Environment Southland is marking a major biosecurity success after eradicating the invasive seaweed Undaria pinnatifida from Broad Bay on Rakiura Stewart Island.
The breakthrough follows the discovery of a single craypot carrying Undaria in September 2020. The pest was immediately removed and treated, triggering a detailed monitoring programme. Extensive follow-up surveillance between 2023 and 2025 found no further trace of the seaweed in the area.
Environment Southland Marine Biosecurity Team Leader Kathryn McLachlan said recent dive surveys confirmed the swift action taken at the time had worked.
“Undaria is a rapidly growing seaweed that disrupts marine ecosystems. Its removal after initial detection shows that eradication can be achieved with swift, coordinated efforts”.
“We can celebrate Broad Bay’s return to Undaria-free status. This result demonstrates what can be achieved when regional authorities, local communities, and marine users work together,” Kathryn McLachlan said.
Despite the success, she warned that continued vigilance remains critical to prevent the pest returning.
“Ongoing vigilance is essential to prevent reintroduction and ensure permanent eradication and we will continue annual surveillance of both Broad Bay and the wider southern Rakiura area, including northern Rakiura and Easy Harbour, where the seaweed is still established,” Kathryn McLachlan added.
Undaria is already established in parts of northern Rakiura and Easy Harbour, increasing the risk of it spreading through vessel movement. Officials are urging boaties and marine users to take simple precautions.
“We want people to take steps not to transport Undaria when moving around Rakiura, especially from the northern parts of the island, where Undaria is established. Remember to maintain a clean hull, and clean gear.”
The ocean plays a central role in the region’s lifestyle. It supports recreation, food gathering and connection with nature. However, invasive marine pests threaten ecosystems, fisheries and biodiversity.
The Protect Our Paradise campaign was created to raise awareness about safeguarding the marine environment. This summer, Environment Southland and Biosecurity New Zealand have partnered to appoint a marine biosecurity ambassador for Southland. John Carter, from Te Anau, will visit boat ramps, wharves and marine events to speak with the public about protecting local waters.
New sightings of Undaria can be reported through pesthub.es.govt.nz. Members of the public are reminded to leave marine pests in place and not attempt to remove them.