Restoring Swan River's Shellfish Reefs for Environmental Health

Two hundred years ago, before the Swan River colony was formed, the river estuary known as the Derbal-Yerrigan to the Noongar people was a unique environment.

"A lot of the base of the river was built of historical shellfish reefs, and that was a limestone base," said Brett Dal Pozzo, marine restoration manager at non-profit organisation The Nature Conservancy.

"We had calcium carbonate shells from thousands and thousands of years growing, and that hard substrate support was a habitat for fish and other species.

"It also was alive with the shellfish that were filtering water. Each shellfish can filter litres of water per day, and obviously that provides a lot of a cleaning service for our river system."

From the 1920s to the 1950s, about 1 million tonnes of limestone that formed shellfish reefs was pulled out of the Swan River bed as a source of building materials.

It left the riverbed severely degraded.

Those soft sediments have high nutrient loads that can be stirred up and flow into the river.

The changing climate is also putting more pressure on the river's health, making it more important than ever to bring those shellfish reefs back to the river, and with it the marine life that can filter the water.

"Shellfish reefs are a key component of dealing with [high nutrient loads]. They will be able to filter out nutrients and address potential algal blooms," he said.

"They will also help the river adapt to changes in our climate and some of the stresses it's dealing with at the moment."

Bringing reefs back

Five years ago, The Nature Conservancy, with the support of philanthropists and the state and federal governments, began a project to bring back those reefs.

In 2023, when they began dumping limestone into the riverbed, they created about 6 hectares of reef in the river around Attadale and Freshwater Bay in what had been areas of soft sediment.

"We were lucky to partner with aquaculture industry and grew 160 tonnes of mussels in Cockburn Sound, and those mussels were translocated onto our reefs in November 2022, to really kickstart it in those first few months," Mr Dal Pozzo said.

"Those mussels began almost immediately filter feeding on those reefs and really providing a foundation."

Due to warm conditions shortly after, there was a big loss of mussels over the following summer, but Mr Dal Pozzo said shellfish numbers had come back strongly.

A bonus benefit has been the impact on fish stocks.

"We're seeing fin fish colonise that reef, and a whole range of other key species for our river actually using that as habitat into those deeper waters," he said.

"They provide a whole range for snapper, and a lot of our breeding, key breeding stock of recreational species, which is fantastic to see."

Heading for shallow water

Ongoing monitoring of the reefs will now be done by the Department of Biodiversity, Conservation and Attractions (DBCA), while the conservancy plans to turn its focus to the health of the river in shallower areas.

"We've just installed a pilot, which is reef structures that are fabricated close to shoreline, which provides shoreline protection," Mr Dal Pozzo said.

"We have around the Swan River the ongoing issue of loss of our sandy beaches and erosion of our shoreline, which obviously impacts infrastructure, but it also threatens habitats as well, such as our riparian vegetation, seagrass and a lot of those key habitats we lost.

"Plus it removes beaches, which are a big part of our recreation for our local community."

By creating artificial reef structures just offshore, the group hopes to stabilise the river foreshore, removing some of the wave energy coming in and reducing erosion.