In 1975, it was estimated that 59 100 tons of the 108 500 tons or about 55 percent of shellfish production of the country was produced through culture. This is based on the calculation that 90 percent of green mussel, 80 percent of cockle and oyster and 25 percent of horse mussel produced in the country were derived from culture operations. The rest of shellfish produced come from wild sources (Table 2).
On the basis of recent production through culture and the hectarage of cultivated areas in Thailand, the annual rate of production for green and horse mussel is about 40 tons per hectare, oyster 13 tons per hectare and cockle 9 tons per hectare. These figures are comparatively low as compared with those of other shellfish producing countries (Table 11). An examination of the culture practices (Andreu, 1969; Chen, 1977; Figueras, 1976; Korringa, 1976) can form a good basis for the introduction of improved techniques.
Mussel culture is practiced in soft mudflats along the coast of Thailand. The method used is almost exclusively the stake method. Bamboo stakes as well as stakes made from the trunk of mangrove date palm locally called “mai ping” (Phoenix paluposa) are used. At present, the use of the palm stake is more widely practiced because the shellfish farmers find them more durable to use than bamboo. The bamboo is hollow and with numerous boring organisms in sea water it is quickly destroyed and can only be used for 1 to 1–1/2-year operation. On the other hand, the palm stakes are solid and are said to last three years in water. The only difference is that you could get longer bamboo stakes (up to 10 m) which can be used in deeper waters while the palm stakes are only from 3 to 6 metres in length. However, the palm stakes are readily available and are very inexpensive (Baht 2 per stake). About 5 000 stakes set at 0.25 metres apart are required for every hectare of shellfish farm. The culture period for mussel is about eight months to obtain a production of about 40 tons per hectare.
Only a small portion (25%) of the present horse mussel production in Thailand is produced through culture. This is done on shallow mudflats characterized by thin layer of soft superficial mud and relatively hard subsurface substratum of sandy-muddy nature. The culture sites may be exposed during the lower low tides. It is observed that the horse mussel bury the posterior part of its body while the anterior is exposed for feeding purposes. Horse mussel seeds 5 to 10 millimetres long are gathered and broadcasted in the culture beds. About 5 tons is used to saturate a one-hectare bed. The rearing beds can be cropped after the sixth month.
Cockles are raised in protected coves where the bottom consist of soft surface mud over a hard and firm substratum. Sites characterized by regular gentle current are suitable for culture purpose. Areas along the coasts directly affected by monsoon winds or currents that can cause radical shifting of the bottom should be avoided.
Cockles are raised on the bottom in shallow waters. The culture site can be exposed during extreme low tides but preferrably for periods not exceeding 4 hours. Small cockles for seeding areas are usually gathered from public cockle grounds or brought from fishermen engaged in gathering these shellfish seeds. About 250 to 600 liters of seed cockles equivalent to about 3.5 to 4 million shells are used in a hectare of cockle farm.
Seeded cockle farms are marked and should be constantly guarded by the cockle farmers for 1 to 1–1/2 years before harvest can be started. During harvest, using manual bottom dredges, the full-sized cockles are selected while undersized ones are returned to the beds. Additional stock of small cockles are added to fully replenish each bed. Thereafter, periodic phased harvesting can be done from these beds. Large variation in productivity is obtained from the culture beds depending on the amount of stock available and suitability of the culture site. Poaching has been a local problem.
Oyster culture, as practiced in Thailand, is unique in Southeast Asia. It is based on the use of rocks, either natural or artificial and also to some extent wood material as a substrate for the growth of the oyster. Even in places where hard, sandy bottom are available and bottom culture may be possible, oyster farmers resort to the use of rocks quarried from the mountain or artificially made concrete blocks for raising oysters. Artificially made blocks are of three kinds: pegs, rods and tube. Two pegs are used to hold in place a rod on the fixed sets of clutch (Fig. 9). A space of 0.50 metres is left between sets to serve as passage for management of the farm.
It is reported that spatting of oyster occurs in two seasons during the year, one in May, June and July and the other in October-November. Oyster farms are, on the average, about 4 rai1 (6 400 m2). The farmer and any additional labour which he hires make and install the concrete blocks that are used to make the farm. Once built, the farm remains permanent save for minor maintenance and periodic harvest of oysters that grow from the farm. Vigilance against oyster enemies like the oyster drill and starfish is taken at this time. The production is fair amounting to about 13 tons/ha/year. Although oyster farmers cultivate only small areas or a fraction of a hectare, the price of oyster is comparatively higher so that the returns are high. Harvest is usually done from October to April and depending on the size of the meat, the oyster cost from Baht 40 to Baht 100 per kg (small - Baht 40/kg, medium - Baht 70/kg, large - Baht 100/kg). Recent problems encountered by oyster farmers is the increasing effect of pollution. For instance, in the village of Tambol Angsila, Ampoe Muang District in Cholburi province, pollution from a lumber yard making wooden boats has affected the oyster grounds in that area.
1 Rai is a unit of land measure in Thailand equivalent to 1 600 m2
Table 11 shows some available figures of production from different countries. It is evident that the production from Thailand is relatively low mainly due to the fact that the culture techniques are simple and traditional. Improvement in techniques based on methods initiated in technologically-advanced countries adapted to local conditions and utilizing locally available materials can greatly improve the production in shellfish farming in the country. Utilization of deeper waters so that higher production per unit area will greatly increase the production and the areas that can be utilized. A more thorough survey and inventory of the different types of areas and the suitable culture method that can be used in each case will be valuable as part of the shellfish culture development programme of the country.
