Oysters - Ecological Superheroes with a Dark Past
Oysters have played an important role in Australia's natural and human history.
Oyster reefs fringed Australia’s shorelines and shaped our marine ecosystems for millennia. They were of great cultural significance to coastal Aboriginal communities for thousands of years, and remain one of our most treasured marine resources.
Oysters are ecological superheros, though their numerous ecological services are seldom known.
Oysters can increase the abundance and diversity marine organisms through the habitat they create, their filter-feeding cleans coastal waters and can enhance neighbouring seagrass, their structures can reduce coastal erosion by attenuating wave energy, and their shell building can provide a carbon sink, helping to slow the rate of climate change.
Knowledge of the important ecosystem services oyster reefs provide is relatively new, and as such oysters have typically only been appreciated as a food resource. The exploitation of oyster reefs over the past two hundred years has devastated wild populations, which are at less than 1% across Australia. In South Australia, native flat oysters are near extinct in the wild, with no known reefs remaining.
Australia's human-oyster history
Aboriginal communities used Australia’s oyster reefs for thousands of years before European colonisation. More than just an abundant source of highly nutritious food, oysters provided an important trading resource for coastal communities, and their shells were used to fashion fishhooks and cutting tools. Enormous shell middens containing billions of shells attest to their cultural significant, with unearthed shell deposits up to 400 meters long and 4 metres high. Carbon-dating has detected oyster remains up to 9,000 years old in Australia’s east coast middens, however middens any older were likely destroyed by rising sea levels. Some Aboriginal communities likely returned oyster shell to the water at their favourite harvesting spots to increase the settlement of baby oysters. Therefore, Aboriginal oyster farming is likely the oldest form of aquaculture practiced in Australia, pre-dating European aquaculture by thousands of years.
In 1770 Captain Cook remarked on the great extent of oyster reefs in Sydney’s harbours, including the largest oysters he had ever seen. Eighteen years later the First Fleet had to navigate a patchwork of oyster reefs when they sailed into Sydney’s shallow bays, with individual reef patches covering up to 10 hectares. As the early colony struggled to grow their European crops, the low-hanging fruit of intertidal oyster reefs provided an essential food source for the colonists, without which they may have starved. The settlers soon started using oysters as a building resource, burning the shell to produce lime with which cement was manufactured. The early foundations of colonial Australia were literally built on oysters.
Wherever new colonies were settled around Australia a thriving, unmoderated industry of harvesting oyster reefs quickly followed. Intertidal reefs were easily exploited with hand tools during the low tide, with oystermen able to work the same reef patch for weeks at a time before moving to the next patch. Advancing dredging techniques soon saw the indiscriminate harvesting of subtidal reefs. Even the underlying bed of dead shells that provide essential substrate for the settlement of baby oysters was harvested for lime burning. Limekilns were built wherever the harvest was good, and even live oysters were burnt for lime until it was prohibited by the 1868 Oyster-beds Act. As oyster reefs disappeared around the major colonial settlements, the exploitation spread out across the coast, with the search for new oyster reefs fuelling the spread of coastal European settlements. By the 1860’s most of the oyster reefs were gone in NSW, with similar declines within half a century of settlement of other major colonies. By the 1860’s most of the oyster reefs were gone in NSW, with similar declines within half a century of settlement of other major colonies. While records of the historic extent of these reefs are limited, in South Australia alone over 1,500 km were lost in the decades following settlement, hence the national loss would likely rival the extent of the Great Barrier Reef. Today less than 1% of our historic reefs remain.
With the near complete collapse of most wild oyster fisheries by the end of the 19th century oyster production transitioned to aquaculture using techniques similar to those used today. Despite the end of wild harvesting, oyster reefs have never recovered from the earlier exploitation. The loss of the enormous breeding populations and shell substrate for young to settle on, combine with the dramatic changes to the coastal landscape, changing the nutrient and sediment loads entering our coastal waters, and the introduction of oyster pests (i.e. mud worm) have all contributed to the lack of recovery.
Learn more about the historic extent, exploitation and cultural amnesia of South Australia’s lost oyster reefs: Loss of an ecological baseline through the eradication of oyster reefs from coastal ecosystems and human memory
- Ecological foundations: housing for the masses
Oyster grow complex habitat where many species live
When large oyster populations grow on mass their aggregations form dense, complex habitat in which a great diversity of other species live. This is called ecosystem engineering, where the shell structures the oysters grow change an environment from being relatively featureless into complex, three-dimensional habitat. The role of oysters as ecosystem engineers is not dissimilar to the role of trees on land or coral reefs in tropical seas. In fact, oyster reefs are often considered the temperate equivalent of coral reefs.
Many of the little animals that live among oysters do so because the convoluted habitat provides a good place to hide from predators. The little animals oysters support are important for the cycling of nutrients in coastal ecosystems, and underpin the broader coastal food-web. Oyster reefs also provide a nursery habitat for many recreational important fish species, some of which lay their eggs on the oyster shells.
- The ultimate filter feeders: the kidneys of our coasts
Oyster filter feeding demonstration
Oysters feed by filtering tiny particles (seston) out of the water, and as such they have a phenomenal ability to improve local water quality. Their filtering decreases water turbidity, allowing sunlight to penetrate to the seafloor which enhances seagrass growth. Also, oysters filter excess nutrients from the water, which result from urban runoff. Algae blooms resulting from excess nutrients are a major environmental issue around the world, and the filter-feeding of oysters can help avoid such catastrophes.
A single oyster can filter 4-5 L of water in one hour, over and 1 million tonnes of plankton in their lifetime. If one oyster can filter over 100 L of water a day, imagine what a reef with millions of oysters could filter!
- Fisheries for the future
Fish eggs laid in a dead oyster shell
Historic tales of our bays teeming with so many fish you could scope them out by hand are difficult to imagine these days. Equally difficult to imagine are the thousands of kilometres of oyster reef that fringed Australia's coast just 200 years ago. These oyster reefs would have supported these enormous fish populations, provide them with abundant food, clean coastal waters and important nursery habitat. Many fish always lay their eggs directly onto dead oyster shell, so the more shell habitat there is, the more fish.
Recreational fishing is an important pastime for over 5 million Australians annually. Restoring our lost oyster reefs can not only help the environment, but strengthen commercial and recreational fishing, and increase tourism for coastal communities.