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Tuesday 9 October 2018

Massive man-made reef aims to revive wild oyster populaton

Parts of the artificial reef
The final construction stage of the largest man-made oyster reef system outside the United States has been completed in South Australia.

The 20-hectare reef in the waters of Gulf St Vincent in South Australia is the first of what is hoped to be five major reef projects in the state to revive wild populations of the almost extinct Ostrea angasi native mud oyster.

The four-hectare stage one of the Windara Reef project was completed in June 2017 and 30,000 adult native oysters were embedded across the 15 limestone structures in January this year.

Work on stage two began in late August and was completed last week. It involved placing almost 10,000 tonnes of limestone boulders, each about the size of a soccer ball, to form 144 reefs across the remaining 16-hectare area.

The final part of the project will include seeding the stage two section with seven million one-month-old native oyster spat in December.

The project, about a kilometre off the coast of Ardrossan on South Australia’s Yorke Peninsula, aims to increase fish stocks for recreational anglers, improve water quality and biodiversity and revive the oyster in the Gulf.

Oyster reefs are considered the temperate water equivalent to coral reefs in tropical waters. Australia’s southern coastline was home to thousands of kilometres of oyster reefs before European settlement but dredging to remove substrate for lime production and the harvesting of oysters for food wiped out all the reefs except for one off the coast of Tasmania.

The Nature Conservancy (TNC) has been involved in dozens of shellfish reef restoration projects, chiefly in the United States and is considered a global expert on their establishment.

TNC project manager Anita Nedosyko said the first dives to measure the environmental benefits on water quality, fish production and biodiversity of stage one were conducted in May and June as part of a six-year study into the success of the project.

She said while the reef system was dominated by turfing algae, the adult oysters placed on the reef were surviving and a number of wild oyster spat had also found its way to the reef, which was an unexpected bonus.

The seven million one-month-old native oyster larvae will be attached at a hatchery near Port Lincoln to 12 tonnes of oyster shells that will be used to host the spat so it can be easily seeded onto the reef, which is in 8-10 metres of water.

The 144 limestone boulder reefs in stage two are roughly four metres wide and 700mm high with lengths varying from 10 metres to 35 metres and are distributed evenly across the area to form one large reef system.

Nedosyko said the individual reef clumps were designed to be close enough together so that native oyster larvae could move across the reef system through tidal flow.

“A successful reef for us will be one where oysters are surviving, spawning and producing new recruits and we’re also starting to see some additional biodiversity,” she said.

Adult native oysters can filter more than 100 litres of water a day and excrete a mucus-like substance that is rich in nutrients and provides food for small shellfish that in turn provide food for larger fish.

Nedosyko said the ability to provide to clean water and provide a food source for small organisms gave native oyster reefs the ability to drive greater fish production than artificial reefs alone.

She said it was hoped the Windara Reef would eventually lead to increased fish production of 5000 tonnes per hectare a year including recreational fishing favourites such as snapper and King George whiting.

“It’s like a well-stocked fridge – it’s really attractive to fish coming in because they can stop, get fed and move on or decide to be residents.”

The Nature Conservancy is working with the South Australian Government to raise additional funds to be able to undertake restoration projects in other South Australian locations including Kangaroo Island, the metropolitan Adelaide coast, Spencer Gulf and the far West Coast.

“This is definitely an important site for us to be able to test the success of the ecological progression of the reef and also to test and trial the construction and deployment methods,” Nedosyko said.

“We’re confident that by building native oyster reefs we’re going to achieve oyster survival but also greater biodiversity and therefore higher fish numbers at these sites.”

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