4.1 Background
4.2 Research needs
4.3 Research approach
The most common oysters on the West African Coast are the mangrove oysters Crassostrea gasar and C. tulipa. It is not yet clear whether they are different species or belong to the same species, showing only local variations in taxonomic characters. They occur abundantly on mangrove roots and stems and the local populations collect and market them in most coastal areas. Owing to intensive exploitation and crowding of oysters on natural substrates, the size of oysters available in most markets is extremely small and consequently the prices obtained are also low.
Attempts have been made in the past to culture the mangrove oysters, using some of the well-known culture techniques. Recent work with the species described as C. tulipa in Sierra Leone has given very encouraging results. Spat collected from nature and grown on rafts produced oysters of 7-10 cm height in seven to eight months. This is about four times the size of wild oysters ordinarily marketed. Preliminary experiments in tray culture of C. gasar in Nigeria have shown a growth of 4-5 cm in size and 30 g in weight, in six to seven months. In raft culture faster growth rates were obtained, but mortalities due probably to predation or sudden changes of salinity, were observed to occur frequently.
Observations made in Nigeria and Sierra Leone show that spat can be collected from nature almost throughout the year, with two peak periods, viz., February to April/May and November to December, coinciding with periods of high salinity. The maximum spat settlement was observed at salinities around 20 ppt. Satisfactory growth of oysters was found within the salinity range of 12-25 ppt.
Studies made so far clearly indicate the potential, and indeed the need for culturing the mangrove oysters. Small-scale farming of these oysters can be developed as a means of increasing the income and nutrition of coastal populations. Attempts to introduce exotic species of oysters have not yet been successful, except on the North African Coast. In view of this, it is recommended that the Centre carry out investigations on the locally available C. gasar, which is probably synonymous with C. tulipa described elsewhere on the West African Coast. While raft culture would be feasible in deeper areas, it may be necessary to adopt rack culture in trays in shallow waters.
(i) Definition of areas suited for raft and rack culture of oysters;(ii) identification and mapping of spat collection areas and development of spat collection methods;
(iii) selection of substrates for hanging culture; design of rafts and hanging structures for raft culture, including methods of preventing predation and poaching;
(iv) development of raft culture procedures;
(v) design Of racks and selection of trays for rack culture;
(vi) development of rack culture procedures;
(vii) economic evaluation and comparison of rack and raft culture techniques;
(viii) monitoring of oyster diseases and contamination;
(ix) development of methods of cleaning oysters for marketing.
Based on available data, supplemented by field studies on Salinity range, tidal range, water depth, currents, water quality, phytoplankton abundance, and safety, it should be possible to identify and describe the areas in the brackishwater zone of the Niger Delta. Spat collection grounds can be easily mapped out on the basis of observations on natural spat settlement and experiments in spat collection. Spat collection experiments should be carried out throughout the year using different types of spat collectors, so that the best substrates for collection and rearing can be determined. In the design of rafts and hanging structures, cost, durability and safety should be taken into account. For prevention of predation, some of the techniques already adopted elsewhere, like net covers, should be used. The same considerations would apply also for rack culture. If diseases are observed, necessary diagnostic and treatment measures should be developed.
Economics of different methods of culture should be compared, with special emphasis on the size of operations needed for a farmer to obtain a reasonable income. The income now obtained by the collection of wild oysters should also be used for comparison.
Besides health, the general condition of oysters and their acceptability for marketing should be studied. In order to meet public health requirements, possible contamination of oysters in the culture grounds should be studied. Although locally the oysters are likely to be consumed only after cooking, cleansing procedures may have to be developed for widening the market, when the technology has been sufficiently developed and the economic aspects determined.
