Other Types of Aquaculture

Mollusks. Other important objects of cultivation in many parts of the world are mollusks. Though few water snails are cultivated, bivalves, especially oysters, are quite important in Asia, Europe, North America. For centuries French fishermen cultivated oysters by placing twigs in the water to which free swimming oyster larvae could attach. In northern Europe, oysters have been cultivated on the ocean bottom, but now winter temperatures limit the extent of this activity. In the Mediterranean, the Romans are said to have been the first to farm oysters. Today, oysters are cultivated on the Pacific coast of North America, as well as on the southern Atlantic coast and the Gulf of Mexico. Australia, the Philippines and South Africa also possess farms and the Japanese grow edible oysters from Hokkaido in the north to Kyushu in the south. Japanese farms are divided into two classes: some cultivate seed oysters only, while others raise them for food, especially for export. The Japanese cultivate oysters on the sea bottom (horizontally) and sticks (vertically). To collect the larvae, which affix themselves to any firm object, such as an old shell or a stone, fishermen place various devices in the water. These may be bamboo sticks with shells attached or a rope with shells hanging from it; limited tiles and wooden plates have been used for the same purpose in Europe. Production is greatest in places with good shelter against rough seas, a tidal current to carry food to the larvae, adequate salinity and optimum temperature.

After some growing time, the larvae are loosened and transported to other areas for maturation under the best conditions. While growing to marketable size, the oysters must be protected against predators, such as starfish and oyster drillers. As starfish damage cannot be completely avoided when growing oysters on the bottom, a vertical system of culture is preferred in many areas; the oysters hang in clusters or in baskets or are fixed on poles in sheltered bays. In an alternative system, the oysters remain in horizontal trays kept at some distance from the bottom. Though such tray-raised oysters are expensive, they generally survive better than those reared directly on the bottom.

Blue mussels are cultivated in Italy, Spain, France, the Netherlands and near Germany in the North Sea and the Baltic. There, too, horizontal–bottom methods have been replaced by vertical culture. Originally, the young mussels, collected from wild stocks, were spread on controlled banks leased by a fisherman from the government. Their capacity to grow in very extensive and dense beds is highly advantageous. Before full-grown mussels are sent in sacks to the market, special purification methods are employed to wash out sand. Today vertical culture is practiced with sticks pushed into the ocean bottom or with lines hanging from rafts. Unfortunately, line cultures may be damaged in winter, thus, experiments have been made with polyethylene net bags and endless tubes of polypropylene netting. These bags must be strong enough to carry the mussels until harvesting.

Many other mollusks are cultivated, including soft clams and scallops. The Japanese even raise octopuses and squid. For bivalves, the problems are roughly the same as mentioned above: collecting the larvae, raising the young mussels under good conditions, protecting them against predators, harvesting the adults without injury, sometimes cleaning for the market.

Дата добавления: 2015-11-14