Seed supply
The Sydney rock oyster industry in NSW and Queensland is dependent on natural spatfall, which has always been abundant and reliable. It takes the Sydney rock oyster an average of 3½ years to reach plate size (50 g whole weight), the most popular size grade. Because of reliable natural spatfall, this industry had not used hatchery technology before 2003. At this time, however, results from NSW Department of Primary Industries' breeding programme using mass selection techniques showed an 11 months reduction in time to grow oysters to a market size of 50 g whole weight. This encouraged the industry to consider hatchery technology. Triploidy is another way of increasing growth rates in Sydney rock oysters. It reduces time to market by six months and kill from winter mortality by half. However triploids should only be used as a winter crop as they may suffer from discolouration over summer, the main marketing season for Sydney rock oysters.
Hatchery production
Hatchery, larval rearing and spat settling techniques for Sydney rock oysters are based on standard hatchery techniques for oysters. Commercial hatchery production is carried out in NSW and Queensland for east coast growers and also in Western Australia for local growers. There has not been any transfer of live oysters from the west coast to the east coast for ongrowing or vice versa. Sales of hatchery produced spat on the east coast have reached 20 million for 2006 or approximately 15 percent of the industry's annual spat requirements. The proportion of hatchery produced spat used by industry is expected to rise rapidly with the use of genetically improved spat. The Sydney rock oyster breeding programme, conducted by NSW Department of Primary Industries' staff at the Port Stephens Fisheries Centre, Taylor Beach, NSW, has reduced time to market size (50 g whole weight) from 3 to 2 years and produced oysters with dual resistance to QX disease and winter mortality.
Ongrowing techniques
Traditionally, the larvae of Sydney rock oysters were set on tarred hardwood sticks. Oysters are knocked off these sticks at 0.5 - 3.0 years of age for growing intertidally on timber frame trays (1.8 x 0.9 m) with plastic mesh bottoms, which are placed on timber racks. Alternative growing systems, such as baskets and tumblers are also being used, and some oysters are grown subtidally on rafts or on floating culture. To reduce the amount of tarred timber used in the construction of oyster racks, high-density polyethylene (HDPE) plastic systems are being developed and used. One example is the intertidal longline system, with horizontally suspended plastic tubes of 41 x 91 cm, developed by South Australian oyster farmers.
Harvesting techniques
The Australian Shellfish Quality Assurance Program (ASQAP) is modelled on the US system and requires that shellfish harvest areas be classified on the basis of a sanitary survey and the results of an ongoing strategic water-sampling programme. The NSW programme currently requires that all oysters harvested in that state are depurated for 36 hours before marketing for human consumption, however with the implementation of the ASQAP, the requirement for depuration will be determined by the classification of a harvest area, as is the case in other Australian states. This ensures that the quality of marketed oysters is very high.
Handling and processing
Live Sydney rock oysters are best stored at 8-10 ºC; at this temperature they can be kept alive in an excellent condition for 2 weeks. Opened oysters, however, should be refrigerated at 4 ºC and have a typical shell life of 7-10 days. The Sydney rock oyster is a gourmet oyster and is best eaten fresh on the half-shell. There are no uniform oyster grade specifications for this species; however common sizes are Plate, Bistro and Bottle grade, with an average whole weight of 50 g, 45 g and 35 g respectively, which equate to shell lengths of around 77 mm, 73 mm and 66 mm, respectively.
Production costs
The industry has learned to cope with reducing profit margins and continues to restructure. In New South Wales, the number of people with permits to farm oysters fell from 474 in 1994/95 to 391 in 2002/03. This fall in numbers has been attributed to producers with little or no production leaving the industry, largely as a result of increasing fees and charges. In Queensland there were 32 Sydney rock oyster farmers in 2002/03. This recent decline in the number of producers has had minimal impact on production. This indicates that farmers are becoming more efficient and are successfully reducing production costs and in particular labour costs.
The major disease problems affecting Sydney rock oyster are included in the table below.
Mudworm |
Polydora websteri |
Spionid polychaete |
Mudworm blisters on the inside of the shell, which may become black and foul smelling; oysters become unsaleable and susceptible to other stresses, such as high temperatures and low salinities and may suffer high mortality |
Intertidal culture; taking oysters out of water in the shade for 7-10 days |
Winter mortality |
Bonamia roughleyi |
Protistan parasite |
Yellow to brown spots on palps, gills, mantle and surface of gonad and ulceration of palps and adductor muscle; survivors appear unaffected by the disease |
Intertidal culture; raising growing height by 150 - 300 mm over autumn and winter; moving oysters to growing areas further upstream in estuaries and rivers in early autumn, before oysters get infested |
QX disease |
Marteilia sydneyi |
Haplosporidian parasite |
Starved and emaciated oysters, with pale brown coloured digestive tract; survivors remain stunted and weak for a long time |
Avoid having oysters in water in autumn when oysters get infested |
Flatworm |
Imogine mcgrathi |
Stylochid flatworm |
Clean empty shells and flatworm hiding from light in dark places |
Restrict use of fine plastic mesh to small spat; take small spat out to dry in the shade for a few days |
Farming methods for the Sydney rock oyster industry have been developed to avoid the mudworm problem. Therefore the industry, which commenced with bottom and dredge bed culture, has developed an intertidal farming system that leaves oysters approximately 70 percent of the time out of water - exposed to air, wind and sun.
Suppliers of pathology expertise
Expertise in pathology can be obtained from:
- Ms. L. Reddacliff, NSW Department of Primary Industries, Elizabeth McArthur Agricultural Institute, Camden, New South Wales 2570, Australia.
E-mail: [email protected]
- Dr. R.D. Adlard, Protozoa Section, Queensland Museum, South Brisbane, Queensland 4101, Australia.
E-mail: [email protected]