OYSTER CULTURE IN THAILAND

presented by

Suparp Pripanapong *

I. Introduction

Thailand is located in Southeast Asia and surrounded by four countries: Burma and Laos in the north, Laos and Kampuchea in the east, Burma in the west, and Malaysia in the south (Fig. 1).
Thailand has more than 2,500 km of coastline along the Gulf of Thailand and the Andaman Sea.
In the National Social and Economic Development plan for 1987– 1991, the fisheries sector has two major components:

1. Fisheries resource development, and

2. Development of fisheries production

Included in the second component there is the development of mollusc culture with particular reference to 3 groups of economically important bivalve species: Cockle (Anadara sp.), Green mussel (Mytilus sp.) and Oyster (Crassostrea sp.).
Oyster culture area increased from 2,042 rai (¹ rai = 1,600 m² or 1 ha = 6.25 rai) in 1975 to 6,173 rai in 1984, about 3-fold in 10 yrs (Fishery statistics). However, from 1980–1984 the culture area increased by almost 1,000 rai (Table 1) but the production decreased in 1982 and 1983 by almost 3 tons. This can be explained by the fact that the production values for 1980 and 1981 included collection from the wild.

* Researcher at the Suratthani Brackishwater Fisheries Station.

Figure 1. Map of Thailand

In 1983, there were 11 provinces where oysters were farmed; 7,046 rai were being actively cultured and a total potential area of about 35,933 rai for oyster culture was estimated (Table 2).

From : Pairoj Brohmanonda, Brackishwater Fisheries Division, Department of Fisheries, information needs for effective aquacultural planning, paper for presentation on aquacultural economics workshop, at Ambassador Hotel, Bangkok, Thailand, May 6–10, 1985, Table 3.

II. Species of Commercial Importance

In Thai coastal areas, there are three dominant species, namely:

• Crassostrea commercialis which is circular in shape and small. In Thailand this species is called Hoy Nang Rom Pun Lek, Hoy Park Jeep or Hoy Joa.

• C. lugubris. It is mostly elongate and can grow up to 12–15 cm. Thai common names are Hoy Nang Rom Pun Yai and Hoy Takrom Kram Khaw.

• C. belcheri. It has a similar shape as C. lugubris but it can be distinguished by the black internal muscle scar. It has a number of Thai common name such as Hoy Nang Rome Pun Yai and Hoy Takrom Kram Dum.

III. Distribution of important commercial species

Oysters are found abundantly in shallow water along rocky shores and intertidal mangrove areas. In Thailand, oysters are found on both the Gulf of Thailand and along the Andaman sea coasts. The species are:

• C. commercialis, very abundant in the provinces of Chonburi, Chantaburi, Trat, Ranong, Prachuab Khiri Khan, Chumpon, Trang, Krabi, Rayong and Satun (Fig. 2).

• C. belcheri, very abundant in Chantaburi, Chumpon, Ranong, Surat Thani, Phang Nga, Trang and Satun.

• C. lugubris, very abundant in Surat Thani, Phang Nga, Songkhla and Pattani.

IV. Background of oyster culture in Thailand

Thailand has abundant oyster resources from natural beds. The oyster beds are located in shallow coastal water of the intertidal mangrove area. Increase in population and demand for oyster have reduced the natural oyster populations to a level insufficient to supply local demand. Oyster culture was started to meet the domestic demand.
It is believed that immigrants from China first began culturing oyster in Chonburi, Rayong and Chantaburi Provinces about 50 years ago. The culture method adopted was the use of stone on the bottom of shallow areas. The stones served as spat collectors and as a grow out surface for the oysters. The system is similar to that used in the southern part of China. In the eastern coast of Thailand oyster farming is a full-time family occupation.
Oyster culture developed later in the southern part of Thailand. There are 12 provinces in the east of Thailand where oyster culture is practised (Chonburi, Rayong, Chantaburi, and Trat), and 8 in the south (Prachuab Khiri Khan, Chumpon, Surat Thani, Songkhla, Pattani, Narathivas, Phang Nga and Ranong) (Table 3). Total number of farms in the east were 1,760, averaging 2.13 rai per farm. In the south there were 496 farms with an average of 4.46 rai per farm.
From Table 3 it is evident that the largest oyster culture area is in Chantaburi with about 1,601.97 rai (26.18%) of total area. Chonburi, Surat Thani and Ranong have culture area of 1,443.36 (23.52), 986.04 (16.11) and 962.50 rai (15.73%), respectively.

Figure 2. Location of oyster beds in Thailand.

From: Fisheries Statistics Section, Fisheries Economics and Planning Sub-Division, List of Oysters Farmers and their raising area by Location Computer printed out, 1984.

V. Selection criteria for oyster culture sites

Physical parameters are the most crucial factors in selecting suitable culture areas. A good area should have the following characteristics:

1. It should have shallow water and be located in a mud flat or soil with sandy loam composition. The mud layer should be thin or should be located over a hard bottom to support the substrate for culture depending on the method of culture. The coastal region of Thailand has many sites that meet this criteria.

2. Natural seed supply should be present or found in a nearby area to ensure that the area can be cultured and to maintain low production costs.

3. There should be no influx of freshwater that would reduce the salinity for long period during the rainy season.

4. Currents and strong winds should not be too strong in order to efficiently manage the culture substrate. A suitable area would be an enclosed bay or protected shoreline.

5. Optimum current in the area would be 3–5 m/min. The water should be rich in nutrients for plankton growth and should be moderately clear with low turbidity and good light penetration.

6. There should be no external source of pollution that can release substances harmful to the oysters or to humans. The area should be located away from mining activities that tend to reduce water transparency. Turbidity reduces oyster growth rate and can cause considerable spat mortality.

7. Substrate material should be available at low cost in the local area. For example, rock, bamboo, empty shell and other materials depending on the availability.

8. The area should have few or no oyster predators such as starfish.

9. The culture site should be located near a market or an area with a developed infrastructures such as roads.

10. The site should have full strength sea water. During the rainy season, oyster can be temporarily transferred to waters of higher salinity.

VI. Oyster Culture Sites in Thailand

There are many environmental factors that affect oyster culture in Thailand related to the suitability of the culture site. Factors of major concern are:

Bottom: The bottom serves as a base to hold the substrate to which oyster attach, or to hold anchor in raft and long line culture.
In Thailand, most oyster culture sites extend up to 3 km from the shore. There are 2 kinds of bottoms:

1. Muddy bottom. This kind of substrate is found in the Gulf of Thailand and in the provinces of Chantaburi, Prachuab Khiri Khan, Chumpon and Surat Thani except Trat, Rayong and Chonburi.

2. Sandy Clay loam bottom. This substrate is typical of the Andaman coast and in some provinces facing the Gulf of Thailand such as Ranong, Satul, Songkhla, Chonburi and Rayong.

Wind: Wind is another factor that can produce harmful effects on the substrate. There are two monsoons in Thailand that can affect oyster farms.

1. Northeast monsoon affects oyster farms in the Gulf of Thailand, and the.

2. Southwest monsoon which affects oyster culture in the Andaman Sea.

Salinity: The salinity of the Thai Gulf sea water is normally lower than the salinity of the Andaman Sea. In the Gulf of Thailand salinity values vary between 10–30 ppt due to numerous rivers flowing into this gulf such as the Wain River in Chantaburi, Tapi River in Surat Thani and other small rivers and canals; values varying between 29–33 ppt are typical of the Andaman Sea.

Tide Levels: The culture grounds in Thailand have varying tidal levels:

1. The east coast provinces which consist of Chonburi, Rayong, Chantaburi and Trat, have a tidal average quite different between highest tide to lowest tide (Table 4).
   The highest seawater in Chonburi is 40 decimeters and the lowest sea water is 5 decimeters. The difference is 35 decimeters or 3.5 m.

2. In southern Thailand there is a narrower range of tide interval. The lowest seawater at zero level, is at Pattani. In Songkhla, tidal interval is only 6–7 dm.

3. The tidal characteristics in southern Andaman Sea is similar to eastern Thailand. In Ranong the highest seawater is 42 decimeters (4.2 m), the lowest 6 dm.

All the figures shown in Table 4 were recorded between 6 am and 6 pm. The tidal fluctuation plays a big role in oyster culture particularly on spat collection, harvesting period, etc.

pH: pH in seawater is normally about 8. According to the Marine Fisheries Division, the pH at the shore is 8.10, while it is 8.30 off shore.

VII. Culture Method

There are various methods used to culture oysters. Each method is biologically suitable at specific locations. The different methods are used depending on site factors. In Thailand the common culture methods are the following:

1. Rock culture method

Rocks are used as substrate for oyster spat settlement. the oysters remain on the rock until harvested. The area should be close to a supply of natural rocks. Rocks are piled in group of 5–10 and spaced in rows approximately 50 cm apart each way (Fig. 3). The bottom should be firm and smooth. For areas with soft bottom mud, a platform or mat or bamboo can be used to prevent the rocks from sinking such as in Amphoe Sawee in Chumpon Province. Rocks are placed on 1×2 m bamboo platform; each pile of rock is about 0.83 m apart while each row is roughly 1.16 m apart. One set consists of three layers.
In Chonburi the rock method is modified with the use of concrete poles because rocks are not easily available. In this method four concrete poles are sunk into the bottom while another two are laid parallel on top of them. Three more poles are placed horizontally above the upper two poles. A pile of stones is then laid on top of the concrete floor (Fig. 4). A distance of 1.33 m is maintained between each set and each row (Table 5).

2. Culture on cement pole

This method is widely used in Surat Thani, Rayong and Chantaburi. Principal material is a 12–16 cm diameter and 35 cm long cement pole with a central cavity at the lower end for insertion on a bamboo or wooden pole. The cement cap is then placed over the bamboo pole. Poles are about 0.5 m in distance, and each row 2.15 m apart (Plate 1).

3. Bamboo stick culture method

One- to two-meter long bamboo sticks are driven 30–50 cm into the substrate (Plate 2) 15–42 cm apart. Rows are about 2–3 m apart.

Figure 3. Rock culture method.

Figure 4. Oyster rock and concrete pole culture method.

Plate 1. Oyster cement pole culture method.

Plate 2. Oyster bamboo culture method.

In Amphoe Bang Saphan, Prachuab Khiri Khan bamboo sticks are used to culture spat to marketable size. However in Surat Thani this method is used exclusively for spat collection while grow-out is by cement pole or the hanging method.

4. Cement pipe culture method

Some farmers believe that oyster spats prefer to settle on cement pipes rather than other substrates. Cement pipes offer a larger surface area for oysters to settle and grow. The pipes are 40–50 cm in diameter and 20–45 cm high. They are rested on 3 bamboo poles stacked in the bottom (Plate 3). The cement pipes are kept 10–15 cm above the bottom. Each set of pipes is 65 cm away from each other and each row 1.5–1.6 m apart.

5. Tray culture method

Tray culture method is commonly used in muddy bottom areas near the mouth of rivers where the water remains brackish all year round. Trays can be built in any convenient size, however a popular size is 80 × 100 cm with an upright side of 25 cm in height. The trays are often constructed from strong wood although bamboo can be used.
The tray is mounted and staked approximately 30 cm above the bottom (Fig. 5).
In Surat Thani the trays are used for stocking oyster in the farm before they are sold.
Pairoj (1986) reported that small oysters 6–7 months in age and 3.5–4.5 cm in length are removed from rocks and placed in the tray. It takes about 12 months to grow these oysters to marketable size.
Large oysters are placed in trays at about 3–4 months of age and 3–4 cm in length. The grow-out period for 7–8 month-old oysters is about 4 months prior attaining market size.

6. Hanging culture method

The hanging method is suitable for ongrowing large numbers of oysters. Plate 4 illustrates the method. Oyster shells (left shell) are attached with cement and a rope is passed through the centre of the shell to string the cultch together.
This method makes it easy to take care and harvest the oysters.

7. Other culture methods

In addition to the types of culture materials listed above, others include car tires, asbestos roof tiles, broken earthen pots, etc.

VIII. Spat collection

Oyster spats in Thailand are mostly collected from the wild or lured from the natural ground. Although spats can be produced from the hatchery in Prachuab Brackishwater Fisheries Station, production costs are. Spat collection methods differ according to species and location.

Plate 3. Oyster cement pipe culture method.

Plate 4. Oyster hanging culture method.

Figure 5. Oyster tray culture method.

Somkit (1987) reported the following observations:

1. In the eastern part of Thailand oyster farmers set the luring materials on 2 periods of the year:

   1. April to June in Chonburi and Rayong, and

   2. August to November in Rayong and Chantaburi (Fig. 6).

Two spatfall seasons can be detected in the east coast of Thailand: October - December and June - August (Fig. 8).

2. In Southern Thailand, the setting of luring material and spatfall depend on the season in each province.

In Prachuab Khiri Khan spat collectors are placed in the sea in November - January (Fig. 8) as the spatfall season occurs between November - March (Fig. 10). Timing is different in Chumpon and Surat Thani provinces (Fig. 7 and Fig. 9).

IX. Harvesting

C. commercialis are harvested by using hammers and sharp steel rods to pry them off the substrate. The harvesting periods vary from province to province.
In Chonburi there are two harvesting periods, from January to April and from November to December.
Chantaburi, Prachuab Khiri Khan, Chumpon and Rayong harvest on April-August, April-October, June-September and October-December, respectively. (Fig. 10).
The large species, C. lugubris and C. belcheri are removed from the substrate after culture for 18–24 months usually with a small ax. The harvest is stocked in the farm or sold immediately. Harvesting of large oysters is done almost the whole year round except in November-January because it is the monsoon season (Fig. 15).

X. Marketing and Handling

Oysters are mainly sold in the domestic market. The small species mostly pass from the collectors or middlemen, to the wholesalers, retailers and on to the consumers.
Marketing is controlled mostly by Chonburi middlemen. The middlemen and collectors from Chonburi collect oyster in Chantaburi, Prachuab Khiri Khan and Chumpon at different months from Chonburi, except in Rayong where harvesting is done all year round.

Figure 6. Percentage of farmers luring spat in the East part of Thailand.
(From: Survey, Somkit, 1987).

Figure 7. Percentage of farmers luring oyster spat in the South of Thailand.
(From: Survey, Somkit, 1987).

Figure 8. Spatfall season in the Eastern part of Thailand (1985).

Figure 9. Spatfall season in the Southern part of Thailand (1985).

Figure 10. Harvesting period of the small oyster Crassostrea commercialis (1985).
(From: Somkit Tugsinavisuiti, 1987).

Figure 11. Harvesting period of the large oyster, Crassostrea belcheri in Surat Thani (1985).
(From: Somkit Tugsinavisuiti, 1987).

XI. Handling of small oyster

The wholesaler removes the shell before sending the product to the retailer or restaurant. Shucked oysters are preferable mainly because it is difficult to open the small sized oyster.
With regard to the large species of oyster, the market price is good all year due to high demand. During the monsoon season, the price increases particularly in November - January. Thus the farmer usually tends to stock oyster near the guard house during these periods.
In handling the large species of oysters (C. belcheri and C. lugubris) the farmer will stock them in the farm near the guard house, or sell them immediately after harvesting. Consumers prefer to eat these species raw, with lemon, chili, garlic, salt and epilleaf. Oysters are transported in jute sac, which allows them to survive for 24–48 hours as long as they are not exposed to direct sunlight or hot air. The farm gate price of this species is about 5–6 Baht for medium size (10–11 cm in length) and 7–7.5 Baht for large size (11–13 cm in length).

XII. Constraints

Rabanal et al. (1977) reported that water pollution has caused oyster mortality in Thailand. Poaching and sedimentation are also believed to be major constraints to oyster culture in Surat Thani.

There are four main constraints to oyster culture in Thailand:

1. Water pollution. Most oyster farmers are facing this problem. In 1985 high mortalities of oyster in Chantaburi and Rayong were reported and related to environmental pollution.

2. Sedimentation. Culture areas are becoming shallower every year. In Surat Thani the oyster farmers are moving to deeper areas due to sedimentation in the old sites.

3. Lack of natural seed. Farmers in Chonburi, Prachuab Khiri Khan and Surat Thani complain that seeds are becoming less every year.

4. Poaching. It is a serious problem in Surat Thani.

XIII. References

Amartayakul, C. (1959). Shellfish of Thailand. Thai Fisheries Gazette. Department of Fisheries, Ministry of Agriculture and Cooperatives. Vol. 12. No. 4, p. 345–354.

Brohmanonda, P. (1962). Spawning Season of Small Oyster. Thai Fisheries Gazette. Department of Fisheries, Ministry of Agriculture and Cooperatives. Vol. 5, No. 3, p. 283–305.

Brohmanonda, P. (1967). Spawning Season and Abundance of Oyster at Leam Tan, Chonburi Province. Thai Fisheries Gazette. Department of Fisheries, Ministry of Agriculture and Cooperatives. Vol. 20, No. 1, p. 116–0132.

Brohmanonda, P. (1971–1972). Some Biological Aspects of Oyster at Klong Na tap Songkhla Province. Annual Report of Songkhla Marine Fisheries Station, Department of Fisheries, Ministry of Agriculture and Cooperatives 1971–1972, p. 107– 129.

Brohmanonda, P. et al. (1986). Oyster Culture in Thailand. Paper preparation for presentation at Marine Biological Centre, Phuket, Thailand. 27 pp.

Rabanal, H. R., Pongsuwana, U., Saraya, A. and W. Poocharean (1977). Shell Fisheries of Thailand: Background and Proposal for Development, SCS/77/WP/61 South China Sea Fisheries Development and Coordination Programme, Manila, Philippines 48 pp.

Tugsinavisuitti, S. (1987). An Economic Analysis of Oyster (Crassostrea sp. Farming in Thailand Asian Fisheries Social Science Research Network, Thailand. Department of Agricultural Economics. Faculty of Economics and Business Administration, Kasetsart University, Bangkhen, Bangkok, Thailand. 109 pp.