Two invasive marine pest species have been found on Stewart Island, one appearing for the first time in Southland waters. 

Carpet sea squirt (Didemnum vexillum) has been found at two locations in the waters around the island, as well as a further discovery of the invasive seaweed Undaria (Undaria pinnatifida) 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 (ES) wants to keep out of the region because they have the potential to adversely effect on the environment or economy.

Invasive seaweed Undaria (Undaria pinnatifida) has been found at a new site on Stewart Island. Photo: Supplied/ES

ES 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.

However, Meade said the council cannot be everywhere, so is grateful to the public for reporting the pest species. 

“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,” says Meade. 

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, on Stewart Island Undaria is present in Oban, Paterson Inlet, with further sightings in Port Adventure and Broad Bay.

ES say 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,” says Meade. 

“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.”

ES say the council needs to know if these pests are elsewhere in Southland, and urged the public to report any sighting to their biosecurity team.

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.

To report sightings, members of the public can email [email protected], or make a report through the PestHub website - https://pesthub.es.govt.nz/