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The systems adopted for the culture of mussels are very similar to those used in oyster culture. Bottom culture is widely practised in the Netherlands and Germany; ‘bouchet’ (posts or stockade) or stick culture in France, Italy and other Mediterranean countries and the Philippines; and raft and longline culture in Spain, Sweden and New Zealand.
On very muddy coasts with gentle slopes and large tidal oscillations, bouchet and stick culture give good results. Bottom culture is suitable for coastal areas with stable and hard bottoms which are submerged for long periods. In shallow estuaries and bays with a low tide coefficient, cultivation on hanging ropes is preferred. The raft method is generally practised in protected areas, with steep coastal profiles and considerable tidal oscillations, at depths of at least 3m at the lowest neap tides.
Bottom culture is entirely based on seed produced in natural beds, and the mussels are more exposed to predators and pests. The mussel parks are located in shallow, enclosed or partially diked areas.
The bouchet system of culture (fig. 27.16) originated in France in the early 13th century and an Irishman, Patrick Walton, who was shipwrecked in the Bay of Aiguillon, appears to
have devised the technique based on his observations of the settling behaviour of the blue mussel on the poles of the equipment he used to capture fish. Rows of wooden poles of about 3–6 m length, called bouchets, are driven into the bottom of intertidal zones, to form the park for growing mussels on ropes. The rows are normally at right angles to the coastline and spaced about 15–20 m apart, the distance between poles in each row being about 20cm to 1 m. The lower part of each pole above the sea bottom is covered with smooth plastic sheets to prevent predators like crabs from reaching the mussels. In stick culture, bamboo or wooden sticks replace the bouchets and are used for spat collection as well as for on-growing.
The raft system of culture is generally practised in protected areas like the rias of Spain, in depths of at least 3 m at the lowest low tide (fig. 27.17). Different types and sizes of rafts are in use, ranging from those constructed with thehulls of old fishing vessels to concrete and steel platforms or pontoons with styrofoam and fibreglass floats. There is a wooden lattice framework over the floats, to which ropes containing young mussels are attached. The outside of the wooden platform and floats is protected from wood-boring organisms (mainly Teredo and Limnoria) by coating with cement, antifouling paints, strips of fibreglass or other synthetic material. The size of a raft varies from about 400 m2, which can carry some 500 ropes of mussels, to 700 m2 or more, carrying 1000 or more ropes. They are built in such a way as to create minimum resistance to surf. The rafts are anchored by long, strong chains (six or seven times longer than the depth of water at the site), facilitating adequate movements and providing strength to withstand bad weather conditions.
The long-line system of culture developed in Sweden (Lutz, 1985) consists of a series of horizontal lines of about 10 m length, buoyed up by a number of suitable floats and anchored down with concrete blocks or other anchoring systems. On these lines a number of vertical ‘substrate’ lines are hung. The length of these lines can vary, but in the shallow fjords (less than 10m depth) where they are operated in Sweden, it seldom exceeds 0.5 m. The lengths are standardized, in order to mechanize harvesting. The system is reported to work well under the freezing conditions and low tidal ranges in Swedish fjords.
The long-line culture system has been tried on an experimental basis in France to improve the quality of the mussels grown in lagoons and other similar sites. One-year-old mussels are raised on long-lines in the sea for about eight months, where they achieve faster growth and better taste.
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