Farming Oysters

History

Oysters were officially reported in Queensland in Moreton Bay in 1822 by W. L. Edwardson. Significant exploitation of these oysters occurred from 1824. Harvesting was either by hand-picking in the shallow water or dredging in the deeper water. Rock oysters ( Saccostrea commercialis ) formed the basis of early oystering. Between 1824 and 1863 many of the oyster banks were over-exploited as a result of using oyster shell to produce lime for mortar. The 1863 Oyster Act introduced certificates for laying down oyster beds and a penalty for burning live oysters for lime.

Protection and encouragement for the industry was provided through further legislation introduced in 1874 and 1886. These changes had the support of industry members. The legislation covered the definition, licensing and lease terms for bank oysters and dredge oysters.

Bank Oysters and Dredge Oysters

Bank oysters were defined as 'those oysters occurring in the intertidal zone and to a depth of 60 cm below low water'. By 1886, there were 178 banks licensed for Moreton Bay and 13 for the Great Sandy Strait. Total leases peaked in 1908, with 849 banks covering 10 100 hectares. The industry declined rapidly to just over 200 leases by 1940. The decline resulted from a complex combination of husbandry mistakes in relation to oyster predators, pests and diseases, and a range of other external influences.

Dredge oysters occurred in deeper water below bank oysters. In 1886 the dredge section leases were extended to cover oysters up to the high-water mark. The total dredge section leases peaked at 64 in Queensland in 1904. Twelve of these dredge sections were in the Great Sandy Strait. The numbers declined slowly until the last section in the Maroochy River was forfeited in 1947.

Oyster Mud-worm Disease

During the 1890s, an epidemic of the destructive 'oyster mud-worm disease' severely affected the industry. It often killed the oysters and in most cases made the rest unmarketable. From 1888 to 1889, the number of dredge sections dropped from 36 to 18 and oyster banks declined from 421 to 292.

Drying oysters out of water for a few days killed the worm but not the oysters. Transferring oysters to fresh water for a short time had a similar effect. Oyster beds with soft mud bottoms were most heavily infected. Oysters on clean firm substrates, on reefs and higher ground usually escaped infestation. This knowledge led to the development of a system of timber and wire trays which raised the oysters above the danger zone. By 1903 the industry had returned to its former production levels.

Oyster Production

Oyster production today is only one-tenth of what it was during the period 1880 to 1910. Queensland markets now rely predominantly on oysters from New South Wales. 

The industry can be divided into two regions: southern and northern. The southern region is from the NSW-Qld border to Baffle Creek (Bundaberg). The northern region is north of Baffle Creek. 

The southern Queensland industry is based on production of the rock oyster ( Saccostrea commercialis ) using aquaculture 'furniture' (e.g. sticks, racks, trays). In 1997 there were 123 licensed oyster areas covering approximately 431 hectares of tidal land.

The seasonal occurrence of the disease QX in south-east Queensland waters restricts the tidal areas where oysters can be viably produced and limits the growing season.

In northern waters, the milky oyster ( Saccostrea amasa ) and the black-lip oyster ( Saccostrea echinata ) are harvested from rocky foreshore areas where they have settled and grown naturally. No aquaculture 'furniture' is allowed to be used in these foreshore areas. The majority of areas cover a maximum of 600 metres foreshore length. In 1997 there were 112 licensed oyster areas incorporating 61 kilometres of foreshore and 59 hectares of culture area.

Oyster Biology

Oysters live in the mid to lower part of the intertidal zone. They are suspension filter feeders and remove plankton and other organic microscopic particles from the water as it passes through their gills.

The life cycle of the oyster begins when eggs and sperm are shed into the water where fertilisation takes place. After 2 to 3 weeks the larvae settle and attach themselves to a surface where they continue to grow. This surface may be artificially provided by oyster growers and is the basis for wild spat collection. 'Spat' is the term used to refer to small oysters, usually less than 12 months old.

Spat Collection

Some Queensland growers purchase rock oyster spat from interstate hatcheries. However, most spat is harvested from wild settlement. Stick placement is one of a variety of proven methods used for collecting spat. In this method 6 to 8 sticks are nailed to cross-beams to form frames which are then grouped to form batches. The batches are placed in the mid to lower part of the tidal range where the spat settle on the sticks. Spat fall occurs throughout the year but peaks in Moreton Bay between November and March.

Maturing Oysters

Oysters are often grown to maturity on the sticks on which they were caught. Thinning encourages healthy growth and good shape. The sticks are separated from the frames and laid 15 to 20 cm apart on racks in the intertidal zone.

Regular inspection and 'culling' (separation) of oyster clumps ensures fast growth and regular shapes. Excess spat are removed or killed by a 3-second immersion in water heated to 82ºC, as markets will not accept oysters covered with spat.

Tray culture is used to produce well-shaped oysters in the final stage of maturation before marketing. Oysters are placed concave side up on the trays.

Market-size oysters are laid out at about 35 dozen to a tray (tray size 900 mm x 1800 mm) where they remain for 4 to 8 months. Trays are then placed on racks in the intertidal zone and are often covered with wire netting to protect the oysters from predators.

Other growout methods, such as suspended bags on longlines, are also used for rock-oyster culture.

Culture of oysters in some areas of southern Queensland is constrained by the seasonal occurrence of QX disease. The risk of infection by the microscopic disease parasite is highest from December to March. To avoid loss of stock, operators generally harvest their crop before Christmas and carry out maintenance of the oyster area over this period.

Harvesting and Marketing

Oysters are generally large enough to market in 2 to 3 years.

The qualities that determine the grade of an oyster are size and condition of the meat. Sydney rock oysters are marketed as 'bottle size' when 29 to 40 grams (whole weight) and as 'plate size' at 40 to 67 grams. Oysters less than 5 cm are returned to banks or trays for further growth.

Bottled oysters are 'shucked' (opened) with a knife and the body of the oyster removed by cutting the muscles joining it to the shell. The oysters are rinsed in fresh water and are bottled in clean fresh water with salt added.

Plate oysters are sold either in full or half shell. Shells are cleaned of silt and any surface growth prior to sale using a scrubbing brush or some form of tumbler.