Chapter: Aquaculture Principles and Practices: Oysters and Mussels

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Cupped oysters

Crassotrea spp. inhabit a wider range of ecological conditions and can grow well in areas of lower salinities, free from some of the common predators and parasites.

Grow-out

In almost all commercial oyster grow-out systems, the main techniques consist of planting spat in protected areas with suitable temperature conditions and high primary production, grading and replanting when needed, providing protection from predators and pests and fattening for the market. The different systems described are mainly designed to facilitate the use of available sites and to enable better utilization of primary productivity for the nourishment of the oysters.


Cupped oysters



Crassotrea spp. inhabit a wider range of ecological conditions and can grow well in areas of lower salinities, free from some of the common predators and parasites. Though they require higher temperatures (above 20°C) during the larval development and settling stages, they can withstand very low temperatures better than flat oysters. Because of these advantages, the overall production of cupped oysters world-wide is much higher than that of flat oysters. Farming of Crassostrea started in Japan, which is still one of the major producers. Though not considered to be of such high gastronomical qualities as the flat oysters, half-shell, cupped oysters in fattened prime condition are well relished.

The basic principles of grow-out of the various cupped oysters are similar and so the following account gives only the salient features of the grow-out practices for the more important species, namely the Pacific oyster, C. gigas, the American oyster. C. virginica, thePortuguese oyster, C. angulata, and the Sydney rock oyster, C. commercialis.

Both on-bottom and off-bottom grow-out methods are followed, depending on local conditions. While in Japan, in some areas of the USA and on a smaller scale in many other countries, cupped oysters are grown on racks and long lines, bottom culture is the common practice in most parts of the USA because oflegal restrictions on the use of floating structures in coastal areas. The availability of hatchery-produced spat has made it possible to adopt off-bottom culture in trays, net bags, etc., of the Pacific oyster, even in places where there are no breeding populations. As spat collection occupies only limited space, suspended collectors can often be legally used in areas where spat-fall occurs.

The traditional system of rearing oysters in parks is widely followed in France, especially in the Arcachon bay, using spat collected on lime-coated tiles. The Sydney rock oyster and the Auckland oyster, C. glomerata, are generally farmed on sticks laid on off-bottom racks. The traditional bamboo stick method of culturing the Japanese (Pacific) oyster continues to be a major source of production in China.

The culture method for the American oyster in the USA consists of catching seed oysters in areas where settling occurs and planting them in areas where conditions are suitable for rapid growth. Unpolluted areas with adequate currents and food production are selected for grow-out. Although many growing areas are in the intertidal zones, the major production comes from sub-tidal beds, often in estuarine areas with low salinities which have few preda-tors and diseases. Harvesting is usually carried out by hand tongs and box and mechanical dredges.

The main predators of the American oyster are starfish, predatory finfish, oyster drills (Gas-tropods), flat worms and crabs. The starfish arecontrolled by the application of quicklime on the oyster beds or by capturing them by dragging special mops and killing by hot water dips. Oyster drills (Urosalpinx), conches (Stylochus) and blue crabs (Callinectes) are major predators of young oysters.

The diseases of American oysters have been extensively investigated and Burrell (1985) listed a number of reviews, including that of Sindermann(1977). Some of the more serious diseases have been caused by protozoans; for example, Minchinia nelsonii causes the salinity-dependent ‘Delaware Bay disease’ and M. costalis causes the ‘seaside disease’. Viral infections and mycosis of larvae have been reported to cause major losses in hatcheries.

 

Although the most common methods of culturing Pacific oysters in Japan today are suspended culture from rafts, long lines or racks, on-bottom sowing is still continued in certain areas (Koganezawa, 1979). The rafts are made mostly of bamboo, or sometimes cedarwood, and measure about 16 m x 8 m. Floats made of styrofoam (covered in polyethylene bags for protection against fouling) are commonly used, although drums or similar material are also suitable. A series of rafts are anchored at distances of 5–10 m and the shell strings or wires bearing the spat are hung from them. For on-growing, the shells are cleaned of all fouling matter and the settled spat thinned where necessary. The spacers between shells are length-ened to provide adequate space for growth and better circulation of water and food organisms. Sometimes the shells are restrung on new wires. The number of floats is adjusted during the rearing period according to the increased weight of the growing oysters.

The long-line system is suited for more exposed areas and can better withstand wind and waves. Essentially, it consists of a series of two parallel wires or fibre ropes, buoyed up with suitable floats (generally wooden or styro-foam). The oyster strings are suspended on ropes between the floats. As in the case of rafts, the floats or the number of strings between floats have to be adjusted as the oysters grow and gain weight. The rack method is adopted in shallow areas and, as described in the earlier section on Production of seed oysters, can be used for hanging culture or stake or stick culture.

The cycle of oyster culture in Japan consists of one or two years. In the one-year cycle, the spat collection lasts about two months, starting in about June, and soon after settling they are transferred for growing on rafts. Harvesting can start from about February to May, giving a grow-out period of eight to nine months. The two-year cycle starts with spat collected later in the season, and the collectors with the spat are transferred to racks in shallow water areas until the following summer, for a period of almost one year, for hardening.

 

During this period there is only limited growth, as the young oysters are usually placed in areas under unsuitable conditions such as low salinity, low tidal flow and fluctuating temperature. For final growing they are moved to rafts during the early summer, and harvesting can start fromabout the succeeding February. The hardened oysters show higher survival rates, although the size at harvest may be relatively small. In order to satisfy the market for large oysters (10–20 cm shell height and meat weight of 10–30 g), a small percentage of the selected one-year-cycle oys-ters may be grown individually in net cages with separate cells for each oyster and suspended from rafts for six to eight months.

Mass mortality in oyster farms in Japan is believed to be related to intensification of farming methods and consequent eutrophication of coastal waters. Starfish and oyster drills (Thais spp., TritonaliaOcenebraRapana, andCeratostoma) are major enemies of oysters, particularly in rack and bottom culture. According to Fujiya (1970) the parasite Polydra ciliata infests 60–70 per cent of the oysters in Japan. On the west coast of the USA, the parasites of the Pacific oyster include the copepod Mytilicola and ciliates such as Ancistrocama and

Trichodina.

The Portuguese oyster (Cangulata), which is very similar and considered by some to be the same as or derived from the Japanese or Pacific oyster (Cgigas), has become an important species in France, especially after the decline of flat-oyster stocks. The Portuguese oyster grows well in a wider range of salinities and can tolerate salinities as low as 15 ppt, making it easier to grow in estuarine areas. It can thrive in rather turbid water and soft bottoms, and grows well at low temperatures, even though it requires a temperature above 20°C for larval development and settling.

Spat collected on shell strings, slabs of slate, scrap iron or plastic tubes are used for on-growing in specially prepared oyster parks. The park is fenced in with galvanized wire netting and often partitioned into several compartments by plastic-covered wire netting. Shells carrying the spat are planted very closely and sometimes rows of stones or sticks are provided as shelter. The grow-out period is generally one year, when the oysters weigh about 10–30 kg per 1000. After the oysters are separated out from the substrate with a heavy knife, they are sorted according to size and held in trays in a basin for some days before replanting.

Replanting is carried out in parks located in areas that are exposed only at spring tides. The oysters are replanted very densely at the rate ofabout 10 kg/m2. The park is fenced with wire-mesh netting to prevent the oysters from being washed away and to protect them from predatory fish such as sting rays (Trygon sp.). As an additional protection from predators, masses of willow twigs are also strewn in the park. All necessary maintenance work, including spreading the oysters, placing willow twigs, removing starfish, scraping the silt and sand from the oysters and harrowing the plot, is carried out during spring tides. Oysters planted in spring are generally ready for harvest in the autumn of the same year, at a weight of about 50–60 kg per 1000. The harvested oysters are manually cleaned of all growths on the shells and sorted according to size. The cleaned oysters are fattened in claires. Only a few oysters (three or four per m2) are planted in each claire, to ensure uniformly high quality. They are fattened there for the whole summer season and if a bloom of Navicula ostrearia develops, the oysters acquire the desired green hue and excellent flavour. By about September the fattened oysters are harvested.

The Portuguese oysters are also grown on trays placed on supporting frameworks. The trays may be made of wood with wire or plastic netting, or made of only wire netting or plastic, divided into a number of compartments. The oysters are spread in the tray one layer thick. Usually the larger oysters weighing 40 kg per 1000 are grown in these trays and, when harvested, they fetch a relatively high price.

Starfish, rays, skates and oyster-eating birds (Haematopus spp.) are some of the predators to be avoided in the oyster parks. Major losses of Portuguese oysters in farms in France were attributed to the so-called gill disease, and another disease of unknown origin. The loss could be made up only by the importation of the Japanese (Pacific) oysters.

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