Carpet sea squirt in Western Australia and New South Wales

Responses to carpet sea squirt (Didemnum vexillum) are underway in Western Australia (WA) and New South Wales (NSW).

Carpet sea squirt (CSS) is an exotic marine pest that has the potential for significant environmental and economic impacts. It is listed on the National Priority List of Exotic Environmental Pests, Weeds and Diseases.

The Australian Government Department of Agriculture, Fisheries and Forestry (DAFF) is working with the Western Australian Department of Primary Industries and Regional Development (DPIRD), the NSW Department of Primary Industries (DPI) and the Department of Defence (Defence) in response to detections of the marine pest carpet sea squirt (Didemnum vexillum) at HMAS Stirling (WA), a Henderson marine facility (WA), Kwinana Grain Jetty (WA) and at Fleet Base East in Sydney Harbour (NSW).

The Australian Chief Environmental Biosecurity Officer, as national contact point of notifications for environmental pest and disease detections and responses, has agreed that both DPIRD and the NSW DPI would work with Defence to manage the marine pest incursion in both states.

Current situation in WA

The first confirmed detection of CSS at HMAS Stirling, WA, was reported to the Consultative Committee on Introduced Marine Pest Emergencies (CCIMPE) in April 2021. This was the first recorded detection of CSS in Australia. In January 2023, CSS was also detected at a marine facility in Henderson, WA.

The CSS incursion at HMAS Stirling, is being managed as a Commonwealth response as it was found at a Commonwealth site.

From March to November 2021, a delimiting survey was undertaken to identify the extent of the pest colonies at HMAS Stirling. Following DPIRD’s expert advice on removal and disposal methodology, remediation works through targeted removal began in November 2021.

Defence under the direction of DPIRD, DAFF and the CCIMPE is implementing necessary actions to manage this pest incursion at HMAS Stirling. Other actions include surveillance, monitoring, and vessel management control procedures.

Foreign warships are required to comply with Australian Government requirements when entering Australian waters.

The CCIMPE met in March 2023 to discuss the technical feasibility of eradicating (TFE) CSS and the consultative committee agreed that it is technically feasible to eradicate CSS at HMAS Stirling. However, the TFE will need to be reassessed with any significant change of the current situation and additional information becoming available.

DAFF has commissioned a cost-benefit analysis to identify the costs associated with the response and potential impacts if CSS was to spread to other parts of Australia.

Annual in water-surveillance at the site commenced in May 2023, and the results will be presented in an annual report later this year.

In January 2023, new detections of CSS were confirmed on wharves at the Australian Marine Complex (AMC) at Henderson (WA).

The CSS incursion at the AMC Henderson is being managed by DPIRD as the lead agency in WA.

In March 2023, DPIRD issued a Quarantine Area Notice (QAN) following the detections of CSS at the AMC Henderson. The QAN outlines vessel management requirements for vessels within or exiting the quarantine area.

The quarantine area covers the waters adjacent to the AMC at Henderson South and encompasses seawalls to the north and west, extending 250 metres down the coastline to the south of the AMC.

DPIRD is working with local stakeholders and the Fremantle Port Authority to manage this incursion.

Additional vessel management procedures will also be implemented to minimise the risk of further spread, including an active marine surveillance program at the Port of Fremantle and Cockburn Sound.

In April 2023, a sample was collected at Kwinana Grain Jetty, Kwinana, WA. The sample initially returned an indeterminate result. Subsequent re-testing indicated the presence of CSS. However, samples taken in both June and August 2023 did not return positive results. Further testing and delimitation are underway.

The CCIMPE will continue to meet to discuss and provide technical support for the CSS outbreak at Henderson and Kwinana once DPIRD has finalised delimitation surveys.

Current situation in NSW

Defence and DAFF are working with NSW DPI in response to a new detection of carpet sea squirt (Didemnum vexillum) at Fleet Base East (FBE) in Sydney Harbour, NSW.

In May 2023, a single colony of carpet sea squirt was detected at FBE. Delimitation surveillance is underway to determine if other colonies are present and if so, the extent of the outbreak.

The Australian Chief Environmental Biosecurity Officer and CCIMPE have been notified of the detection.

In June 2023, a delimiting survey using divers commenced to determine the extent of the outbreak. The survey is anticipated to take upwards of six weeks to complete, with ongoing surveillance planned after the delimitation survey is complete.

Surveillance and treatment are being overseen by Defence with guidance from DAFF and DPI.

Further inspections at other Defence sites may be required in addition to targeted surveillance within the broader Sydney Harbour area.

Government response

The Consultative Committee on Introduced Marine Pest Emergencies (CCIMPE) is providing technical and scientific advice on the CSS response.

The consultative committee is chaired by Australia’s Chief Environmental Biosecurity Officer and comprises marine pest managers from each state and the Northern Territory and involves other specialists across government.

The DPIRD is continuing the collaborative marine pest surveillance program (SWASP) with WA Port Authorities, which provides marine pest surveillance at ten port locations in WA.

Defence has drafted a Biosecurity Response Management Plan detailing actions to manage this marine pest. It is implementing a national marine biosecurity surveillance program in partnership with state and territory governments.

Foreign warships are required to comply with Australian Government requirements when entering Australian waters.

About carpet sea squirt

CSS is a highly invasive marine animal that can overgrow rocks, shellfish, sea sponges and algae. It can also overgrow man-made structures such as wharves, jetty pylons, pontoons, buoys and vessels.

CSS reproduces both sexually and asexually, where it can grow new colonies from small fragments that may break off from larger colonies.

It is native to east Asia and has been introduced to New Zealand, North America and Europe. The incursion of CSS at HMAS Stirling is the first confirmed detection of this marine pest in Australia. 

More detailed and technical information on CSS is found at the National Introduced Marine Pest Information System (NIMPIS).

CSS growing on an underwater marine structure in New Zealand. The CSS is light cream-yellow in colour and surrounded by other marine organisms.
Carpet sea squirt in New Zealand. Photo courtesy of C. Woods, NIWA

CSS can have potentially devasting impacts on aquaculture industries due to its invasive nature. It can alter the marine environment by competing with and growing on native marine species.

The main impacts of CSS are fouling of vessels, marine infrastructure or aquaculture equipment.

The pest is transported to new locations mainly as biofouling on vessels, or sometimes from natural dispersal through ocean currents.

CSS does not present any known risks to human health.

What to look for

CSS is usually a yellow-grey mass but colour is variable. It has holes that are used for siphoning water to feed on small phytoplankton and organic matter from the sea water. CSS is a filter feeder and not a predatory animal.

It will grow as a covering over surfaces but may get quite thick (up to 10 cm), producing 'arms' or tendrils up to 1 metre long that hang below the main mass. It is known to grow rapidly and aggressively on substrates.

Close-up photograph of CSS showing many small colonial animals and larger pores for filter feeding.
Close-up photo of carpet sea squirt in New Zealand. Photo courtesy of C. Woods, NIWA
CSS growing on an underwater structure in New Zealand and forming ‘dripping’ tendrils. The tendrils are light yellow in colour.
Carpet sea squirt in New Zealand. Photo courtesy of C. Woods, NIWA

There are many native species of colonial ascidians in Australia that look similar to CSS. Accurate identification of CSS is made using a microscope by an expert or molecular techniques to confirm the species’ DNA.

Reporting marine pest sightings

If you find unusual marine species or suspected marine pests attached to vessels, submerged infrastructure or in the marine environment, report it to the Commonwealth, or relevant state or territory authority.

To assist with your report:

Take photos

  • Take a few close-up photos of the suspected marine pest from different angles.
  • Where possible, include an object in the photo for size reference (such as a coin, bank note, or ruler if available).
  • If your camera has GPS, turn it on.
  • Record relevant details including the number of suspected marine pests you saw.

Check the location

  • Record your exact location. Include as many details about where you found the marine pest as you can, such as the habitat (e.g., harbour, beach, rocky reef).
  • Screenshot your map app or use photo geotagging on your phone. You can turn geotagging on in your phone settings.
  • Note the date and time of the sighting.
  • If the organism was found underwater (i.e., while SCUBA diving), include the approximate water depth if known.

Stop the spread of marine pests

There are simple steps you can take to stop the spread of marine pests and help protect Australia’s marine environment and industries.

  • Recreational boaters, fishers and divers should keep their boats, trailers, wetsuits and equipment clean.
  • Check wheel arches on trailers, boat propellers, fishing tackle and footwear for bait, debris and seaweed.
  • Use soapy water to clean your boat and trailer, fishing rods and other equipment, and allow them to dry completely before using them at another location.

More information

More detailed and technical information on carpet sea squirt (Didemnum vexillum) can be found at the National Introduced Marine Pest Information System (NIMPIS).

For further information on marine pests in Australia, please visit:

To report a suspected marine pest, contact your local biosecurity authority at: