Limestone‑Based Shellfish Reefs Return to Swan River

The Swan River, which runs through Perth’s suburbs of Attadale and Freshwater Bay, once supported extensive shellfish reefs built on limestone foundations. Those reefs supplied habitat for fish, filtered nutrients from the water column and helped stabilise the riverbank. Extraction of roughly one million tonnes of limestone between the 1920s and the 1950s removed the natural base, allowing soft sediments to dominate the riverbed and increase nutrient loads.

In 2022 the Nature Conservancy, in partnership with the Department of Biodiversity, Conservation and Attractions (DBCA) and local Noongar groups, launched a five‑year restoration programme. The first phase involved dumping about one million tonnes of limestone into the river to recreate a calcium‑carbonate base. Six hectares of artificial reef were formed in Cockburn Sound and the lower reaches of the Swan River.

Simultaneously, a hatchery operation grew 160 tonnes of native mussels. Those mussels were translocated into the new reef structures in November 2022. Early monitoring recorded substantial filtration activity, with the mussels removing litres of water each day and improving clarity. The reefs also began to attract other shellfish – oysters and scallops – and fish such as snapper were observed colonising the structures.

A warm summer later in 2023 triggered a decline in mussel numbers, highlighting the sensitivity of the translocated population to temperature extremes. Nonetheless, the artificial reef continued to provide habitat for a range of species, and the limestone base began to reduce shoreline erosion by dampening wave energy along the river’s edge.

DBCA will keep a close watch on the pilot reefs, with particular emphasis on shallow‑water structures where soft sediments still dominate deeper channels. Future steps include refining the design of reef modules to improve stability and exploring additional mussel seeding events under more favourable climatic conditions.

The project aligns with broader Australian goals for marine restoration and coastal water‑quality improvement. By restoring a functional shellfish reef system, the initiative seeks to re‑establish a natural filter for nutrient loads, curb algal blooms and reinforce the river’s resilience against erosion.

Stakeholders such as the WA government, the federal government and the aquaculture industry have expressed interest in scaling the approach if the pilot demonstrates lasting ecological benefits. Continued collaboration with the Noongar people ensures that traditional knowledge informs site selection and long‑term stewardship.

Overall, the Swan River pilot represents a tangible step toward rebuilding lost ecosystem services, offering a model that could be adapted to other estuarine systems across the continent.