Thailand is richly endowed with extensive coastal areas which are suitable for shellfish production. Of the 360 000 littoral and sublittoral mudflats, about 300 000 are mangrove areas while over 60 000 are clear mudflats which can be good shellfish grounds. The Gulf of Thailand including the Inner Gulf and the outer areas have scattered areas which are well-known shellfish grounds. Likewise, in the Andaman Sea coast, there are extensive sites that are hardly exploited for their shellfish resources. In varying extent, the different coastal provinces have existing as well as potential shellfish grounds.
In 1971, the peak year for shellfish production which was perhaps mainly from natural beds, the major production provinces as gauged from their production in tons were as follows (Table 3): Samut Sakhorn (79 500), Samut Songkhram (64 200), Cholburi (50 000), Petchaburi (29 900), Samut Prakharn (26 700), Chachoengsao (8 200), Surat Thani (7 600), Trat (4 000), and Chanthaburi (500). In 1976, when culture practice has already started and certain beds were reduced due to pollution, there was a slight shift in the production areas. In that year (1976), the major production beds were in Cholburi (1 000 ha), Chanthaburi (800 ha), Chachoengsao (700 ha), Samut Sakhorn and Samut Songkhram (500 ha each), Petchaburi (400 ha), Satul (300 ha), Trat (206 ha), Samut Prakharn (200 ha), and Ranong (100 ha) (Table 9). In general shellfish farming is being done in small farms averaging 0.6 hectares per farm in 1973; 0.7 hectares in 1974; and 1.3 hectares in 1975 (Table 4).
For potential shellfish areas, the main provinces where expansion sites are highest, with estimated areas are: Trat (16 300 ha), Pangnga (10 000 ha), Chanthaburi (7 200 ha), Ranong (6 000 ha), Surat Thani (4 900 ha), Samut Sakhorn (3 300 ha), Samut Prakharn (3 200 ha), Samut Songkhram, Petchaburi and Nakhornsrithamaraj (2 100 ha each), Satul (1 500 ha), Trang and Rayong (1 000 ha each), Chumporn (800 ha), Chachoengsao and Narathiwat (500 ha each), Pattani (400 ha), Cholburi and Krabi (200 ha each) and Prachuap Khirikhan and Songkhla (100 ha each) (Table 9).
The different species of economic shellfish require slightly different habitats for existence. The green mussel can survive in soft, muddy bottom in waters of about 2 to 8 metres. The horse mussel can also survive in soft, muddy bottoms but in shallower waters with depths usually less than 3 metres as well as in mudflats that may be exposed during the lower low tides. In general, mussel can survive in waters with lower or brackishwater salinities and can therefore exist in estuarine areas.
The major areas where green mussel is presently produced, arranged in importance, are: Chachoengsao, Cholburi, Samut Sakhorn, Samut Songkhram, Samut Prakharn and Petchaburi. Potential grounds for this species are extensive in Trat, Pangnga, Chanthaburi, Ranong, Surat Thani, Samut Prakharn, Petchaburi, Samut Sakhorn, Samut Songkhram, Nakhornsrithamaraj, Chumporn and Trang (Table 9 and Fig. 3).
Horse mussel is produced mainly in Cholburi, but potential areas for this species exist in Pangnga, Samut Sakhorn, Ranong, Samut Songkhram, Nakhornsrithamaraj, Chachoengsao, Samut Prakharn and Surat Thani (Table 9 and Fig. 4).
The ark shell or cockle require harder bottoms with good mixture of sand for their existence and they are always confined to the bottom. It is noted that they are either spawned in the same site or they have definite nursery areas separate from their growing beds. It is apparent that the cockles require relatively high or marine salinities so that they are not common in areas affected by freshets.
Cockles are presently produced in two provinces, Petchaburi and Satul. Potential areas for this species are located in Chanthaburi, Ranong, Pangnga, Samut Songkhram, Surat Thani, Satul, Trat, Samut Prakharn, Nakhornsrithamaraj, Samut Songkhram, Chachoengsao, Chumporn and Trang (Table 9 and Fig. 5).
Oysters require a hard substratum for attachment and can grow on wooden, stone or rock substratum. They could also exist in hard, sandy or sandy-muddy stable bottoms. Their culture in Thailand is presently very confined in medium-sized areas mainly in the provinces of Trat, Cholburi and Ranong and in small patches in the provinces of Surat Thani, Chumporn, Pattani, Prachuap Khirikhan and Songkhla. However, potential areas for this species are found in Trat, Chanthaburi, Rayong, Pangnga, Narathiwat, Ranong, Trang, Satul, Pattani, Nakhornsrithamaraj, Surat Thani, Krabi, Chumporn, Cholburi, Petchaburi, Prachuap Khirikhan and Songkhla (Table 9 and Fig. 6).
The Inner Gulf of Thailand, mainly those within the provinces of Samut Sakhorn, Bangkok, Chachoengsao, Samut Prakharn, Petchaburi and Cholburi used to be very suitable grounds for the natural growth of shellfish and also for culture operations. The rapid growth of population in coastal communities and the development of industries have decimated a large portion of these sites for shellfish production.
According to recent reports there are at least 1 500 individual factories of various kinds scattered along the coast of Thailand. The main outlets of wastes from these factories are the four main rivers that flow into the Inner Gulf; these are Chao Phraya, Ta Chin, Maeklong and Pran rivers (Fig. 1). The areas along the coast seems to be the most affected. The factories are varied and include distilleries, fruit canneries, fish sauce plants, slaughter houses, petroleum refineries, sugar refineries, tapioca starch mills, textile mills, sugar centrals, lumber sawmills, veneer factories, matress factories, match factories, noodle factories, fishmeal plants, etc. Besides the possibility of harmful chemicals from some of the factory effluents, heavy load of organic matter reduce the biological oxygen demand (BOD) of the water, besides the accompanying excessive silting and sedimentation which are harmful to shellfish.
As evidence of the deleterious effects of pollution and related problems along the coast of the country, many maritime provinces which used to be the major production grounds for some species have practically disappeared. For instance, Chachoengsao province which produced about 8 000 tons of green mussel in 1971 dropped to 800 in 1974; Cholburi from 37 000 in 1971 to 3 000 in 1974; Petchaburi 18 000 to 4 000; Samut Prakharn 19 000 to 700; Samut Sakhorn 61 000 to 500; Samut Songkhram 63 000 to 2 000 (Table 5).
In the case of the horse mussel, the production of Samut Sakhorn of 13 000 and Samut Prakharn of 7 000 and Chachoengsao of 500 practically disappeared in 1974. Likewise, with the cockles, the production of 2 800 in Samut Sakhorn disappeared in 1974 and that of Petchaburi of 7 000 went down to 700 in 1974 (Table 6).
For cockles the areas which afforded fairly good production in 1971 and 1972 did not produce any in 1973 and 1974, like Samut Songkhram which produced about 1 000 to 2 000 tons in 1971–1973 only had about 700 tons in 1974 (Table 7).
In the case of oysters, there has been no decline since oyster production is mainly through culture and culture operations gradually increased using selected areas in the Inner Gulf that were not heavily affected by human population and industrial development (Table 8).
