3.1 Background
3.2 Research needs
3.3 Research approach
Brackishwater fish culture is still in the early stages of development in Africa. The present practices are based on the availability of natural fry and often several species are either purposely stocked or they gain entry with the tidal water, in spite of preventive measures. The more common species in brackishwater ponds are mullets, tilapia and catfish. The combination of species is only incidental and not actually based on their compatibility in terms of the use of food available or other habits. In so far as tilapia are concerned, the problem of prolific breeding, over-population and stunted growth occurs at least when all available species gain entry. Mullet fry are generally available in required numbers, but the growth and production achieved so far are not high. This is largely due to the nature of the soil on which brackishwater ponds are built. Coastal swamps and mangrove areas used for pond construction generally have peaty acid soils. Besides the difficulties of building water-tight dikes with peaty soils, serious problems of maintaining favourable pH of water and soil for the growth of benthic as well as planktonic fish food organisms, are faced during the first few years of pond operation. Till the toxic salts are leached out from the soil and its texture improved by the deposition of silt brought in by high tides, which may take as much as two or three years, the growth of adequate quantities of natural food cannot be relied upon for high density culture of mullets or Other fish.
Different species of tilapia are cultivated in brackish water in several areas. Initial culture experiments with the catfish, (Chrysichthys) have given very encouraging results when fed with artificial food. It would appear that a high production cannot be expected with only natural food.
The economic viability of brackishwater pond culture, as freshwater pond culture, has yet to be convincingly demonstrated, and future work has necessarily to be focussed on this requirement.
In view of the above factors, the task force recommends that initial studies should concentrate on monoculture of three species, viz., Tilapia melanopleura, T. nilotica and Chrysichthys nigrodigitatus.
The problems of culturing tilapia in brackishwater ponds are essentially the same as in freshwater ponds. It is known that salinity would affect the breeding of T. nilotica, but the effect of different salinities on breeding has not yet been experimentally verified. If separate breeding and fry production facilities with supplies of fresh water are developed, this will not become a major constraint. Whether salinity will affect growth' rates of these species also has yet to be determined.
Although the culture of Chrysichthys has been attempted in a number of places in West Africa, very little scientific research has been done on this. C. nigrodigitatus, which is the more desirable species for culture, is a brackishwater species, which migrates up-river to freshwater areas for breeding during the rainy season (generally from July to September). This species seems to have only one restricted breeding season, unlike the other two slow-growing species of the genus (C. walkeri and C. auratus) which breed throughout the year. Even though the fish would attain up to the fourth stage of maturity in ponds they have not been observed to breed naturally in such water or in brackish water. Preliminary experiments show fast growth and the possibility of high production, when fed with artificial feeds such as oil cakes.
The main constraint to the farming of this species appears to be the scarcity of fry in adequate numbers. It is difficult to find enough fry in natural habitats. Induced breeding and larval rearing techniques have yet to be developed. Similarly nutritionally and economically suitable feeds have to be developed for larvae, fry and adults. Though the fish grows relatively fast, the growth and production that can be obtained in high density culture in short growing periods, have yet to be determined. In fact the whole farming system remains to be developed. The need for demonstrating economic viability applies to Chrysichthys culture also, even though it is a high-priced fish and there appears to be no major difficulties in growing them to a uniform marketable size with artificial feeding.
In addition to the problems related to the species cultured, there is also the inherent problem of design and management of brackishwater ponds built on mangrove soil. Methods of controlling the acidity and leaching out of toxic salts from the soil have not yet been adequately developed.
The research needs for tilapia culture in brackish water are essentially the same as for their culture in fresh water and has already been described on page 3. In addition to these it will be necessary to determine the effect of salinity on the breeding of the two species selected and whether freshwater hatcheries will be needed for the production of fry for brackishwater culture.
Other factors that may need some special attention are the entry of extraneous species into the ponds and the maintenance of optimum salinity.
In regard to the culture of Chrysichthys, the following research needs were identified:
(i) Study of maturation and breeding;
(ii) development of brood-stock rearing methods and induced breeding;
(iii) hatchery systems and rearing of larvae and fry;
(iv) nutritional studies and development of suitable feeds for larvae, fry and adults;
(v) determination of stocking rates, feeding regimes, growth and yield in production ponds;
(vi) diseases and. disease control;
(vii) economic evaluation of culture system.
Of equal importance to both Tilapia and Chrysichthys culture in brackish water would be the design of ponds, methods of leaching pond soils and the control of pH of pond water.
As regards tilapia culture in brackish water, besides the research described under freshwater pond culture, the effect of salinity has to be studied. This can be tested in the laboratory and based on the results obtained, breeding, larval rearing and production experiments should be carried out, using different salinity ranges as a major variable.
In research on Chrysichthys culture, field observations along with laboratory examinations would provide the basic information on maturity and breeding seasons. Induced breeding experiments have to be carried out with fish pituitaries as well as mammalian and synthetic hormones, to determine their relative efficiencies and the dosages to be used. Suitable hatching and larval rearing techniques have to be evolved by trying the methods already adopted in catfish hatcheries and making modifications found necessary.
The approach to be adopted for nutritional studies and feed preparation and testing, would be basically the same as for Tilapia and Clarias. A basic formulation may have to be used initially and necessary modifications incorporated when more precise information on nutritional needs and feed conversion ratios become available.
Well-planned production experiments using different stocking rates, feeds and feeding procedures and pond management, including salinity and pH control and growing periods, should be carried out. Full evaluation of cost-benefit ratios will be necessary to determine the most economic farming methods. Even though the species can probably be bred only once a year, it may be possible to obtain more than one crop a year by stock manipulation. Partial cropping from a very densely stocked pond, say after four or five months, may give the first crop of fish or minimum marketable size, and later in about ten months a second crop of larger fish. This possibility and related economics should be studied.
The introduction of improved designs of brackishwater pond farms should be considered. Improvement of the texture and acidity of the peaty soils of fish ponds built in mangrove swamps of ten takes two or three years. This period can probably be reduced by regular leaching of the soil with tidal and rain water. Suitable deep trenches may be built on the pond bottom to facilitate the leaching and draining of ponds. This, combined with liming should be tried to accelerate the improvement of pond soils. Inexpensive methods of consolidating pond dikes, and the prevention of pH changes of pond water due to rain water washing down the acidic soils of dikes, should be developed.
