THAILAND

Current status

Thailand ranks among the top ten fishing nations of the world with marine products accounting for more than 90% of the annual production. Molluscs account for a modest percentage of this production, however there is great potential for expansion, and the Government of Thailand, through the Department of Fisheries (DOF) is emphasizing coastal aquaculture development especially with regard to bivalve culture. This development activity is the responsibility of the Brackishwater Fisheries Division (BFD) of the DOF, which is assisted by several agencies including the Food and Agriculture Organization of the United Nations (FAO), the UN Development Programme (UNDP), the Japan International Cooperation Agency (JICA), the Asian Development Bank (ADB), and the World Bank.
Up to about 50 years ago Thailand had abundant resources of oyster from natural beds. These resources have gradually decreased due to over-exploitation and pollution brought about by a substantial population increase in coastal communities. To meet the growing demand for oysters in the country as well as to exploit foreign markets, oyster culture was first initiated in Chonburi, Rayong and Chanthaburi Provinces, later spreading to the southern part of Thailand (Prachuab Khirikhan, Surat Thani, Songkhla and Pattani Provinces).
Three commercially important species are captured from natural beds and cultured: Saccostrea commercialis, Crassostrea lugubris and Crassostrea belcheri. The first, a small species, is commonly cultured along the east coast of the Siam Gulf. The latter two larger species are produced naturally and cultured mainly on the west coast of the Siam Gulf and on the west coast of Thailand (Table 11 and Fig 24). They obtain a higher market value than S. commercialis mainly because they are consumed fresh.
Oyster production figures from 1980 to 1985 are shown in Figure 25. Although the production for 1985 (8,757 MT) was lower than in previous years (possibly due to the limited supply of seed) the output from 1980 to 1984 had steadily increased with an annual growth of 20.5 %. The total area under oyster cultivation is reported to be just under 1,000 hectares.

Culture practices

Sites for oyster culture are usually on the hard, stable bottom of intertidal flats which support heavy collectors for spat attachment and growth. However, a variety of coastal environments have been explored.
Various culture methods are practised in Thailand. The most suitable for a given area depends on factors which are site specific, the most important being bottom and sea conditions. Oysters can be grown in three basic ways : bottom, suspension, and midwater culture. All these techniques are currently being practised in Thailand and for each type a number of different methods have developed.

Rock culture. This method is unique to Southeast Asia. It is based on the use of natural rocks as a substrate for spat settlement and growth until harvest. The rocks are usually piled in groups of 5–10 and spread in rows approximately 50 cm apart in each direction so as to facilitate management and harvest. This technique is used in areas with hard, sandy or sandy-mud bottoms firm enough to support the rocks. Bamboo mats or platforms are commonly used in soft bottom areas to prevent the rocks from sinking.

Tray culture. This technique is usually practised in muddy bottom areas. Trays are usually built in any convenient size; there are no standard sizes but a popular dimension is 100 x 80 cm with a height of 25 cm. Trays, which are mounted on stakes approximately 30 cm above the bottom, are constructed from strong wood (bamboos are often used) and are fitted with a nylon net bottom. Oysters used for tray ongrowing are removed from rocks or other spatted materials at 2 cm in length.

Culture on cement poles. This method is suitable for muddy substrates. Its advantages are its durability and resistance to wind and wave action. The cement poles (Fig. 26) which are manufactured by the fishermen constitute two separate units: 1) a supporting pole roughly one meter in length (sometimes substituted with wood poles) (Plate 8) and 2) a concrete “cap” 50 to 70 cm in length (Plate 9) on which the young spats (2 to 4 cm in diameter) are fixed with cement. The concrete support pole is buried in the sediment as deep as half of its length, depending on bottom conditions. The cap, which has an opening for insertion in the pole, is then fixed over the top of the previously buried pole (Plate 10).

Culture on wooden poles. This technique is suitable for areas with strong but not destructive currents and wave action. Poles are set in soft or muddy bottoms similar to the system used for green mussel culture. Some producers allow spat to settle on the poles and use the initial spat set for grow out to market size; others purchase spat and mechanically fix them on the poles. Low-priced locally available bamboo or wood is used. Poles are firmly driven into the bottom at a depth that depends on the sediment composition. One disadvantage is the brief life of the poles (about one to one-and-a-half years).

Culture on cement pipes. This system is similar to cement post culture. However, the pipes can be acquired from local dealers and do not have to be especially manufactured for oyster culture. The pipes are fitted on strong wooden or concrete stakes driven into the bottom of the culture area Plate 11 and Plate 12).

Suspension culture. Suspension or hanging culture is widely practised in Japan, United States and Europe because the oysters grow fast and give high yields. This method has been tested in Thailand but, although good growth was observed, a number of problems were encountered, mainly with regard to material durability and the relatively high initial investment as compared to other types of culture materials. This method however, would be most suitable for sheltered deep waters.

Oyster spat collection uses all kinds of materials on which the oyster larvae will attach. However, not every material is suitable as cultch from the practical and economic points of view. Shells of Placuna placenta are commonly used and experiments are being carried out to determine whether cement coated shells or plain shells are more suitable (Plate 13 and Plate 14). It is reported that spatting of oyster occurs in two seasons during the year, in May to July and in October to November. Oyster farmers need a simple way to determine the best time to place the collectors, avoiding attachment of fouling organisms and maximizing spat collection. As suggested by BFD, farmers should place a few collectors several months before the spatting seasons, monitoring them for occurrence of oyster spat. When suitable numbers of spat are found on the test collectors, the bulk of the collectors can be placed. Oyster farmers have on the average, 7 rai (11200 m²) and usually hire one or two permanent workers who perform a number of duties ranging from spat collection to harvest. Temporary labour is necessary during peak harvesting periods when market size oysters have to be removed piece by piece. An average of 2.0 × 10³ kg/rai (19t/ha) is removed annually.
Molluscs are sold either fresh or cooked. S. commercialis are usually shucked and frozen so as to exploit distant markets. C. lugubris and C. belcheri are commonly sold shell-on for consumption. At present there is little health control because molluscs are not dangerously affected by pollutants. However, with the increase in industrial activities, monitoring flesh quality will be essential. Fishermen remain interested in oyster culture as supply does not meet demand.

Problems and constraints

The constraints to oyster culture in Thailand are mainly limited seed supply, especially of C. belcheri, limited suitable culture areas and poor quality control.
Environmental problems (coastal pollution) are limited mainly to the production areas in the inner Gulf of Thailand where 70 % of the coastal aquaculture production is derived. Pollution in the Gulf is generally the result of a major conflict in the use of the coastal zone. Industrial plants release materials that are detrimental to aquaculture production such as pesticides, herbicides, heavy metals and fertilizers. Problems of low oxygen content in the water caused by the decomposition of organic matter are becoming chronically serious in the inner part of the Gulf and in the estuaries of main rivers. The “red tide” is increasingly a threatening problem as it is also associated with the nutrient richness and organic waste materials in the sea.

Future plans

Thailand is expanding considerably and projects are underway to introduce oysters culture in different parts of the country. At the moment oyster spats are harvested naturally from the sea, however, the recently built bivalve hatchery at the Prachuab Khirikhan Brackishwater Fisheries Station is producing oysters which are utilized for a number of demonstration and on-growing trials.
On average 2 million oyster spat size 2 mm are produced monthly, but only a small percentage (5 %) reaches 3 cm which is the size preferred by the fishermen. More research is required to improve survival of young spat, and efficient nursery practices need to be introduced, developed and tested to reduce spat production costs. Various experiments are now underway at the hatchery.
For many reasons capture fisheries is becoming less feasible, and as a consequence, coastal aquaculture is receiving a great deal of attention. Mollusc culture is one sector that is rapidly developing. Traditional culture methods are being improved through the introduction of techniques from technologically advanced countries. However, one major factor which will influence the expansion of this activity is the ecology of the coastal areas where mollusc culture is mainly practised. More and more areas are becoming dangerously polluted by industrial and domestic wastes and therefore, in addition to farm management, careful monitoring will be required to maintain healthy environmental conditions.
Although at present depuration of shellfish is not practised in Thailand, the DOF is conducting experiments on technical and economically feasible depuration systems.

Table 11. Distribution and main culture areas of commercially important oyster species in Thailand.

Figure 24: Major farming areas of the three commercially important oyster species in Thailand.

Figure 25: Oyster production data in Thailand from 1980 to 1985.

Figure 26: Cement pole (a) for oyster culture as practiced in Thailand;
cement pole (b) with attached adult oysters.

Plate 8: Cement pole culture. Cement cap supporting pole, Ranong, Thailand.

Plate 9: Cement pole culture. Manufacturing site of the cement caps, Ranong, Thailand.

Plate 10: Harvesting of marketable size oysters cultured by the cement pole method, Surat Thani, Thailand.

Plate 11: Cement pipe culture, Surat Thani, Thailand.

Plate 12: Cement pipe culture. The bamboo fence defines individual private farms. Small guard-houses are visible in the background.

Plate 13: Plain (left) and cement-coated (right) windowpane shells (Placuna placenta) used as oyster spat collectors, Phangnga, Thailand.

Plate 14: Fixing of windowpane oyster spat collectors on a raft, Phangnga, Thailand.