Shamsuddin Elias
Marine Fisheries Survey Management and Development Project
I. INTRODUCTION
Bangladesh has a coastline 480-km long and approximately 1 million hectares of territorial waters, and an abundant resource of marine bivalves. The major bivalve groups in order of abundance are clams, edible oysters, windowpane oysters and mussels. These are mainly distributed in the South Eastern coastal region of Bangladesh, more specifically in the Cox's Bazar coastal area. The present exploitation is far below the level of natural production and confined to wild stocks; bivalve culture is not commercially practiced.
Few Bangladeshi tribal people traditionally consume shellfish as part of their diet. The bivalves, mostly clams and oysters, are gathered from their natural habitats as no culture system has yet been introduced. Recently two oyster demonstration farms have been set up by the Department of Fisheries with the assistance of an FAO shellfish expert. Sarker et al., (1985) described the status and potentials of bivalve culture in Bangladesh, while Alam et al., (1985) reported preliminary observations on the culture of the blood cockle (Anadara granosa).
At the moment bivalves collected from natural fisheries are not depurated using standard techniques, although prior to their consumption they are held in clean seawater for 24 hours. Post harvest operations are carried out manually, and bivalves are sold to the local markets in fresh form soon after collection.
II. SPECIES COMPOSITION, DISTRIBUTION AND STATUS OF UTILIZATION
The major species of bivalves found in the coastal waters of Bangladesh and the location of the natural beds are shown in Table 1 and Figure 1.
Edible oysters. These are sedentary organisms that grow attached to rocks, gravel, tree roots or any hard object in the intertidal zone. During low tide they are exposed to air and therefore become inactive, however the gills are kept moist by the water retained in the mantle cavity. Most species of oysters can tolerate low salinities and are therefore found in brackishwaters in estuaries and marshes. In the Bagkkhali estuary they are commonly attached to boulders, concrete posts and sluice gates of shrimp ponds. In Moheshkhali and Teknaf oysters are abundantly found on rocks, bricks, sluice gates, bridge posts and mangrove roots. In St. Martins they abound on coral rocks around the entire island. Four species of edible oysters have been identified from various natural beds. They are: Crassostrea gryphoides, C. madrasensis, C. belcheri and Saccostrea cucullata. Up to the 1940's large oyster beds existed in the Bagkkhali river, however after the war their number declined partly due to the removal of boulders on which the oyster settled, the rapid siltation of the river bed, and also indiscriminate harvesting. It has been estimated that over 900 hectares are available for culturing oysters which could produce 100 MT per hectare. The succulent oyster meat should be promoted locally as human food and could also be exported if the sanitation standards required are met by the industry. Oyster shells, which are the only by-product, could be used for lime production or as an additive to poultry feed.
Table 1. Status of the marine bivalve fisheries in Bangladesh
| MAJOR BIVALVE GROUPS | Scientific name | Natural beds | Present mode of utilization | Estimated annual production (MT) | Prospect | Available culture grounds (ha) |
|---|---|---|---|---|---|---|
| Edible oysters | Crassostrea Madrasonsis Crassostrea belcheri Crassostrea gruphoides Saccostrea cuccullata | Cox's Bazar Moheshkhali Teknaf | Little amount of meat consumed by coastal people; some sold to Burma; shells used for lime | 50–70 | Culture for meat & shells | 900 |
| Windowpane oysters | Placuna placenta | Moheshkhali Cox's Bazar Teknaf | White pearls gathered and shells used in the craft industry | 50 | Culture for pearls & shells | 400 |
| Clams | Anadara granosa Anadara rhombea Meretrix sp. | Moheshkhali Cox's Bazar Teknaf | Shells are only used for making of lime | 80–100 | Culture for meat & shells | 400 |
| Mussels | Mytilus edulis | Teknaf Moheshkhali | Limited use of shells in the craft industry | 2–5 | Culture for meat & shells | Floating rafts in sea & estuaries |
Figure 1. Map of the coastal area of Bangladesh showing the distribution of important bivalve beds. O= oyster beds; ǒ = windowpane oyster beds; C= clam beds; U= mussel beds.
Windowpane oysters. Only one species (Placuna placenta) of this oyster has been identified in the waters around Cox's Bazar, Gotibhanga (Moheshkhali) and Teknaf. Adult specimens are found at depths of 0.5–10 m preferably covered by a thin layer of mud. The white pearls produced by this species have a commercial value. Wild specimens of windowpane oysters are collected by local people for the pearls and for the shells which are used in the handicraft industry. There is a bright prospect for culturing it, and extensive culture areas have been identified in canals located between Moheshkhali, Sonadia and Futkhali near Sahpuridwip.
Clams. Two species of cockles have been identified in Bangladesh. They are the blood cockle Anadara granosa and A. rhombea. Relatively large natural beds have been found along the Bagkkhali, Gotibanga, Chakaria and Teknaf estuaries. These species of clams typically live in muddy bottoms composed mostly of clay (65%) and silt (25%) usually 5–10 cm into the mud. The recorded values for salinity and temperature in natural beds have been in the range of 0–30 ppt and 20–30 °C respectively. Cockles are usually harvested at a premature size for the preparation of lime, while the flesh is totally discarded. The thick shell clam Meretrix sp. is also found and could be cultured in numerous areas along the Moheshkhali and Teknaf estuaries.
Mussels. Mytilus edulis has been identified in the estuary of the Naf river and in St. Martins. They are firmly attached to hard substratums with the aid of their byssus. This mussel species requires full strength seawater for culture and therefore raft culture could be introduced in the Teknaf and St. Martins estuaries.
III. CONCLUSION
Bangladesh has rich natural shellfish resources which are randomly exploited by coastal communities. Present level of demand is rather low and no sign of stock depletion is yet visible. However, due to the sedentary life style of these organisms, stock depletion can easily be caused if harvesting pressure increases. It is therefore recommended that strategies be developed for the judicious exploitation and utilization of this marine resource.
The Bay of Bengal coastline of Bangladesh has not reached any serious level of pollution and therefore local people tend to consume shellfish with minimum concern to sanitation measures. No significant sickness from shellfish poisoning has so far been recorded in the country.
Notwithstanding the above statements, the sanitary quality of bivalves is becoming a matter of concern. The uncertainty of the quality is believed to adversely affect sales. It is believed that the national market demand could be promoted through setting up of adequate harvesting and handling measures as well as sanitation and quality standards. Sanitation and quality control programmes are necessary due to the feeding mode of these organisms. Bivalves are filter feeders and therefore can concentrate and accumulate in their bodies pathogenic micro-organisms as well as pollutants particularly heavy metals.
In Bangladesh all post-harvest operations are carried out by hand. Steam is used to facilitate the opening of the bivalves (oysters, cockle, etc.) which are then consumed either cooked or pickled. Bivalves are popularly consumed by few tribal communities along the coast. In general the Bangladeshi people prefer other foods and oysters themselves are not widely consumed. In addition to human consumption, boiled cockle meat is now being used as feed for cultured shrimp. The low market price of bivalves in general is the result of the poor local market demand. Bivalves collected from the wild are not currently depurated and are usually sold fresh in the local market, immediately after collection from the natural beds.
Information exchange and interaction between scientists and personnel involved in bivalve culture, sanitation and marketing in the region should be encouraged. The many constraints affecting the bivalve culture industry in Bangladesh are due to the fact that it is still in the preliminary stages of development. Production data, trained personnel, market demand and sanitary regulation are limited, if they exist at all.
Some recommendation which could help promote the development of the mollusc industry include the following:
