoring and adaptive management strategies will be essential in addressing this invasive species and protecting local marine ecosystems.
Verbatim Quotes
- “When we saw these multi-hectare carpets appearing, it was very, very concerning … I think people are right to be cautious about making assumptions that if the caulerpa has disappeared, it’s going to continue to be that way. We’ve got a window to breathe, that’s awesome, but it may well grow back. We don’t know.” — John Walsh, Biosecurity NZ
- “So we are definitely being cautious. We’ve got our guard up because we’ve seen what it’s done in the past, how it’s just taken off and carpeted everything over about 30ha within three months.” — Rana Rewha, Ngati Kuta Kaitiaki
- “Where the ecosystem is thriving, where it’s protected, it’s the most resistant to caulerpa.” — Professor Barry Scott, Tai Tu Moana Steering Group
