Flat oysters growing in Botany Bay after more than a century of local extinction

Flat oysters growing in Botany Bay after more than a century of local extinction

Just a kilometre off Sydney Airport’s runways, a species of oyster long extinct from Botany Bay is growing on 3,600 tonnes of rocks.

The Nature Conservancy used a construction barge to sink the rocks five metres into the bay to build bases for 23 reefs in a mission to bring the Australian flat oyster back.

Australian flat oysters (left) have a larger shell than rock oysters (right).(Supplied: Kirk Dahle, The Nature Conservancy)

One million baby oysters from Merimbula were placed on the site inside oyster shells gathered from the oyster farming industry in Port Stephens.

This new 3-hectare haven, dubbed Kurnell Reef, was built in the middle of 2023 and has since become a functioning habitat in an effort to re-establish the flat oysters, which used to be a feature of Botany Bay and the Georges River estuary.

The 23 reefs cover 3 hectares at the bottom of Botany Bay.(Supplied: The Nature Conservancy)

The Nature Conservancy’s Kirk Dahle says it is the first time in 100 years that the oysters have called Sydney home.

“They’ve always been a really important part of the ecosystem in this location,” he said.

“We’ll be doing another two to three million oysters this year on the Kurnell Reef site.

“Australian flat oysters are back in beautiful Botany Bay, where they will support biodiversity and water quality, and the recovery of shellfish reef ecosystems.”

Oysters have a long and storied history in Botany Bay, Mr Dahle says.(ABC Radio Sydney: Declan Bowring)

From a 2,500-tonne industry to one farmer

Oyster shells found in middens on the Georges River show that First Nations peoples used oysters as a food source for thousands of years before European settlers arrived.

Early explorer records indicate Botany Bay was bountiful with oysters, which Captain James Cook himself described as the “largest oyster shells” he had ever seen.

More than likely, he was referring to the Australian flat oyster.

The Georges River produced a quarter of NSW’s oysters in the 1970s.(ABC)

The flat oyster has a large flat open shell, in contrast with the smaller Sydney rock oyster, which is found in some seafood restaurants.

Both oysters became a food source and a building material for the colonists. Many of the earliest buildings, including Sydney’s first Government House, were built using shell mortar produced from oyster shells.

Overharvesting, disease and a decline in water quality led to the local extinction of flat oysters by the late 1800s.

Oyster farms were a feature of the Georges River for decades.(ABC)

When native reefs were depleted, the Georges River became home to an aquaculture industry that produced 2,500 tonnes of oysters, or a quarter of the state’s produce, at its peak during the 1970s.

But the disease QX wiped out about 90 per cent of Sydney’s oyster population by 2001. Resistant strains existed but needed multiple generations to become commercially viable — longer than most farmers were able to hold on for.

Remnants of oyster farms are still evident around Botany Bay.(ABC Radio Sydney: Declan Bowring)

Robert Hill is the last oyster farmer on the Georges River.

He operates on leases in Quibray Bay, Woolooware Bay and Oatley. He is also deeply involved in the project to bring oysters back.

Oyster farmer Robert Hill (left) says he is happy to be part of the conservation effort.(ABC Landline)

The project is using Mr Hill’s lease in Woolooware Bay to keep the baby oysters and then helps ferry them to the Kurnell site.

“When they get big enough, they’ll come out here,” Mr Hill said.

“We had eight tonnes of oysters on this site in my boat.”

Robert Hill is the last oyster farmer on the Georges River.(ABC Radio Sydney: Declan Bowring)

Unlike the tonnes of oysters that used to be farmed, the project’s oysters are purely for conservation purposes. 

Mr Hill finds it fitting that he is involved in both farming and conservation as he wanted to get into oyster farming to look after the river.

“It gives you a chance to be out on the river and I’m pretty happy about that,” he said.

The new reefs were placed on what was previously a soft sand bed that could not naturally sustain oysters.(Supplied: The Nature Conservancy)

More reefs on the horizon

The project in Botany Bay is a partnership between The Nature Conservancy, the NSW Department of Primary Industries Fisheries and the Greater Sydney Local Land Services.

The Nature Conservancy also has approval to build two more reefs at Taren Point and Audrey Bay in the Woronora River, and is currently seeking funding to get those projects off the ground.

Another reef has been approved off Taren Point in the Georges River.(ABC Radio Sydney: Declan Bowring)

Mr Dahle hopes the return of the oyster populations will also improve water quality and the biodiversity of the river.

“You get a lot of benefits in water quality improvement, but they also provide rearing habitat and nursery areas for juvenile fish,” he said.

“Native flat oyster filters up to 50 litres of water an hour.

“If you look at an area where you’ve got millions of those working in unison, they filter the equivalent of multiple Olympic-sized swimming pools per hour.”

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