Annex XV
STATUS OF SHELLFISH CULTURE IN THAILAND

Songchai Sahavacharin

Prachuab Khirikhan Brackishwater Station

I. INTRODUCTION

Thailand has more than 2,500 km of coastline along the Gulf of Thailand and the Andaman Sea. There are about 1.5 million MT of annual marine products, of which 150,000 MT are shellfish. The most economic shellfish species cultured are the green mussel, blood cockle, oyster and horse mussel (Table 1).

Table 1. Annual production of shellfish in Thailand by species for the period 1981–1986. (From: Fisheries Statistics of Thailand 1988. Department of Fisheries).

The total area under shellfish cultivation in Thailand for the period 1981–1986 is given in Table 2.

Table 2. Area under shellfish production in Thailand for the period 1981–1986. (From: Fisheries Statistics of Thailand 1989. Department of Fisheries). 6.25 rai= 1 hectare.

II. MOLLUSC CULTURE

2.1 Green mussel

The green mussel (Perna viridis) is the most important mollusc species in Thailand. It has been cultured has been practiced in Thailand for more than 60 years (Juntarashote et al., 1987). The culture method was developed from a type of stationary fishing gear called “bamboo stake trap”. The bamboo poles or palms are driven into the muddy bottom of shallow water areas (about 4–8 m in depth) specifically as collectors of mussel spat. Another adaptation is to extend the wings of fish traps to collect mussel spat.

Pole culture is the only method used for green mussel culture in Thailand. The bamboo poles serve as cultch for the setting of mussel larvae, where they are left to grow until they attain marketable size. Culture period lasts for 6–10 months. Culture areas are located in the east and west coasts. The major mussel culture areas are located near the mouths of large rivers such as Bang Pakong, Chao Praya, Mae Klong and Tha Chin in the inner Gulf of Thailand. The expansion of the culture area and production by transplantation of mussel seed to the south was successful in Sawi Bay and Pattani Bay where production exceeded 8,000 MT in 1983, from a total culture area of 3,113 rais. The potential total area available for mussel culture is about 62,000 rais.

Green mussel culture in Thailand has a good potential for development from both the biological and economical points of view. However, for such development to occur, the basic scientific knowledge of and data on the biology of green mussel, how to fatten it and how to determine the best period of harvest must be available. Spat collection techniques using ropes or other material should be introduced. New culture methods such as the hanging method which has proven to be the most efficient in Europe should be investigated for their biological and economic feasibility.

One of the major constraints in green mussel culture development is the sanitary condition of the final product. At present, the careless manner in which farmers handle green mussel during harvesting is liable to cause serious contamination. To solve this problem, farmers should de-cluster and clean the mussels at the culture area where clean seawater is available, instead of carrying out these operations at the landing sites where the water is generally polluted. Also, improvement of the techniques for packaging should be considered because about 51% of the green mussel production is marketed fresh shell-on while the remaining 49% is processed.

2.2 Cockle

Cockle is a popular mollusc consumed in Thailand. The culture of a local species (Anadara nodifera) began about 90 years ago in Phetchaburi province. Cockle seed were collected in the natural settling grounds and sown in suitably selected areas fenced out by bamboo poles. The fencing was 50 cm high usually enclosing an area of 5–10 rais in order to prevent them from being washed away. The farmers sow about 2–3 million spat in each rai (spat size: 3,000–4,000 seed/kg). It takes about 2 years to reach the marketable size of 60–70 pieces/kg.

Cockles are highly favoured in Thailand and the production from culture is not enough for the local market demand. Some culturists began importing cockle seed (A. granosa) from Malaysia since 1973. About 20,000 MT were imported annually and cockle culture was also introduced in the south of Thailand such as Satun, Trang, Nakorn Si Thammarat and Surat Thani. Cockle farming in the south of Thailand has since developed considerably, where a single farm extends for 200–900 rais. The culture method was the same used in Malaysia (Tookwinas, 1987).

2.3 Oyster

The production of oyster culture in Thailand is about 5,000 MT per year, 60% of which is derived from culture practices. The main oyster species cultured are Crassostrea commercialis, C. luqubris and C. belcheri; the former is a small sized species, while the latter two are large sized species. C. belcheri is the biggest with regard to size and the most economical among the above. One of the most important culture ground for C. belcheri is Surat Thani.

Oyster culture in Thailand has been practiced for about 50 years (Brommanonda, 1987). Farmers used stones and branches for collecting spat and then grew them in shallow waters. The east coast is the culture area for C. commercialis and the southern coast mainly for C. luqubris and C. belcheri. The culture methods are semi-traditional. The culture methods practiced in Thailand are the following:

Rock culture. Rocks are used as substrates for oyster spat settlement. Oysters remain on the rocks until they reach the marketable size. The rocks are piled in groups of 5–10 and spaced in rows approximately 50–80 cm apart. Some farmers use concrete poles instead of rocks. This method is extensively used in Chonburi and Chantaburi provinces.

Concrete tube culture. The dimension of a concrete tube is about 15 cm in diameter and 40 cm in height. The concrete tubes are fitted on bamboo stakes driven into the bottom of the culture area. The tubes are placed in rows approximately 1 meter apart. This method is widely used in Surat Thani province.

Bamboo pole culture. This method uses bamboo poles or wooden stakes 2–4 m in length driven into the bottom. These poles act as spat collectors as well as culture supporting structure. The number of poles per rai ranges between 1,600–2,000. This method is very similar to the mussel pole culture; the only difference is that the oyster culture grounds are usually located nearer to shore.

Other methods. Hanging and tray methods are also used in some localities, depending on the availability of local material and type of substrate.

The culture period from small-sized spat to marketable sized is from 1.5–2 years. The culture of the large oyster C. belcheri in Thailand has good potential from both biological and economical points of view. However, the following problems still remain to be solved:

• The quantity of oyster spat collected from the wild is not sufficient for the needs of the oyster growers

• The water depth in numerous oyster culture grounds throughout the country has become shallower as a result of heavy siltation

• Existing spat collection and culture techniques need to be further improved and new ones developed.

In order to overcome the spat supply problem the Government established a bivalve hatchery in Prachuab Khirikhan province which produces oyster spat as well as conducts research on other commercially important bivalves.

2.4 Horse mussel

The production of horse mussel in 1986 was 8,406 MT from a culture area of 541 rais. The farmers collect the mussel seed when these are 5–10 mm long and transfer them to selected mud flats at a stocking rate of 9–10 MT/rai. Following a culture period of 8–12 months the mussels are harvested by dredging when they have reached a size of 2–3 cm in length. The production per rai ranges between 12–35 MT. Horse mussels are sold for human consumption and as animal feed.

2.5 Short-necked clam

The production of short-necked clam (Paphia undulata) has gradually increased from 62,220 MT in 1981 to 101,232 MT in 1986. All the short-necked clams derive from capture fisheries. The world market demand for this clam species is showing an increasing trend and therefore Thailand should promote its clam industry for the export market. To control the stock depletion problem, the Department of Fisheries is trying to develop an appropriate management scheme. A regulation on boat size and type of gear used has been issued.

III. SANITARY CONTROL AND DEPURATION

Environmental pollution is one of the major problems affecting the molluscs and their culture grounds (Virulhakul, 1988). Pollution is generally caused by conflicting uses of coastal areas and resources. Heavy metals and waste products from industrial establishments and agriculture practices are extremely detrimental to marine organisms. The environmental quality of the oyster, mussel and cockle culture areas vary considerably according to geographical location and season. The concentration of faecal organisms is usually low during the dry season while it tends to increase during the rainy season. The near-shore and river waters are usually more heavily polluted than coastal waters (Saitanu, 1988). Phillips and Mutarasin (1985) studied the concentration of trace metals (Cd, Cr, Cu, Fe, Pb, Ni, Hg and Zn) in four commercially important bivalve species (short-necked clam, cockle, mussel and oyster) from the Gulf of Thailand. The results indicated that all elements were in acceptable levels. Uttarapong et al. (1989) reported the total DDT level in oysters, cockles and short-necked clams. Oysters from the Andaman Sea were found to have more Zn, Cu and DDT compared to the oysters from Surat Thani. However, the level of these residues was below the safety concentration value.

One preliminary study on depuration of some economical bivalve species carried out by Sangrungruang and Songchai (1989) showed that faecal coliform in oysters, cockles and green mussels was acceptable (lower than 230 MPN/100 g) after 24 hours in a recirculating system with sea water sterilized by ultraviolet light.

It is recommended that besides monitoring the heavy metal and bacterial level in the sea water from the culture grounds as well as in the bivalve themselves, sanitary control measures should be generally improved. Therefore, strict sanitation controls from the farm level to consumer should be enforced to ensure that the quality of molluscs are within international acceptable limits. Depuration should be a way to protect the consumer from undesirable health hazards.

REFERENCES

Brommanonda, P. (1987). Present status of coastal aquaculture in Thailand. Southeast Asian Fisheries Department Centre. p. 61–69.

Brommanonda, P., Mutarasin K., Chongpeepien T. and S. Amornjaruchit. (1987). Oyster culture in Thailand. ICLARM Technical Report, 19: 31–39.

Department of Fisheries Statistics of Marine Shellfish and Other Products. (1988). Department of Fisheries, Ministry of Agriculture and Cooperatives. 88 pp.

Juntarashote, K., Bahrometanarat S. and H. Grizel. (1987). Shellfish culture in Southeast Asia. Southeast Asian Fisheries Development Centre. p. 6–14.

Philipps, D.J.H. and K. Mutarasin. (1985). Trace metals in bivalve molluscs from Thailand. ICLARM Technical Report 19: 124–147.

Tookwinas, S. (1987). Commercial cockle farming in Southern Thailand. ICLARM Technical Report 19: 158–170.

Saitanu, K. (1988). Bacteriological quality of molluscs in Thailand. Technical Paper, Chulalongkorn University. 8 pp.

Sangrungruang, K., Sahavacharin S. and S. Ramonudom. (1989). Depuration of some economically important bivalves in Thailand. Asian Food Journal, 4 (3): 101–106.

Virulhakul, P. (1988). Environmental effects of mollusc in Thailand. Technical Paper, Department of Fisheries. 14 pp.

Utarapong, M., Somsiri C. and P. Taintong. (1989). Toxicant residue in sediments and aquatic organism in coastal areas. Abstracts of a seminar on Fisheries, 1989. Department of Fisheries. p. 66.