Chapter: Aquaculture Principles and Practices: Oysters and Mussels

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Flat oysters

Compared to cupped oyster, flat oysters are less hardy and thrive best well below the low-water line.

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.


Flat oysters

Compared to cupped oyster, flat oysters are less hardy and thrive best well below the low-water line. Their growth rates are lower and they require three to four years to reach market size. They normally suffer greater mortality during the long culture period and so the overall production of flat oysters is relatively less. Korringa (1976a) has given detailed descriptions of grow-out methods followed by selected enterprises in Europe and the USA and these show significant differences based on local hydrographical and market conditions. Some of the common basic practices are summarized below.

As indicated earlier, the spat-collecting area is not always the best area for on-growing. Besides the water quality and primary production, the nature of the bottom, depth and tidal regimes are of considerable importance, particularly in view of the length of time required for flat-oyster culture and the need for over-wintering. Though in principle off-bottom culture can be practised on muddy beds, a firm bottom is preferred. In oyster parks in France, sand or fine gravel is spread every year after the bottom is levelled to maintain the sub-soil structure. The shallower areas of the park used for rearing spat to young oysters are protected by net fencing.

 

Spat collected during the summer over-winter in the collecting ground and are brought to the park for rearing in the succeeding spring. The spat are detached from the collectors (usually lime-coated tiles from which they can easily be scraped off) manually or by special mechanical devices. During early spring, the detached spat are planted in the deeper areas of the park (about 80 cm water at low tide), since predation by crabs is not a major problem then. Later in the season the spat are planted in the fenced area and special care taken to eliminate crabs with baited crab traps and other devices. In order to reduce excessive algalfouling, some farmers introduce periwinkles (Littorina) which graze on the algae. The young oysters over-winter the second time in the park. After the second winter they are dredged out and replanted in fenced parks in intertidal zones, which are exposed for only short periods during spring and neap tides. A stocking rate of 10 tons/ha of second-year oysters (weighing 6– 7 kg per 1000) is believed to be ideal in typical areas in Brittany (France). Some farmers allow the young oysters to grow in the same park, without replanting. Constant care is essential to maintain the parks and protect the stock. After the third over-wintering, the oysters are gathered early in spring and replanted in a clean plot. The third-year oysters can be marketed, but after a further over-wintering the oysters fetch a higher price. The oysters are harvested by dredging followed by hand picking, and are then carefully graded according to size.

Newly detached spat often suffer high mortalities due to predation and therefore many farmers, particularly in the Netherlands, grow them first in trays in areas with a rich supply of food. The trays are placed on racks and filled densely with spat. Under favourable conditions, the growth rate is fast and thinning is necessary to maintain their growth. When they have grown to a weight of 3–4 kg per 1000 by the end of the summer, they are ready to be planted in the growing plots. In some areas, as in Italy, the spat may be fastened to ropes for suspended culture. Instead of the old practice of setting spat on pieces of wood or twigs and inserting them in ropes for hanging, some farmers in parts of southeastern Europe cement the spat on wooden boards (7.5 cm x 4 cm) in groups for on-growing. A common method of oyster culture (for both flat and cupped oysters) on the Atlantic coast of France at present is rack culture. The spat are held in synthetic bags (1 m long and 0.5 m wide), which are fastened by rubber bands to wooden or metal racks standing 0.5 m above ground. A density of not more than 6000–7000 bags (each containing 5 kg in the case of 1.5–2-year-old flat oysters and 5–10 kg cupped oysters of the same age) per ha is considered suitable for satisfactory growth.

In Maine, in the USA, the flat-oyster farmers usually purchase hatchery-produced spat and grow them in trays or nets off-bottom from rafts or long lines. The oysters are repeatedlygraded during the summer. The trays are lowered to the bottom or suspended low from rafts for over-wintering and then raised in spring to continue growth.

The hanging method of culture has become very popular on the Mediterranean coast of France in recent years. Ropes laden with oysters are suspended in protected areas from metal or wooden frames, in such a way that the oysters are totally submerged. Seed oysters are stuck on synthetic ropes or specially made wooden poles, using quick-setting cement. On a2 m long pole or rope, about 75–80 oysters are stuck. Fouling organisms are regularly removed by hand to allow the oysters free access to the water flow. Harvesting is fairly easy, as the ropes can be brought ashore and the oysters detached. Though the growth rate and yield (5 kg per rope or pole) are high, the shell is often fragile and tends to open after harvesting.

The technique of fattening oysters in special oyster ponds called ‘claires’ in France has been briefly described. This practice is of special importance in areas where there is a scarcity of fattening grounds but is now used mainly for greening Portuguese oysters.

Predators, pests and diseases As indicated earlier, once the spat are planted the major effort of the farmer is to protect the stock from predators and pests to the extent that is feasible under open-sea conditions. The vulnerability of spat and young oysters to shore crabs (Carcinidas sp.) makes it especially important to provide all possible protection from them. Another major predator is the starfish (Asterias sp.), which may settle on the collectors and prey on the spat. Repeated hand picking is the common method of control, although application of quicklime on the oyster bed can also be effective.

Barnacles and other fouling organisms not only compete for space on collectors, trays, nets, etc., but also affect the growth and appearance of the oysters. In addition to manual clearing, which is commonly done, Korringa (1976) described spraying of a low concentration of DDT on spat collectors as a means to prevent fouling.

Large-scale mortalities of the European oyster occurred on the coasts of France from 1968 for over a decade. These epizootics are believed to have been caused by three or four protistan parasites, starting with Marteiliarefringens and then Bonamia ostreae. The onlymeans of control appears to be to avoid planting seed oysters during the period when infections occur, which is reported to be July and August for M. refringens.

Shell disease caused by the fungus Ostracoblabe implexa has caused serous losses ofyoung oysters in the Zeeland oyster area of the Netherlands in the years following 1930(Korringa, 1976). Another reported disease is the ‘pit disease’, which is described as a congestion caused by the rapid multiplication of the flagellate Hexamita. It generally occurs when oysters are kept too long in a storage basin at low temperatures.

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