Good signs for man-made oyster reef near airport runways in Botany Bay

Baby oysters planted on the reefs are surviving while a huge increase in fish species has been observed.

More than 3600 tonnes of rock was deposited on to the floor of the bay to create 23 individual reef patches over three hectares, equivalent in size to two football fields.

The reefs have been "seeded" with two million young Native Flat Oysters to kickstart shellfish population establishment.

The baby oysters were settled on to recycled oyster shell, called cultch, which was taken out and spread across the reefs.

The Nature Conservancy Australia is carrying out the project with federal government funding and in partnership with Greater Sydney Local Land Services and NSW DPI (Department of Primary Industries) Fisheries.

Two other reefs are planned upstream in the Georges River - at Taren Point and Audrey Bay at the entrance to the Woronora River. A funding application has been made to the federal government.

Kirk Dahle, from the Nature Conservancy Australia said, "For the first time in a hundred years, Australian Flat Oysters are back in beautiful Botany Bay, where they will support biodiversity and water quality, and the recovery of shellfish reef ecosystems".

"We are still waiting for the results of the official monitoring but anecdotal monitoring and recent observation from underwater vision shows that the reefs are healthy, with kelp growing in multiple places on the reef, and good survival of the oysters," he said.

"Observation of the reef also shows abundant wildlife on and around the reef, with more than fifty species observed in the last three months around the reef, including Snapper, Silver Trevally, and multiple species of wrasse, compared to only one or two species of fish observed in the area before the restoration began."

The goal of the Botany Bay project is to restore up to five hectares of the locally extinct Australian Flat Oyster (Ostrea angasi) and depleted Sydney Rock Oyster (Saccostrea glomerata) reefs, across the three sites.

The Nature Conservancy explains the project:

Botany Bay was historically a place of shallow waters and extensive shellfish reefs.

Early historical records recall "great quantities of oysters, mussels, cockles, and other shellfish" (John Hawkesworth, official record keeper HMS Endeavour).

On the surface, it was a place rich in native plants and swamplands. Underwater - it was a thriving ecosystem supported by shellfish reefs.

Records show that oyster reefs were once plentiful in the Bay, with James Cook himself describing the "largest oyster shells" he ever saw. These oysters were likely to have been the Australian Flat Oyster (Ostrea angasi), which is now locally extinct in Botany Bay.

For European settlers, oyster reefs provided easily accessible food and building material. With a lack of limestone quarries, early colonial buildings such as the first Government House were built using shell mortar produced from shells and living oyster beds. The new colony was literally built on a foundation of oysters.

The first sign of overfishing and over-dredging was seen in the rapid depletion of the Botany Bay oyster stocks. Unfortunately, by 1896, the natural mud oysters of Botany Bay were declared extinct.

With the exhaustion of natural oyster reefs, the local aquaculture oyster industry developed to meet demand. However, by 2001 disease killed up to 90% of the oyster population. This outbreak marked the fall of the local industry, with many farmers closing their business in the bay.

In recent decades, the river's water quality has been recovering. With the introduction of eco-friendly practices, land re-development, and regulations on sewage. The natural populations of Sydney rock oysters in the estuary have shown an incredible resilience with signs of good health and growth.

Shellfish reefs have multiple benefits both for people and for nature, which include: