Two marine pest species discovered at Rakiura Stewart Island

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Photo of Didemnum vexillum - supplied by Environment Southland

Photo of Didemnum vexillum - supplied by Environment Southland

Photo of Undaria - supplied by Kath Blackmore (Department of Conservation)
Photo of Didemnum vexillum on a mussel line - supplied by Environment Southland

Environment Southland has discovered Didemnum vexillum, commonly known as carpet sea squirt, at two locations in the waters around Rakiura/Stewart Island, as well as a further discovery of the invasive seaweed Undaria pinnatifida (Undaria) at a new site.

Carpet sea squirt is an exclusion pest under the Southland Regional Pest Management Plan and this is the first time it has been found in Southland. Exclusion pests are those that Environment Southland wants to keep out of the region because they have the potential to adversely effect on the environment or economy.

Environment Southland biosecurity and biodiversity operations manager Ali Meade said council has a number of work programmes, including the Jobs for Nature Fiordland Undaria Control Programme, that aim to tackle these kinds of invasive pests, but we can’t be everywhere all the time, so we’re grateful to the public for reporting these things when they find them.

“At this stage we’re unclear if the pest organism is more widespread in Stewart Island waters so a plan to undertake surveillance to determine the extent of the spread is being developed."                                                                                                                                                                                                                                         “Carpet sea squirt is a problem because it forms dense colonies that displace native species and damage fisheries. It can smother beaches as well as rock pools and tide pools,” Ali Meade said.

In New Zealand, the carpet sea squirt is generally found on structures such as wharves, mooring lines and vessel hulls.

Carpet sea squirt can tolerate a wide range of environmental conditions, has a high reproductive ability, and high population growth rates.  

Following a report from the public of Undaria in Easy Harbour, divers from the Jobs for Nature project in Fiordland were tasked with an initial survey of Easy Harbour to look for the invasive pest.

The divers discovered the spread was bigger than expected, with more than 300 juvenile Undaria plants found between Easy Harbour and False Easy Harbour. Very few mature plants were found.

Although this is the first find in this area, Undaria is present in Oban, Paterson Inlet, with further sightings in Port Adventure and Broad Bay.

A team of divers will return to further assess the situation, before additional actions are considered.

We will send divers in to do further surveying on the south west coast of the island to confirm the spread,” Ali Meade said.

“We’re disappointed to find more Undaria on Rakiura, however we are thankful to the member of the public who alerted us to the incursion in such a remote area. We ask the public and industry to continue to be vigilant and ensure their boats and gear are clean so we can avoid any further spread of these pests.”

Environment Southland needs to know if these pests are elsewhere in Southland. If you think you have seen either, please contact Environment Southland's biosecurity team.

If you can, please take a clear photo of it and email it to with your contact details, or report it through the PestHub website -