Oysters: Farming and the Environment

In last month’s blog post we explored the issues facing our local oyster farmers and the industry as a whole. This month we’re delving deeper into oysters, how they’re farmed and their environmental impact.
Australia has been commercially farming oysters since the late 1800s when native stocks of the shellfish were almost depleted due to the oyster ‘meat’ being a reliable food source and the use of the shells in lime production, a cement replacement and an important building material in the early colony.
Fun Fact: Did you know that you can still see oyster shells in the mortar between bricks of historic buildings – look closely the next time you visit the Rocks in Sydney where some of the oldest buildings remain.
By the 1860s the government had to put controls in place to manage the dwindling stocks and early cultivation started to take hold. Thomas Holt established an oyster farm in 1872 at Gwawley Bay on the Georges River which demonstrated the potential of the Sydney Rock Oyster as a commercial opportunity.
Fast forward to today and the industry now has 3200 aquaculture leases in NSW, spread across 41 coastal estuaries with three species of oysters grown - the Sydney rock oyster (Saccostrea glomerata), Pacific oyster (Crassostrea gigas) and the native, flat oyster (Ostrea angasi). In 2018-2019, the NSW oyster industry produced about 76 million oysters worth $59 million at the farm gate.
Historic photo of oyster stick farming - image credit to Bradley Australia
Oysters are bivalve molluscs that change sex. Their first spawning is male with subsequent spawnings as female. Millions upon millions of eggs and sperm are dispersed in the water when the temperature and tides are optimal for the widest dispersion.  Fertilisation takes place and development continues for up to 3-4 weeks as the larval stages of the oyster settle on a suitable hard clean surface and are then known as spat.
There are various distinct cultivation methods that have evolved in NSW including:
Stick Culture
Oyster larvae settle on tarred or cemented hardwood sticks approx. 1.8 metres long and 25 mm square, where they have been placed to catch spatfall – usually near river mouths. They are then moved to avoid overcrowding and are grown to maturity on horizontal racks in the inter-tidal zone which can take three to four years.
Tray Culture
Oyster trays are usually one metre wide and from 1.8 - 2.7 metres in length, of timber and wire or plastic construction. They are portable, easy to manage and allow precise stocking densities which encourages more uniformity of shape and size.
Single seed culture
This is when spat are removed from the catching surface very soon after settling. They are then placed on specially constructed trays or in plastic mesh cylinders or baskets which protect them from predators. This system also prevents the oysters becoming misshapen or clumped together and it promotes faster growth rates resulting in better grading and higher market price.
Rock oysters
Oysters and the Environment
Oysters feed by filtering the water and straining organic material. They filter up to five litres of water every hour and as such, are an excellent indicator of the health of the aquatic environment they are a part of. They are often referred to as the canary of the waterways and oyster farmers are among the first to become aware of environmental issues and estuary health.
There are four major environmental benefits to oysters and oyster farming:
Water Filtration
Feeding oysters and shellfish consume solid particles from the water (excess nutrients, sediment and pollutants) as they filter it through their gills and expel clearer, unused water that enables sunlight to penetrate and benefit the ecosystem.
Nitrogen Removal and Cycling
Oysters cycle nitrogen by absorbing it and once harvested, removing it from the ecosystem. They also cycle it back into the water through their own waste which feeds phytoplankton and other organisms.
Carbon Recycling
Our waterways contain some carbon dioxide from the atmosphere, which is absorbed and processed by the oysters, expelled as waste and turned into sediment. This oyster-processed carbon then contributes to the lives of many species of deposit-feeding organisms.
Habitat Building
Oyster reefs provide a valuable home to many forms of aquatic life. Mussels, barnacles, anemones, and other shellfish often require a hard floor on which to attach in order to thrive and many species of fish use the oyster reefs as a place to lay eggs and to be protected from predators.
In addition to the above environmental benefits, oyster farming is a low impact and sustainable industry that provides valuable sheltered habitats for fish populations, especially juveniles.
Sydney Rock Seed Oysters
Here at Dawson's we love oysters and so do our customers. We process both stick and seed oysters and do as much as we can to support our local growers. It’s a fascinating industry and we’re proud to be a part of it. Why not include more oysters in your diet in support of this important industry? Drop by the shop and plan a special meal with our delicious oysters.