Any fisho who has ever taken a good look at the chompers on a bream would know that they are perfectly suited for crushing the shells of molluscs, like oysters. In fact they form a really important part of not just the diet of bream species, but of a host of other fish species as well.
Oysters are not just good tucker for fish, they are also ecosystem engineers, forming habitat that other species live on and around. They’re also the kidneys for our estuaries; a single oyster can filter a bath tub of water in a single day. In fact the economic importance of the 'services' provided by oysters through providing cleaner water, a source of food and habitat for fish we love to catch, has been estimated at between $5,500 and $99,000 per hectare. So the take-home message is that oysters are a foundational building block for our estuarine ecosystems and recreational fisheries.
Given how important they are, it's really concerning to learn how much of our vital oyster reefs we have lost. Researchers which had examined the condition of oyster reefs all around the world, past and present, estimated that 85% of the earth’s oyster reefs have been lost. That’s enormous, and it actually suggests that oyster reefs may be the most threatened of all shallow-water structure habitats – more so than coral reefs, mangroves or wetlands. And the weird thing is, whilst there are endless protests by people to save the great barrier reef, there aren’t many people calling for our oysters to be saved. Why is that?
Now if you’re wondering what has caused this enormous decline, there are a few factors. Overharvesting, water contaminants such as pesticides etc, disease, but also habitat degradation.
Siltation is a particular problem for oysters. That’s because they grow on hard substrate, so if the entire ocean floor is covered in silt, then the new planktonic oyster spat have nothing to attach to, and so it only takes one event such as a disease outbreak to knock the adult population back, and it can make it very hard to recover.
And a good example of this is the Richmond River. An aquatic veterinarian from Northern NSW, Dr Matt Landos has been highlighting this as an issue for a long time. Talking about his local area he explained that the town of Ballina probably got its name from the indigenous word ‘Boolinah’ which means ‘place of many oysters’. And from the 1850’s to the 1970’s there was a booming oyster collection industry for the lime in their shells. But disease struck in the early 70’s, and progressively this system has lost oyster reefs, until now you are lucky to find them at all. So just imagine what the bream populations were probably like back in the day in the Richmond compared to now…
But the good news is that they can come back – to the Richmond and elsewhere - and there are places overseas where this has happened, and so some fishers from the Albert Park Yachting and Angling Club, as well as researchers from Fisheries Victoria and conservationists from the Nature Conservancy are doing a pilot together to look at bringing oysters back to Port Phillip Bay, Victoria
The 3 year project that started in late march 2015 begun trials on a few different techniques for rehabilitating oyster reef habitats that have been shown to work overseas. They used scallop shells recycled from the seafood industry and restaurants as substrate for building two experimental reefs at Geelong, Hobsons Bay. They reintroduced native flat oysters bred in hatcheries at the Victorian Shellfish Hatchery that are then deployed to these reefs.
The study, that is still in early stages, have identified that oyster spat survival is directly successful when deployed on a rubble base. Preliminary results indicate that elevation is important for both the survival and growth of oysters.
Further research is yet to be planned for the Tasmanian site to complete a biological assessment. With the complete data the project aims to be able to produce long-term restoration targets and reef design at Port Phillip Bay that could be basis for similar projects all around Australia in areas where oyster reefs have been lost.
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