Introduced Marine Pest: Carpet sea squirt

Page last updated: Friday, 15 March 2024 - 10:24am

Please note: This content may be out of date and is currently under review.

Carpet sea squirt (Didemnum vexillum) is an invasive marine pest that can overgrow native marine species and increase biofouling on commercial and recreational vessels. 

It is a noxious fish listed under the Fish Resources Management Act 1994 and a declared pest under the Biosecurity and Agriculture Management Act 2007. It was detected in Western Australian waters in early 2023.  

Situation update

In early 2024, the Consultative Committee on Introduced Marine Pest Emergencies concluded that eradication of the marine pest was not technically feasible. DPIRD will continue to actively monitor and manage this pest, with the primary aim of minimising spread into other areas of Western Australia.

Carpet sea squirt (CSS) has been detected in two WA locations; at HMAS Stirling Garden Island, and the Australian Marine Complex (AMC), Henderson. In 2023, CSS was also detected at Fleet Base East (FBE) in Sydney New South Wales.

Throughout 2023, DPIRD worked with the Australian Government Department of Defence, the Department of Agriculture, Fisheries and Forestry, Fremantle Port Authority and local stakeholders to respond to the detection of CSS.

DPIRD has undertaken delimiting surveys, vessel management controls, and ongoing marine pest surveillance as part of the response.

As ongoing surveillance indicates that CSS is continuing to persist at known locations, and eradication is not possible, DPIRD will transition the response to management and continue to undertake marine pest surveillance and management activities in collaboration with stakeholders at key locations.

Quarantine area

A new Quarantine Area Notice (QAN) has been issued for the AMC Common User Facility (CUF) Southern Harbour which will be in place until 11:59 pm 14 March 2025. The previous QAN, effective from 20 March 2023, was revoked on 15 March 2024.

A copy of the full notice and supporting documents can be found to the right-hand side of this page.

Under the new QAN, the Quarantine Area (QA) remains unchanged. The QA includes waters adjacent to the AMC CUF and BAE Systems Australia sites and is encompassed by seawalls to the north and west and extends 250 m down the coastline south of the AMC CUF. The QA covers waters that are restricted access waters, and recreational vessels should avoid the area.

A map of the QA can be found to the right-hand side of this page.

The aim of the QAN is to:

  • Mitigate the risk of vessels spreading the pest outside the QA to other parts of Western Australia when they leave the QA.
  • Strengthen the requirements for biofouling inspections and treatment for vessels that have been in the QA for a period of 30 days or more.

There are no restrictions or requirements for vessels berthed in the QA for less than 30 days.

About Carpet sea squirt

CSS is native to east Asia, likely Japan, and has been introduced to New Zealand, North America and Europe. 

CSS is a marine invertebrate animal that can overgrow and smother native species. It can overgrow rocks, shellfish, sea sponges and man-made structures such as wharves, jetty pylons, pontoons, buoys and vessels. 

What can I do to prevent CSS from spreading in WA?

Recreational boaters, fishers and divers can assist in stopping the spread of marine pests and aquatic disease by keeping their boats, trailers, wetsuits and equipment clean. 

When you clean your equipment, make sure that any bait, debris and seaweed is removed and not returned to the water. Check wheel arches on trailers, boat propellers, fishing tackle and footwear. Do not clean in the water or allow run off to enter the ocean. 

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, even if it is on the same day. 

There are many native species that look like carpet sea squirt so expert taxonomic identification and molecular techniques are required to confirm the identity of the species. 

If you find unusual marine species attached to vessels, sub-merged infrastructure or in the marine environment, report it to​

More information

carpet sea squirt factsheet and FAQ documents are available for download. More detailed and technical information on CSS is found at the National Introduced Marine Pest Information System (NIMPIS​).​

Contact information

Phone 1800 815 507 (FishWatch 24-hour hotline)