Harvesting of molluscs
Methods of harvesting molluscs like oysters, mussels, etc., for market depend on the culture systems employed. In bottom culture, where the sea bed is planted with spat for on-growing, the most common harvesting method is dredging. From intertidal areas which are fully or partly drained at low tide, oysters or other molluscs can be easily collected by hand. But, as will be described, off-bottom methods of culture are commonly practised in modern oyster and mussel farms. The oysters and mussels are grown on sticks planted in the intertidal areas or hung on long lines, special nylon bags or plastic containers. In such cases, harvesting is relatively simple and can be done by hand or from a boat when necessary. When they are grown on special platforms, as for example mussels in New Zealand (fig. 11.8), mechanical lifting devices installed on a large boat may become necessary.
In countries like France, where there is a special market for greened oysters, fully grown flat or Portuguese oysters may be reared in small, shallow coastal ponds known as claires for up to six months in order to increase their weight and develop the green coloration. Claires are drained, dried and then fertilized before filling with salt water to a depth of about 25cm. Water is exchanged at spring tides. Oysters are stocked in these ponds at a high density. They grow in weight rapidly (almost doubling in a six-month period) and the bluish-green diatoms (Navicula), which grow in the claires, impart a green colour to the meat. Such large greened oysters fetch a high price in French markets.
When molluscs are dredged out from sea beds, some of the sand and silt swirled up during the operations may enter their mantle cavity. In such cases, the animals can be spread out on a cleansing plot or basin and covered with gently flowing sea water for varying periods of time. This helps them to eliminate any silt or sand and also recover from the fishing stress. They are transported alive to distant markets.