Reefs are made with both living and non-living materials. First, limestone is laid down on the seafloor to give the reef height and new homes for fish and other reef animals. Then, live baby oysters are placed on top of the limestone, which will eventually grow and form a new living reef.
Rapidly growing turfing algae are the first to colonise the reef. Baby oysters are growing under the algae, whilst common fish such as Little Weed Whiting (Neoodax balteatus) and Southern Pygmy Leatherjackets (Brachaluteres jacksonianus) also inhabit the reef.
Oysters are beginning to emerge from the undergrowth and can be easily seen on the reef. The reef has now been colonised by more complex canopy-forming macroalgae, and has become home for small, juvenile fish such as gobies and blennies and other animals living in the rock crevices, such as worms, seastars, abalone and hermit crabs.
Other shellfish like mussels, abalone and scallops continue to grow and add to shellfish diversity on the reef. The oysters provide food to other animals by filtering microscopic algae (phytoplankton) from the water and depositing the waste into tiny 'poo pellets'. Millions of worms and small crustaceans growing on the reef consume this waste, and are prey for larger fish and other marine life.
Several generations of oysters are now living on the reef. Turfing algae is disappearing as slower growing oysters, sponges, corals and sea squirts take over. Larger schools of fish such as Snapper are more commonly seen and reproduce on the reef.
Oyster 'towers' or oysters growing on top of each other, have now begun to form, creating more complexity on the reef. The reef is becoming a more attractive place for larger predatory fish such as Snapper, Bream and Salmon to live as more food is available, such as juvenile fish, shellfish and crustaceans.
Even more oysters populate the reefs, as they grow, spawn and recruit new oyster hatchlings every year. There are now at least two generations of oysters producing baby oysters. The reef is highly diverse with many fish, squid, cuttlefish, crabs and macroalgae reproducing and sharing this restored ecosystem.
By year seven, the restored oyster reefs look and feel much like a natural reef. Although it will still take another decade or more for the reef to fully mature, many key species and ecological foundations are established. Millions of oysters are filtering the surrounding water. The reefs act as nursery grounds for important fish and invertebrate species, and provide homes for many others. Biodiversity has increased several-fold compared to the soft-bottom habitat that used to exist at the site.
Razorfish are the last refuge for oysters and reef associated animals
Razorfish (Pinna bicolor) are important habitat for native oysters. Their shells provide a hard surface for oysters and other reef animals to attach to. A single Razorfish can have up to 50 species living on it.
The sandy seafloor
Underwater cameras are used to identify suitable sites, such as those with sandy substrates, that are best for building new shellfish reefs. This photo shows the sandy bottom where the reef was built.
Shellfish reefs under construction
Large barges and excavators are used to build the reefs 10 metres below the surface. Limestone rocks and boulders are placed on the seafloor with accurate precision to form a reef base measuring four metres wide, one metre high and up to 30 metres long.
Blue swimmer crabs (Portunus armatus) are oyster predators that congregate on the shallow reefs during the summer. Many baby oysters will be eaten in the first year by crabs and other predators such as Snapper (Chrysophrys auratus) and sea stars. Reefs are purposely seeded with millions of oysters allowing for high levels of oyster loss in the first few years.
Young oysters grow several millimetres every few weeks during the first year and reach the size of a five cent piece after 12 months. Nearby wild adult oysters have spawned and underneath the rocks young oysters have settled onto the reefs. Millions of these young oysters can settle during the peak spawning period between September and May.
Rocky reef fish
Magpie Perch (Cheilodactylus nigripes) are common on rocky reefs along the southern coast of South Australia and feed on small invertebrates like worms. This species was not seen in the area before the reefs were built but have now made this reef their home.
An oyster reef symphony
Listen carefully to the sound of the ocean with a one year old oyster reef. What can you hear? Sounds can be used to measure reef biodiversity . A healthy reef has lots of different snapping and crackling sounds whilst a 'degraded reef' or sandy bottom area has very little sound.
Juvenile cuttlefish living on the reef
At this stage, some species move onto the reef from the surrounding environment in search of new places to live. Cuttlefish are generally solitary predators. They can be seen inhabiting the reefs and nearby seagrass areas where they hide in crevices from larger predators and visit the reef to hunt for small fish.
Small turfing and colourful algae continue to grow on the reefs. Brown, red and green macroalgae species, such as kelp (e.g. Sargassum sp.), are also growing on the reef and will eventually grow to form a canopy, much like a forest, that will provide important shelter and food for fish and crustaceans.
Juvenile and small fish on the reef
If you look closely there are many small fish living in the rock crevices. Small fish such as Leather-jackets, Southern Goatfish (Upeneichthys vlamingii) and Wavy Grubfish (Parapercis haackei) are foraging and finding shelter on the reefs.
Oysters are sexually mature by year three and their babies will recruit onto the reefs. Several times a year in the warmer months, female oysters release larvae into the water column where they are free-swimming until ready to settle onto a hard surface. One oyster can release up to 2 million larvae at a time.
Schools of fish
As the reefs produce more food, schools of fish such as King George Whiting (Sillaginodes punctatus), Leather-jackets and occasional Snapper can be seen more frequently: eating, spawning and sheltering from predators.
Herbivorous fish, crabs and abalone are turf mowers
As the reef matures, it attracts herbivorous fish and grazing invertebrates like crabs, abalone and sea urchins, which eat the turf algae and help to expose bare rock. This creates new space for young oysters, as well as other shellfish, colourful encrusting algae, lace corals, sponges, sea squirts and seaweeds, which attach to the reef and out-compete turf algae for the available space.
Snapper on the reef
Snapper is highly prized by recreational fishers but their numbers have decreased in the Gulf St Vincent over the last few decades. These reefs will attract Snapper over the summer months when they come into shallow waters to spawn and feed on the plentiful sources of food.
Reefs promote healthy seagrass
More seagrasses begin to grow around the reefs, such as Paddleweed (Halophila australis) and Eelgrass (Heterozostera nigricaulis). The oyster reefs slow water flow, creating calmer conditions and cleaner water, which allows more sunlight to reach the seafloor for these plants to grow. There is also more life growing under the seafloor, as mounds pop-up created by small burrowing worms and other crustaceans attracted to the abundance of food near the reefs.