Returning Oyster Reefs to the Shoalhaven & Crookhaven Rivers
Shoalhaven Riverwatch is embarking on a fascinating project to construct oyster reefs from cages and recycled oyster shells to be placed at target sites to assist improving the health of the Shoalhaven and Crookhaven Rivers.
Efficiency - LEP23-014-LLC08-3
The issue
Natural oyster reefs were once abundant in many NSW estuaries. Historically, large beds of Sydney rock oysters were a dominant feature in most south coast river systems, however over many decades these oyster reefs have been largely lost due to overharvesting of oysters for lime production for brick mortar (lime mortar).
Natural oyster reefs provide structural habitat for over 300 marine species including many native fish. The restoration of this important habitat will improve local water quality, enhance foreshore protection and help support more native fish, including bream and flathead, and other marine wildlife in the area. It is well recognised that each oyster filters between 50-150 litres of water per day.
The solution
The most common method used in Australia to re-establish oyster reefs is using hard rock. This method is expensive, as it involves expensive materials and equipment to build and locate the artificial reefs.
In recent years, a new method has been developed by OzFish which involves recycling used oyster shells, placed in steel baskets and located in the intertidal zone. The benefit of this method is that oyster shells targeted for landfill, are reused in the baskets. The steel which holds the shells rusts away within 3yrs, and the new oysters that colonise the shells hold together the shell mass creating a new natural oyster reef.
With the support of grant funding, Shoalhaven Riverwatch is working with Macquarie University to trial and monitor both methods. This project will scientifically test the advantages and disadvantages to both the hard rock and constructed steel-reused oyster shells methods with an aim to determine the benefits to using the caged -reused shell reefs.
The impact
The construction and placement of these oyster reefs will return oyster beds to these rivers where they have been historically depleted. The oyster reefs will assist in improving local water quality through additional filtration by the newly established oysters. Oyster reefs are considered ecosystem engineers. This means they create habitats that support a variety of marine life and provide extensive services to the local ecosystem. Other benefits of the oyster reefs will be to enhance foreshore protection and provide habitat for native fish, and other marine life in the Shoalhaven and Crookhaven rivers.
Key facts
- The Shoalhaven and Crookhaven rivers have been depleted of oyster reefs over time
- Shoalhaven Riverwatch in partnership with Macquarie University will trial both hard rock and reused shell cage reefs
- Results aim to determine the benefits of using the reused oyster shell cages