on 11/11/2024
A lost part of the coast
Most people are unaware that just 200 years ago, South Australia was home to some of the largest reef ecosystems on the continent. South Australia’s large gulfs and sheltered bays and estuaries were filled with enormous shellfish reefs formed by billions of oysters, mussels, and razorfish cemented together on the seafloor. This living crust of seafloor could be meters thick and full of large native Flat oysters (Ostrea angasi) the size of dinner plates (Captain Cook called Australia’s Flat oysters “Ostrea gigantus” on account of them being the largest he had ever seen). In South Australia, these reefs characterised at least 1,500 km of coastline, but they were also part of a continent-wide network of shellfish reef ecosystems that spanned across southern Australia’s coastline from the east coast to Perth, with reefs as deep as 40 m and right up into the intertidal.
It is hard to fathom the biodiversity and fish productivity that these reefs would have supported. Shellfish reefs function like trees in a forest or coral reefs in tropical seas, supporting highly biodiverse invertebrate communities and acting as the natural fish factories of the sea. The loss of these reefs has fundamentally changed the function, biodiversity, and fish productivity of Australia’s coastlines.
Ostrea Angasi Reef - the only surviving natural flat oyster reef in Australia (Tasmania)
A long love affair with oysters
Shell middens attest to the important food and cultural value of these oysters to Aboriginal communities across Australia. Some shell middens (culturally significant piles of used shell) stretch for kilometres and would have accumulated over thousands of years; the oldest Flat oyster shells have dated middens back to 10,000 years ago, when sea levels began to stabilise (i.e., older middens were submerged by rising seas). This interaction with oysters over thousands of years is not unique to Australia, with Indigenous communities across the globe harvesting oysters over long periods of time. The human-oyster story goes deep.
Following the European settlement of South Australia in 1836, these reefs were the focus of one of the new settlement’s most lucrative and widespread fisheries. Oysters were scraped from the seafloor by dragging dredges (metal chain bags) across the seabed by sailboat. This dredging was very effective at removing oysters, but as they indiscriminately removed all live and dead oysters, big or small, eventually all the shell material was lost from the seafloor. Baby oysters are free-swimming larvae that settle on the smell of other oysters, but with all the shells removed, these reefs were unable to regenerate. From covering thousands of kilometres of seafloor, South Australia’s oyster reefs were all but destroyed within 100 years of European settlement, and have not returned since wild oyster harvesting ended. Today, Australia’s oyster reefs are considered functionally extinct, with over 99% gone.
A comparison of what the seafloor may have looked like at the time of European settlement compared to now.
Bringing back oyster reefs
For the past seven years, we have been working to bring back South Australia’s oyster reefs. Collaboration between the University of Adelaide researchers, the South Australian Government, and The Nature Conservancy has led to some bold restoration efforts involving the construction of boulder reefs to reintroduce the hard structure that oysters need to settle down and form reef habitat. At some of these restoration sites, we have placed underwater speakers through which we play the natural sounds of healthy marine habitats to attract baby oysters to the newly built reefs. Sounds ridiculous, but it really works! Myself and Prof Sean Connell (University of Adelaide) first assessed which sounds oysters responded to the most in our research aquarium – the healthier the marine sound, the more they moved towards it. When we played their favourite sounds at the restoration sites, we had, on average, 5,000 more baby oysters settle on the reef per m2, compared to sites where we didn’t use speakers.
These reef restorations were bold projects to initiate because native oysters had never been restored at large scales in Australia (the largest reef, Windara Reef, is 20 ha and the largest in the Southern Hemisphere). We didn’t know if they were going to work, or if any oysters would find and settle on the reefs. But they did, and in incredible numbers. The reef built at Glenelg is one of Australia’s greatest conservation success stories because the high density of oysters that have grown to form a new reef habitat tells us that we can restore these lost reef ecosystems from functional extinction.
Arriving to work at one of the large reef restorations off the Adelaide Coast. Credit: Stefan Andrews
Community leadership
I am particularly excited at how local communities have embraced these projects and looked to get involved. One exciting project is underway in Coffin Bay, where I work with a local Scuba tourism business (Coffin Bay Scuba) and local high school students to collect key ecological data to inform future restoration projects. As part of a citizen science project, we engaged local high schools and recruited 10 students who we certified as scuba divers, so they could help drive this community project. The students helped deploy a restoration experiment at 8 sites across Coffin Bay, through which we learnt where and when the native oysters are settling. This is key information for informing new restorations, which the community are ready to rally behind. The opportunity to engage high school students on the Eyre Peninsula in marine science and conservation has been inspiring. Hopefully it can spark their interest to pursue further study in science and marine conservation.
Dr Dominic McAfee
Future Making Fellow at the University of Adelaide
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