A number of oyster species occurs in Indian waters (Rao, 1974); those that are commercially exploited are Crassostrea madrasensis, C. gryphoides, C. rivularis and Saccostrea cucullata. The first three species occur mainly in estuaries, backwaters and creeks and are all exploited to some extent by local fishermen. On the other hand S. cucullata, a purely marine form, is predominantly found in shallow areas with rocky substratum.
The native Indian oyster (C. madrasensis) occurs throughout the coast of India, whereas the other two Crassostrea species are restricted to the northwest coast regions and have not been reported so far anywhere on the east coast (Alagarswami and Narasimham, 1973). C. madrasensis dominates the entire east coast and Kerala. It is also found in various localities of Karnataka State. C. gryphoides is mainly present along the Maharashtra coast and in several localities of Goa State. C. rivularis occurs along the coast of Gujarat State and to a lesser extent along the coast of Maharashtra State. S. cucullata occurs all along the Indian coast, however only the settlements along Maharashtra and Gujarat coasts are large enough for exploitation (Table 4 and Fig. 7).
Oyster fishing grounds along the west coast include Dahanu creek, Jaytapur, Versova, Satpuri, Alibag, Boiser, Marve, Ratnagiri, Cuff Parade, Mahd, Utsali, Navapur, Aramra creek, Gagwa creek, Gomati, creek and Azad island. Small oyster beds exist in Kerala and Karnataka, but little exploitation occurs. Along the east coast, exploitation concentrates in the backwaters of Orissa, in Gokulapalli (Andhra Pradesh) and in Courtallayar and Adyar estuaries and Tuticorin (Tamil Nadu).
Oyster culture in India is not as well developed as in other Indo-Pacific countries, however it has received considerable interest within the last decade or so. The Central Marine Fisheries Research Institute (CMFRI) under the Indian Council of Agricultural Research (ICAR) is the main institution involved with bivalve culture. Through applied research aimed at developing suitable and low-cost culture methods and through training and demonstration courses they are trying to popularize oyster farming.
Small scale bottom culture of oysters by transplanting the spat from the natural beds to shallow areas of convenience has been practised for some time along the west coast. However, only recently have extensive oyster culture trial been executed by a number of scientists. Oyster culture at Tuticorin (Nayar and Mahadevan, 1983) and some experimental culture carried out at Athankarai estuary (Rao et al., 1983), Bheemunipatnam backwaters, Andhra Pradesh (Ruben et al., 1983), Mulki estuary (Joseph and Joseph, 1983), Goa (Parulekar et al., 1983) and Cochin backwaters (Purushan et al., 1983) indicated good prospects for oyster farming along the Indian coasts. Oysters are cultured in intertidal regions, bays and estuaries and available data indicate that many of these environments in the country are suitable for oyster farming. Apart from bottom culture techniques the rack method has been developed and successfully practised at Tuticorin. This method can be profitably employed in shallow coastal waters at several locations along the Indian coast. Each rack is composed of two rows of six poles (2.4 m in length) driven into the muddy bottom at an interval of 2 m apart. Each set of six poles are fixed together by a long pole placed horizontally on top of them; the two rows are connected to one another by a series of short poles placed horizontally between the two long poles. This rack system, which covers an area of 25 m3, constitutes a suitable platform for suspending the oyster trays. Each rack can accommodate 20 rectangular trays which have a holding capacity of 3,000–4,000 oysters (Nayar, K.N., 1987)
Although oyster spawning tends to occur throughout most of the year, it has been reported that C. madrasensis has two peak spawning seasons, April-May and August-September. A variety of techniques and cultch materials have been tested in India (Muthiah, 1987). Cultches tested were oyster shell rens, coconut shells, lime coated asbestos sheets, mussel shells, Velon screen and polythene liner sheets, PVC tubes and lime coated tiles. Among the above coated materials, tiles are preferred due to the high setting rate, although more expensive than oyster and coconut shells.
A new dimension has been given to oyster culture by the successful production of seed of the C. madrasensis at the Tuticorin Research Centre hatchery (Nayar et al., 1984). The development of hatchery techniques for the mass production of oyster seed is certainly a major breakthrough as considerable quantities of oyster seed can be supplied all year round for culture. Conditioning and spawning of the oyster broodstock have been successfully accomplished. Conditioning is carried out by supplementing the diet of the adult oyster with a variety of cultured phytoplankton species such as Chaetoceros affinis, Skeletonema costatum, Thalassiosira subtilis, Isochrysis galbana and Chlorella salina (Nayar et al., 1987). Spawning has been accomplished by thermal stimulation of the conditioned oysters.
The interest in oyster culture is relatively recent and although production-orientated techniques have been developed, commercial culture is still at an infant stage. One major constraint is that bivalves are not a popular food item and are consumed by small communities along the coast. The low demand results in low prices. In some cases, adequate economic returns on investments cannot be assured except by setting a price slightly higher than that of non-cultured bivalves. The integrated development of increased production, increased consumer demand, and a market strategy for molluscan products is required. To further stimulate oyster culture among coastal people extension activities are essential. The CMFRI has organized several training programmes in bivalve culture, but it is generally felt that those efforts are localized and are not sufficient (Nayar and Mahadevan, 1987).
Availability of oyster seed is essential for large-scale production. For this reason the Tuticorin Research Centre hatchery was constructed. Although this hatchery is currently producing spat of C. madrasensis it is recognised that research activities are required to further improve production techniques. Other major problems are related to post-harvest handling and quality control. Because of several cases of contaminated bivalves appropriate depuration and other sanitary measures should be taken to make bivalves safe for human consumption before they are marketed. Cultured bivalves and those meant for export undergo depuration, however this practice should also be extended to bivalves collected from the wild, especially if harvested from areas not considered safe.
It appears that the of the oyster resources potential of maritime
States other than Tamil Nadu and Karnataka have not been properly
assessed. The CMFRI has recognised the need to undertake a
programme of resource survey in the coastal States and Union
Territories (James, 1987).
Another area for future investigations is the identification of suitable areas and sites for undertaking extensive culture of edible oysters. The advances made on oyster culture technology have stimulated considerable interest among several entrepreneurs and agencies in States like Kerala, Goa and Maharashtra. Unfortunately, precise information on the suitability of sites in these States is lacking.
Future research priorities include the development of culture technologies suitable for each geographical and topographical zone along the Indian coast.
The development of oyster culture activities also depends on finding new markets. Initial national market survey indicates a growing demand in the country (James, 1987). The export market for oyster is known, however, the quality of the product in competition with other exporting countries would be a deciding factor.
Table 4. Distribution and main culture areas of commercially important oyster species in India.
|Crassostrea madrasensis||West Bengal||Little information available.|
|Orissa||Bahuda estuary||Approximate area of 5 ha. Three distinct beds observed.|
|Andhra Pradesh||Sarada estuary Bhimunipatnam||Oyster beds reported. Oyster beds subjected to annual depredation due to fresh water influx.|
|Upputeru canal||Approximate area of 2.25 ha of rich oyster beds.|
|Godavari and Krishna estuaries Gokulapalli||Low density beds. Rich oyster beds regularly exploited.|
|Tamil Nadu||Pulicat Lake||Rich and extensive oyster beds regularly exploited.|
|Courtallayar and Adyar estuaries||Approximate area of 50 ha in each locality. Regular fishing occurs.|
|Mudasodai and Chinnavaykal Muthupet swamp Vaigai estuary||Small areas. Limited exploitation. Patchy settlements. Approximate area of 2 ha. No exploitation has been reported.|
|Tuticorin||Approximate area of 20 ha. Exploitation occurs.|
|Tamparaparni estuary||Approximate area of 2.5 ha. No exploitation has been reported.|
|Kerala||Ashtamudi Lake||Approximate area of 5 ha. Oysters are sparsely distributed & exploited.|
|Anchengo backwaters||Highly populated oyster beds.|
|Karnataka||Nethravathi, Kali & Sharavathi estuaries Venkatpur, Bhatkal, Mulky, Uppunda and Coondapoor estuaries||Oyster beds of limited extent.|
|Oyster beds of same extent. Regular exploitation occurs.|
|Crassostrea gryphoides||Maharashtra||Dahanu creek, Mahim creek, Purnagab, Malwan, Alibag, Palghar, Satpuri, Boiser, Malad, Worli, Ratnagiri, Jaytapur, Versova, Gobbunder, Cuff Parade, Marve, Madh and Bandra||Oyster beds of same extent. Regular exploitation occurs.|
|Utsali, Navapur and Kelwa||Oyster beds of same extent. Bottom culture is practiced.|
|Goa||Ribanden, Siolim and Curca||Oyster settlements reported.|
|Crassostrea rivularis||Gujarat||Aramra, Poshetra, Port Okha, Porbander, Sikka, Gagwa creek, Singach creek, Beet Kada, Khanara creek, Laku Point, Gomati creek, Navibander, Harsad, Balapur and Azad island||Oyster beds of same extent. Regular exploitation occurs.|
|Maharashtra||Mahim, Ratnagiri and Jaytapur areas||This species is found along with C. gryphoides.|
|Saccostrea cucullata||Maharashtra||All along the coast||Oyster beds of same extent. Regular exploitation occurs.|
|Gujarat||All along the coast||Oyster beds of same extent. Regular exploitation occurs.|
Figure 7: Major farming areas of the four commercially important oyster species in India.