Stripped of marine life and ground up for bricks and mortar, South Australia’s natural oyster reefs have been devastated since colonisation, but a newly built reef off Glenelg is demonstrating a speedy revival.

Key points:

  • Australia’s natural oyster reefs are estimated to cover just 1 per cent of their pre-colonial extent
  • A major shellfish reef restoration project in South Australia is showing positive results
  • Further oyster reefs are to be established off Port Noarlunga and Kangaroo Island

The two-hectare reef was only built in November, but already Angasi oysters the size of 10 cent pieces are in their hundreds per square metre and are expected to reach up to 70 millimetres wide in two to three years.

It follows the successful rebuild in 2019 of the much larger, 20-hectare Windara Reef near Ardrossan, Yorke Peninsula, which was constructed with limestone in about 10 metres of water.

“Southern Australia’s flat oysters [Angasi oysters] were pretty much completely overfished, basically to extinction,” Adelaide marine biologist Anita Nedosyko said.

“The rock underneath them would have been thousands of years of oysters growing on top of each other and condensing, but the colonists discovered it was a really rich source of lime, so they used it to create bricks and mortar.”

Marine biologist Anita Nedosyko is an associate lecturer at Flinders University.(ABC Radio Adelaide: Malcolm Sutton)

Just 1 per cent of oysters left

Ms Nedosyko is SA’s oceans coordinator for The Nature Conservancy (TNC), a global not-for-profit organisation that has been funded by both state and federal governments to restore a series of natural shellfish reefs in SA.

It builds on work by a team of University of Adelaide marine ecologists, including Heidi Alleway, who have been researching Australia’s lost shellfish reefs.

A four-year-old Angasi oyster compared with an Angasi spat.(ABC Rural: Nick Bosly-Pask)

The university team, which was last year awarded an Australian Museum Eureka Prize for its contribution to science, reported that 1,500 kilometres of Southern Australian coastline reefs had been lost due to population growth by colonial settlers.

It further found that today’s oyster populations are at less than 1 per cent of their pre-colonial extent.

“Oysters played a large role in Australia’s colonial history,” Dr Alleway said in December.

A remnant clump of native oysters found attached to a razorfish near Windara Reef.(Supplied: Anita Nedoysko)

‘Ecosystem engineers’

Ms Nedosyko said restoration was a “two-pronged process”, starting with the reinstallation of the hard “substrate”, or the limestone rocks the local oysters prefer, followed by the deposit of oyster shells seeded with spat, or baby oysters.

Once the hard substrate was established, more hatchery-raised oysters would be released on top, where they benefited from their elevated position above the seabed sediment.

Ms Nedosyko said the shellfish themselves would become the foundation for a broader ecosystem.

Angasi oysters have the capacity to increase marine biodiversity.(Supplied: Dr Chris Gillies)

Adult oysters are able to filter 100 litres of water a day, excreting a nutrient-rich mucus that becomes food for smaller shellfish, which in turn become food for fish.

“What we’re trying to do is make the oysters and the shellfish reef the main ecosystem engineer on both of those reefs,” Ms Nedosyko said.

“We’ve already started to see marine diversity increase at our site on the Yorke Peninsula.”

Signs of life at Windara Reef eight months after limestone was placed in the water.(Supplied: Anita Nedoykso)

Windara results

Ms Nedosyko said Windara Reef, which was built over two stages in 2017 and 2018, was the largest restored native oyster reef in the Southern Hemisphere.

It started as a four-hectare trial by Primary Industries and Resources SA, until TNC came onboard in a partnership for the 20-hectare expansion.

Windara Reef was given a boost in November 2019 with the release of 7 million native oysters, produced in a hatchery from adult broodstock, collected in the wild by the SA Research and Development Institute.

Windara Reef’s construction was finished about a kilometre offshore in late 2019.(Supplied: Anita Nedosyko)

It is expected to take about a decade to reach maturity but Ms Nedosyko said the smaller site at Glenelg — chosen so it could become one of the world’s first reef restoration projects near a major urban centre — could get there sooner.

“We may see Glenelg reach maturity a little bit sooner because we’re seeing oysters start to grow a lot quicker and lot larger in a shorter period of time,” she said.

This is believed to be due to a higher availability of food resulting from nutrient-rich water discharged into the ocean.

Watching the reefs grow

TNC last year received a $20-million investment from the Federal Government to expand the Glenelg reef to five hectares, as well as build more reefs of a similar size off Port Noarlunga and Kangaroo island.

For Ms Nedoskyo, a keen scuba diver who has dived some of the world’s best sites, watching these reefs grow from scratch is incomparable.

“Some of my favourite places to dive now are these newly created reefs,” she said.

“I could go out there so frequently, it’s just so interesting. I encourage others to go out there and see it for themselves.”