The Sydney rock oyster (Saccostrea glomerata) is considered a gourmet's delight and is the main focus of oyster production in the State. With a current annual production of over 106 million oysters worth over $35 million, oyster farming has been the most valuable aquaculture industry in New South Wales for over 100 years.
Cultivation of oysters is far from a recent innovation, the Japanese were raising oysters as early as 2000 BC and the Romans from about 100 BC. While New South Wales can trace its origins to the 19th century, use of natural stocks of oysters in the State has a much longer history. The Aborigines on the coastal regions feasted on oysters and shell beds can be found in the many kitchen middens along the coast. Some of these middens have been carbon-dated to ten thousand years.
With European settlement of the State and a rapidly increasing population, the demand for oysters grew quickly. The use of oyster shell as a source of lime in cement production resulted in natural oyster stocks being near depletion by the 1860's. Government controls were introduced and this precipitated the introduction of early cultivation practices, the first of which was the establishment of Claires (ponds) based on French cultivation techniques, by Thomas Holt in Gwawley Bay in 1872. Although this technique did not lend itself to local conditions, it demonstrated the potential of the Sydney rock oyster as a commercial species.
Oyster farming now employs many different techniques, all of which take place on selected sites held under about 3200 aquaculture leases, with a total current area of about 4300 hectares, which are administered by NSW DPI. Commercial production in the State occurs in 41 estuaries between Eden in the south to the Tweed River in the north. Wallis Lake and the Hawkesbury River are the main producing areas. Pacific oysters (Crassostrea gigas) have been commercially cultivated in Port Stephens since 1991, but are declared a noxious fish in all other NSW waters.
Most oyster species, including Sydney rock oyster, change sex during their life. The first spawning is usually as a male and subsequent spawnings as a female. During spawning, adult females disperse up to 20 million eggs and males hundreds of millions of sperms into the water when the tide and current are optimal for the widest distribution. Pacific oyster females can produce between 30 to 100 million eggs per spawning. Spawning is so intense during this period that the surrounding water can take on a milky appearance. Fertilisation takes place in the water column and development continues for up to 3-4 weeks as the larval stages of the oyster swim and grow, ultimately settling on a suitable hard clean surface. Survival rates during this phase is less than 0.1%. The surviving oysters are then called "spat" and will grow to maturity in about 3 to 4 years never again leaving their chosen position.
Oysters, which are bivalve molluscs, obtain food by filtering and extracting minute marine plants (algae), bacteria and nutrients from the surrounding water.
Three distinct cultivation methods have evolved in New South Wales over the years; rock culture (now seldom practiced), stick culture; and various tray type cultures.
This method has been the mainstay of the industry since the 1930's and commences with oyster larvae settling on tarred (and sometimes additionally cemented) hardwood sticks 1.8 metres long and 25 mm square, which are placed in areas of estuaries where spatfall (settling of spat) is most reliable, typically near river mouths. The sticks are then moved to low spatfall areas to reduce "overcatch" (further spatfall on growing oysters) and are grown to maturity on horizontal racks in the inter-tidal zone. The process takes from 3 to 4 years with great care required in the first two years to protect the oysters from excessive heat and predators (bream, octopus and stingray). At maturity, the oysters are removed from the sticks and graded into various sizes prior to marketing. The largest (15 to 25 oysters/kg) are sold as first grade oysters and the next grades (25 to 35 per kg) are sold as "bistro" or "bottle" grade. Oysters too small to meet either of these criteria are usually placed onto trays and returned to the same or other estuaries to develop to a marketable size. While this method has proven to be the most efficient for the industry relying on natural catch, it may be less significant should commercial oyster hatcheries establish and produce single seed oysters.
Oyster trays are usually one metre wide and from 1.8 - 2.7 metres in length, of timber and wire or plastic construction. They have many advantages as a cultivation method over earlier methods and in some cases even stick culture. Trays are more portable, easier to manage and allow precise stocking densities to encourage oysters to grow in a more uniform and marketable shape. Oyster farmers have devised techniques to further exploit these advantages culminating in the "single seed technology".
Increasingly, oyster farmers are removing oyster spat from the catching surface (usually sticks or PVC slats) very soon after settlement when the oysters are still only 3-8 mm in diameter. Spat are then either placed on specially constructed trays or in recently developed plastic mesh cylinders or baskets. These systems provide excellent protection from predators and the early removal of the stick prevents oysters becoming misshapen or clumped together. Faster growth rates have also been reported. Whilst single seed techniques require substantial capital investment, faster growing, better shaped oysters generally allow more precise grading and the oysters generally receive a higher market price. Research into the commercial production of "triploid" oysters which grow fast and hold market condition longer, is aimed at further enhancing the viability of single seed culture.
Apart from the production of the unique Sydney rock oyster, fledgling industries farming other species are becoming established in several NSW estuaries. Pacific oysters, which are the most commonly farmed oysters in the world, are now grown in Port Stephens. Production is increasing each year with strong demand bringing high prices.
"Flat" or "dredge" oysters (Ostrea angasi) cultivation is developing in some south coast estuaries. While production is limited by available growing areas, initial returns are good and there is potential for expansion in the cultivation of these indigenous oyster and possible future export opportunities to the large European flat oyster market.
Water pollution is one problem that oysters are subject to however, environmental stress can manifest itself in many forms, all of which may affect oysters. As such, oysters are a valuable environmental indicator, sometimes referred to as the canary of the estuary. Monitoring wild and cultivated oysters can reveal environmental damage before it is otherwise apparent. Oyster farms can also affect the environment, but when properly managed, oystering is a low impact and sustainable industry relying, however, on a well managed catchment. Oyster leases provide valuable sheltered habitats for fish populations, especially juveniles.
Few other foods can compare with the nutritional balance of oysters. The oyster is a well-balanced, easily digestible and nutritious food, rich in minerals (zinc, selenium, magnesium) and vitamins (A, B1, B2 and C). They are low in cholesterol, and contain approximately one quarter of the cholesterol of prawns and squid, equal to most other fish. They are well below the cholesterol levels in red meat and some poultry. The Sydney rock oyster is unique in its ability to live out of water for up to three weeks in cool moist conditions. This is longer than any other oyster variety in the world. Even in a country as vast as Australia, this means that fresh, unfrozen product can be available almost anywhere throughout the year.
Mandatory depuration of all oysters before sale has been introduced in all NSW estuaries since 1978. This process makes use of the fact that food taken in by the oyster, including bacteria, will be excreted within a short period of time. A depuration plant provides a controlled environment in which oysters spend the final 36 hours before sale in high quality water, allowing any possible contaminants to be removed by purging. The most common method of obtaining water of appropriate quality is by exposing the water to high intensity, germicidal ultra-violet light twice every hour. In a well-designed depuration plant, salinity, temperature and dissolved oxygen levels of this water are controlled for maximum efficiency.
The NSW Shellfish Program regulates the harvesting of all oysters in the State. The Program aims to provide high quality product to consumers. This can be best achieved by rectifying potential pollution point sources in shellfish producing areas, assessing and controlling production methods at all levels of industry and educating shellfish producers in their responsibilities. This is a mandatory industry funded program designed to ensure that oysters are only harvested under strict water quality and product guidelines that seek to ensure that public health and high industry standards are observed and promoted.
The first 75 years of the NSW oyster industry saw production grow to approximately 60 million oysters per year. It laid the foundation for unprecedented growth to approximately 175 million oysters per year during the latter 25 years of its history. Production peaked in 1977 and has dropped back to about 101 million oysters per year since 1990. The oyster industry and NSW DPI are working closely to improve sustainable production levels. For current production figures see the NSW DPI aquaculture production reports.
Maintaining production levels in an industry, which is centred on the most populated section of the Australian coast, is a challenge for the oyster farmers and governments at all levels. Research programs underway, jointly funded by industry and government are focusing on improved cultivation techniques, stock genetics, habitat improvement and protection and better disease control which are essential to ensuring the continued viability of Australia's oldest, and New South Wales' most valuable aquaculture industry.
For further information, call NSW DPI
Port Stephens Fisheries Centre 02 4916 3900