In the final grow-out to market size, different systems of varying degrees of sophistication are used, ranging from simple sowing on the bottom to recirculation systems. A cost-effective system developed in the USA involves the use of covered sub-tidal areas in which baffles, pens or net tents are used together with crushed stones for protection of the seed and prevention of siltation. A stocking density of up to 4300 seed/m2 is possible in such beds. However, thinning of the stock to about half the original density is necessary as the clams grow in size. The grow-out period is generally two to three years, depending on local conditions, and the market size is 4 to 4.5cm shell length. Tray culture allows higher densities of stock and many farms in the USA have used them successfully in the grow-out of M. mercenaria.
Clams of the genus Meretrix, especially M.meretrix and M. lusoria, are cultured in the FarEast. The common species cultured in Taiwan is M. lusoria. Its culture is somewhat unique anddifferent from those described above in that it is often grown in association with milkfish in the inlet and outlet channels of pond farms. It is also cultivated on sandy flats and tidal estuaries.
Seed clams are collected from coastal areas with rakes operated from small boats, mainly during summer, although seed can be found throughout the year. The seed clams of about 0.5mm length are sold to seed clam growers, who rear them in shallow brackish-water ponds. If the ponds are not fertile enough, organic fertilizers are applied to promote growth of phytoplankton. The water is exchanged every three to four days. The stocking rate is about 30–50 million seed/ha. Where possible, the growing clams are sorted according to size and replanted separately. In about six months they reach a size equivalent to 800–1000 clams per kg and are ready for sale to farmers who grow them to market size.
Grow-out may be in sandy tidal flats, estuarine areas or in fish pond facilities. Beds with a high content of sand (at least 50 per cent) are selected as such bottoms provide for the bur-rowing of the clam and also seem to promote the attractive pink coloration which is important in marketing. In such areas it is usual to install fences of net mesh to prevent the escape of clams and the entry of predators. The stocking rate generally varies from 2000–5000kg seed clams of 600 per kg size in ponds, to as low as 100kg per ha on sandy flats. They are spread evenly on the bottom. A size of 35 per kg may be reached in about 18 months. Harvesting from ponds is done by hand and from estuaries and tidal flats with rakes fitted with a net bag for holding the collected clams.
The clam that is important in several countries of Asia is the cockle or the blood cockle Anadora granosa. It is cultured on a limitedscale in most countries of the region, including China and Taiwan, and is found on muddy estuarine flats and bays with weak tidal currents and waves. The spawning season varies with the locality, e.g. in China from July to September and in Taiwan from January to April. The spat settle on fine, sandy mud flats in the lower intertidal areas.
Cockle culture is relatively simple and mainly consists of collecting natural spat and planting them in protected beds for rapid growth. The sites generally selected have soft muddy bottoms with 2–2.5m water at high tide. In Thailand, shrimp farmers often use elevated parts of their ponds for cockle growing. Exposure of the bed for more than about six hours and sudden changes in salinity due to heavy rains can cause serious mortalities. Areas under cockle culture are often fenced in to prevent poaching. In Taiwan, the spat are generally nursed in specially prepared mud flats enclosed by fencing of nylon netting. They are grown to a size of 5000 per kg and then sold to farmers who grow them to market size. Some of the seed producers operate their own grow-out farms. Stocked at the above size, the cockles grow to 500–600 per kg in one year in Taiwan. The minimum market size is 120 per kg and to reach that size it usually takes two to three years or more. During this period the farmer tries to eradicate predators and pests, such as wild ducks, crabs, sea snails and puffer fish. Harvesting is generally done manually.
Relatively fewer clam or cockle diseases have been reported. Many of the known diseases of juveniles and adult clams are caused by the haplosporidian Perkinsus marinus, the coccidian Hyaloklossia and Pseudoklossia, the gregarine Nematopsis and ciliates like Trichodina and Ancistrocoma.
Copyright © 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.