Two marine pest species discovered at Stewart Island

Source: 1News

Two bodies of water around Rakiura/Stewart Island have revealed species of a type of marine pest commonly known as carpet sea squirt along with a further discovery of the invasive seaweed Undaria pinnatifida (Undaria) at a new site.

Didemnum vexillum, commonly known as carpet sea squirt

Environment Southland said in a statement on Tuesday that 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 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,” Meade said.

In New Zealand, the carpet sea squirt is generally found on structures such as wharves, mooring lines and vessel hulls and 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,” 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 says it needs to know if these pests are elsewhere in Southland.

"If you think you have seen either, please contact our biosecurity team a clear photo of it and email it to with your contact details, or report it through the PestHub website.

Carpet sea squirt is leathery or spongey and light mustard in colour. It can look like yellowish wax dripping over a structure such as a rope or mussel line.

The appearance of Undaria differs depending on its maturity. Mature plants are a brown, green and yellow colour and grow up to one to two metres. They have a very visible midrib up the plant. They have a holdfast (which anchors them), a stipe (or stem) and a sporophyll (a spiral shaped reproductive structure which produces spores) found at the base of the stipe.

Juvenile plants have a holdfast and stem and an undivided blade (they appear as a single leaf). The distinctive midrib starts becoming apparent once the plant grows over five centimetres.