Western Australia declares invasive marine pest Didemnum vexillum

The Minister for Agriculture and Food has declared the colonial tunicate Didemnum vexillum a Declared Pest across the whole of Western Australia, including offshore islands. The declaration, published in the Western Australian Government Gazette No. 114 of 4 September 2026, assigns the species Category 3 (Management) control and prohibits its keeping throughout the State.

The measure was made under section 22 of the Biosecurity and Agriculture Management Act 2007 through the Biosecurity and Agriculture Management (Declared Pest) Declaration (No. 2) 2026. It was signed by Hon. Jackie Jarvis, MLC, Minister for Agriculture and Food, and dated 1 September 2026.

A declared pest under State biosecurity law

The declaration is the latest step in Western Australia’s biosecurity framework, which classifies organisms by the level of control and management required to protect agriculture, fisheries and the environment. Under the Biosecurity and Agriculture Management Regulations 2013, each declared pest is assigned to control and keeping categories for the areas specified.

For Didemnum vexillum, the declaration applies to the whole of the State, including offshore islands, with no geographic carve-outs. The organism is assigned:

• Category 3 (C3) — Management control, requiring active management of the pest across the declared area.
• Prohibited keeping, meaning the species may not be held, kept or moved within Western Australia.

The declaration also revokes all previous declarations made under the Act relating to the organism, consolidating the pest’s status under a single instrument.

A threat to marine industries and aquaculture

Didemnum vexillum, a colonial tunicate also known as the “marine sea squirt,” is a fast-growing invasive species that can smother natural and artificial surfaces, including aquaculture infrastructure, moorings, vessel hulls and shellfish beds. Its capacity to overgrow and outcompete native marine life makes it a significant biosecurity risk for Western Australia’s marine industries.

By listing the species under Category 3 management, the State signals that the pest is established or likely to establish and requires coordinated control. The prohibited-keeping classification tightens the legal obligations on operators, who must now take steps to prevent the species from being introduced, kept or spread through their activities.

What the declaration means in practice

The declaration places a legal duty on anyone who encounters or handles the species to comply with the management and keeping requirements set out in the Act and Regulations. This has direct implications for:

• Aquaculture operators, who must guard against the tunicate attaching to farm infrastructure and stock.
• Commercial and recreational vessel owners, whose hulls and gear can transport the pest between locations.
• Marine industries and port operators managing moorings, jetties and other submerged structures.

The measure strengthens Western Australia’s biosecurity defences against a species that can spread rapidly through fouling and ballast water, protecting both the marine environment and the economic value of the State’s coastal and aquaculture sectors.

Source: Western Australian Government Gazette, No. 114, 4 September 2026, Agriculture and Food section, p. 1859 (official reference: AG401).