The rapid spread of an invasive seaweed in Fiordland waters has forced a regional council to adjust how it deals with the plant.

Undaria is a fast-spreading pest that has established itself around the country over the past 30 years, including in Fiordland’s Te Puaitaha/Breaksea Sound and Tamatea/Dusky Sound.

Environment Southland has now revealed it is shifting away from large-scale control at those sites because it cannot contain it under existing budgets.

Instead, it will target new infestations and concentrate on high-risk anchorages and moorings.

“This change will enable an increase of surveillance trips to high-risk areas and to act quickly if any new infestations are found — within existing budgets and resources,” council team leader marine Kathryn McLachlan said.

The decision was made by a governance group which includes the council, the Ministry of Primary Industries and DOC, with support from Fiordland Marine Guardians.

The new approach had already been successful at Taiari/Chalky Inlet in 2019, with the area declared free of the pest last year, McLachlan said.

Guardians chair Dr Rebecca McLeod said while the previous goal was to contain the plant to an area until eradication technology could be developed, it had now spread extensively throughout the two sounds.

“We appreciate that this incursion has now reached a size where efforts to reduce its spread and reduce the biomass are no longer effective.”

The focus was now on containing Undaria within the two fiords and making sure new incursions were detected quickly, she said.

Undaria competes with native species and travels by attaching onto ship hulls.

Fiordland Marine Guardians urged operators entering Fiordland to check their hulls and gear were clean, and reminded that all vessels entering Fiordland were required to hold a clean vessel pass from the council.

The plant was not believed to have reached Fiordland until 2010, when a single mature plant was found in Te Puaitaha/Breaksea Sound.

Environment Southland said it was also doing control work at Easy Harbour on Rakiura/Stewart Island.

LDR is local body journalism co-funded by RNZ and NZ On Air