2.1 Tilapia culture in freshwater ponds
2.2 Clarias culture in freshwater ponds
2.1.1 Background
2.1.2 Research needs
2.1.3 Research approach
Being native to Africa and widely distributed throughout the continent, tilapia have been the most commonly cultured species in the region for the last three decades. The first experiments were made with T. macrochir and T. rendalli, collected from the Luapula-Mweru (1946-47) and reared in south-eastern Zaire. From there tilapia culture rapidly spread into western and eastern Africa, using also other species.
Although more than 90 species of tilapia have been recorded, only four species, shown below, are commonly cultured in Africa on a relatively large scale:
|
Group |
Species |
Biological Characteristics |
|
Tilapia |
T. melanopleura1/ |
Macrophagous herbivores; spawn on substrates |
|
Sarotherodon |
T. nilotica |
Microphagous omnivores; incubate eggs and hold larvae in the mouth of female parent |
1/ This species is described by some as T. guineensis
Experience so far has clearly shown the importance of the choice of species for culture. The best results have been obtained with T. nilotica which readily feeds on plankton and on various artificial feeds, grows well and can provide high yields under pond culture. T. melanopleura has also been popular because it feeds on higher vegetation and grows relatively well, yielding high production under intensive culture. T. mossambica and T. macrochir have slower growth rates. They reproduce when young and small, causing retarded growth, overpopulation and consequently stunted populations in ponds. On the basis Of experience gained, T. nilotica has been identified in most African countries as the most suitable species for intensive culture in freshwater ponds. The culture of this tilapia should therefore be given the highest priority for research at the Centre. As T. melanopleura also gives satisfactory results and probably has certain advantages over T. nilotica in brackish water, this species should also be used for studies at the Centre.
To produce tilapia in ponds as an economic crop one has two basic options, either culture without sex separation or the culture of all-male (or predominantly male) populations.
Failure of tilapia culture in the past, has often been due to uncontrolled spawning, producing large numbers of fry and stunted populations. It has been shown in experiments and limited farm trials that this can be overcome in two ways:
(i) appropriate management which enhances the growth of fingerlings in order to attain a commercially acceptable size in as short a period of time as possible, and preferably before spawning occurs. For this to be successful, the pond should be completely drained after each cropping. It should then be treated, so as to destroy any fish left. The stocking of the pond should be done with healthy fingerlings of one age class, and intensive feeding should be done with a suitable protein-rich diet. The selection of strains with fast growth rate and late maturity would obviously contribute greatly to the success of this system;(ii) the addition of a predator species to control the population by feeding on fry and fingerlings. Predator species such as Lates niloticus, Hemichromis fasciatus and Clarias lazera have been used for this purpose in research stations. In most cases, the results have been inconclusive. The application of this method outside the research Stations has rarely been successful. Observations on tilapia-predator relationship show that it is rather unclear. Stocking and size ratios may affect results very considerably. In general, it appears that the adoption of this method is unlikely to be successful, unless combined with some other method of reproduction control.
Monosex culture can resolve the problem of uncontrolled reproduction in rearing ponds. Since the males have higher growth rates than the females, production of larger fish and higher yields are possible when all-male stocks are grown. All-male tilapia populations can be produced in one of three ways:
(i) manual sexing: skilled personnel and relatively large-size fingerlings (50-80 g) are required for separating sexes manually. It is time-consuming and often results in a loss of 40-50 percent Of the fingerling production, due to the discarding Of females. Errors can easily be made even by skilled workers;(ii) monosex male hybrids can be produced by the crossing of some tilapia species, but not all crosses will produce 100 percent males. The cross between T. hornorum males and T. nilotica females is reported to give all-male offspring, although some females may also be produced if the brood stock is not of pure line. The F1 generation, being fertile, may backcross with the female parent fish in rearing ponds and produce fry of both sexes. Culture of hybrids has not yet been adopted for farming to any appreciable extent in Africa. Several problems have been encountered in producing hybrids, such as maintaining the purity of brood stocks and the limited fecundity of parent fish which restricts fry production. Difficulties in preventing the entry of wild tilapia into culture ponds has also been one of the other major constraints;
(iii) sex reversal techniques have been adopted with some success. Feeding hormones (e.g., methyltestosterone) to the larvae between the second and sixth Week after hatching has been observed to reverse females into males. Though reported to be successful in experimental work, this technique has yet to be tried in pilot- or large-scale farming operations. Larvae have to be given specially prepared artificial feeds containing the hormone for about six weeks, excluding all natural food. The presence of hormone residue in adult fish has not yet been studied.
Of the three techniques described above for controlling tilapia reproduction in the production ponds, the last two seem the most promising, although not yet adequately developed and immediately applicable for commercial production. Since hybridization for the production of all-male tilapia will be a major item of research at the Latin American Regional Aqua-culture Centre in Brazil, with which this Centre will establish close liaison, it is recommended that research on these lines be considered only during the second phase of its research programme, if the results of work done in Brazil warrant it. The Centre should concentrate on detailed investigation on sex reversal.
Recent experience in a pilot-scale farm in the Ivory Coast indicates the relative efficiency of combining sexing with the use of predators, for successful control of reproduction. The production cycle in the farm is based on three separate age-classes, fry (5 g), fingerling (25-30 g), and adult fish (250 +g). The fingerlings are sexed manually by trained workers and only males are retained. The sexing error is estimated to be about 10 percent. To control the reproduction of residual females in the population, a small predator (10-50 g H. fasciatus) is used in the production ponds at a low 'stocking rate (3-10 percent of the weight of tilapia stocked). The predator may be re-used several times, as its growth is relatively slow and the size rarely exceeds 100 g. It can be bred easily in small ponds. This method deserves to be investigated further through critical experiments using either H. fasciatus or C. lazera as possible fry predators. Mechanical means of size grading for separation of sexes may facilitate the adoption of this method in large-scale farming.
Although wild reproduction has been the major problem in tilapia culture, in actual practice the scarcity of fry and fingerlings in sufficient numbers has, somewhat strangely, been also a major constraint, particularly in small-scale rural farming. Mass production of fry and fingerlings cannot yet be considered a well-developed technique. Methods of brood stock selection and rearing, design and preparation of spawning ponds, breeding, larval rearing and nursery management, need to be developed. The most efficient sex ratio to be used for breeding has yet to be determined for each species. It has been reported that a ratio of 1 male to 2-3 female T. nilotica is the most appropriate. Recent evidence, however, suggests that a higher number of females (6-10) would increase the number of hatchlings in the breeding ponds. Another aspect of interest is the reported effect of water temperature on the sex ration of progeny. In the Ivory Coast, it has been observed in very shallow ponds (30-50 cm) - and consequently relatively warm water - that the proportion of males among the progeny of T. nilotica could be as high as 70-80 percent. If this is proved to be true, the design of breeding ponds could be modified to attain predominantly male population of fry for more productive farming. The importance of intensive feeding with nutritionally adequate feeds to obtain rapid growth of tilapia to marketable size in a short period of time, has already been indicated. Experience so far shows that it will be extremely difficult to obtain such growth rates through the production of natural foods by fertilization in ponds. A variety of feedstuffs have been used in pond culture of tilapia, including cassava and brewery wastes can be fed to them from demand feeders, minimizing loss of feeds. This provides some basis for future work on the formulation of efficient and economic feeds.
Based on a review of the state of technologies relating to the culture of T. nilotica and T. melanopleura in freshwater ponds, the following research needs were identified:
(i) development of suitable methods of brood stock selection and management, including design of spawning ponds or tanks;(ii) determination of appropriate ratios of male to female pond brood fish in spawning ponds, and the influence of water depth and temperature on the sex ratio of progenies;
(iii) development of suitable hatchery systems; hatchery production of fry;
(iv) testing and standardization of sex reversal, using hormone-fortified feeds; monitoring of hormone residue in adult fish;
(v) development of easy and practical methods of sexing fry/fingerlings, including mechanical graders;
(vi) determination of the efficiency of controlling over-population in tilapia ponds by the use of selected predators (Hemichromis fasciatus or possibly Clarias lazera);
(vii) study of digestion physiology, with special reference to the production and character of digestive enzymes, in order to determine response to dietary patterns;
(viii) studies on nutritional requirements of different life stages and age groups;
(ix) formulation, preparation and testing of suitable feeds for larvae, fry, fingerlings and adult fish, based on locally available ingredients and testing their effectiveness in terms of cost and fish production;
(x) development and testing of suitable feed dispensers;
(xi) determination of the feasibility of producing marketable tilapia as a short season crop by intensive feeding;
(xii) economic evaluation of different culture practices and the determination of the most profitable methods of tilapia culture in ponds.
A review of the research needed will clearly show that a systems approach is essential to obtain results that can be readily applied in farming. Almost all the studies need experimental work. However, each of the investigations would not need separate experiments. Many of the factors involved are inter-dependent and so through factorially designed experiments, it should be possible to study a number of parameters simultaneously. Monitoring of water conditions, including temperature, dissolved oxygen, pH, salinity if applicable, fish health, growth and behaviour, will have to be done on a routine basis to enable proper interpretation of experimental results.
In view of the observed differences in the growth potential, fecundity and breeding characteristics of tilapia populations from different environments, it is necessary to obtain brood fish from as many representative locations as possible and breed pure lines in order to describe their characteristics and evaluate the extent of variability of observed characters. It would be useful to segregate sexes and rear them separately on special feeds. The effect of such rearing methods on the breeding behaviour of the fish has to be observed.
As it is believed that the development of separate hatcheries, either on a centralized basis or at the farm level, would contribute to the availability of good quality fry and fingerlings for farming operations, suitable spawning and hatchery facilities have to be developed. Spawning ponds or tanks should be designed based on the breeding habits of the fish; the differences in the habits of T. nilotica and T. melanopleura make it necessary to have different designs for their spawning. A simple hatchery design can be developed for mass production of larvae.
Breeding experiments can easily be designed to test the hypothesis that water depth or consequent temperature regime in the spawning ponds would have a determinant role on the sex ratio of progenies. This could be further verified through laboratory-scale experiments where water temperature and other conditions can be controlled.
The efficiency of larval feeding with cultured zoo-planktonic organisms and artificial feeds will have to be compared. It is believed that a larval diet based on whole eggs, dispensed in the form of minute globules of yolk encased in albumin, would form an efficient and economic feed for larvae. This has to be tested under laboratory and field conditions.
The study on sex-reversal can be done largely through laboratory-scale experiments based on work already done in the Philippines, using methyltestosterone-fortified feeds. If found successful, pilot-scale production experiments will have to be undertaken, incorporating observations on the growth and behaviour of the treated stock and residue of the drug in adult fish. Comparative experiments may be required to determine the best dosage of hormones and feeding procedures for sex-reversal.
Although the development of a sex-reversal technique can render other methods of reproduction control in tilapia ponds unnecessary, parallel studies of other procedures should be undertaken because (a) one cannot be sure of the results of sex reversal studies and (b) even when found to be feasible, other methods may have to be adopted in rural areas where centralized fry production is not developed. It will be useful to try different methods of manual sexing to separate females and males in fry and fingerling stages. Mechanical grading by size is believed to be possible as males of a given age are larger, and the small percentage that cannot be separated can be either discarded or grown in separate ponds along with an adequate number of predators. Sexing procedures, as well as the efficiency of selected predators to control populations of tilapia fry in ponds, will have to be determined through production experiments. The species that are recommended to be tried as predators are Hemichromis fasciatus and possibly Clarias lazera.
As stated earlier, the development of suitable feeds is considered to have special importance in successful culture of tilapia. This has been found to be the case in most other culture systems that show potential for development in Africa. It would therefore be necessary to make a detailed survey of the availability of feed ingredients and their costs and to undertake their proximate analysis to determine their nutritive values, if the necessary information is not already available. Laboratory studies of the digestion physiology of the two species of tilapia, with special reference to digestive enzyme characteristics and production, should be carried out. Through experiments with test feeds prepared on the basis of available information, the nutrient requirements of the species can be roughly outlined. Based on these data appropriate least-cost feeds should be formulated and tested initially on a laboratory scale and then in production trials. Data on nutritive value and cost of feed ingredients should enable adjustments and substitutions to be made in diet formulae, if the need arises, either in the Centre itself or in the countries where the feed is to be used.
A series of factorially designed production experiments are suggested to determine the feasibility of producing marketable tilapia as a short-season crop and to evaluate the economic viability of such farming. The culture variables of significance, besides limnological conditions, would be size and sex composition of fry or fingerlings, as well as their genetic features, stocking rates, feeds, feeding procedures, health of stock, ratio of predators and type of predator(s) when present, and duration of growing period.
2.2.1 Background
2.2.2 Research needs
2.2.3 Research approach
Although not yet widely cultivated and not included as a priority species in the development plans of all African countries, the freshwater catfish Clarias lazera, has come to achieve some special importance in recent years because of a number of reasons. The success of catfish culture elsewhere in the world has encouraged the search for an African species that has culture potential and consumer acceptance. Experimental culture of catfish in some countries of the Region have yielded encouraging results. When compared to tilapia, which is the most widely cultivated group, this catfish can more easily be grown to a marketable size in a relatively short period of time. When cultured in combination with tilapia, Clarias is reported to serve sometimes as a predator of tilapia fry produced by wild spawning. It gives high yields in ponds and appears to be quite hardy and able to grow well even in relatively poor pond conditions. Because of these reasons, the task force suggests that the culture of Clarias should form a subject of research at the Centre.
At present this species is cultured in some countries of Africa, alone or in combination with Tilapia nilotica. It is an omnivore, and cannot therefore be considered as complementary in its feeding habits with T. nilotica. However, as mentioned earlier, it is believed that Clarias can keep tilapia populations in ponds under control, by feeding on larvae and fry produced by wild spawning. It must, however, be noted that this belief has not been fully substantiated. In relatively turbid water it has been observed that Clarias does not feed on tilapia fry; even in clear water its efficiency as a predator has yet to be fully proved.
The growth rate of the species in ponds is relatively rapid, reaching a weight of 400-500 g in less than six months, providing yields of up to 7-15 t/ha/year.
The two major constraints to the wider cultivation of this species appear to be the availability of (a) fry and (b) suitable feeds. It has been demonstrated that Clarias can easily be induced to breed by the administration of fish or synthetic hormones. Breeding appears to be possible throughout most parts of the year under tropical conditions. However, methods for mass-rearing of larvae have not yet been developed. A suitable artificial feed may be one of the major requirements for successful larvae rearing, as apparently in the absence of proper food, large-scale loss of larvae occurs due to cannibalism.
Preliminary studies have shown that adult Clarias would feed on compounded feeds. They seem to be rather slow feeders as they did not feed from demand feeders. Because of this, the conversion ratios were always high, but in fertile ponds, with natural food, they seemed to need only supplemental feeds of relatively low protein content. These observations have to be verified through appropriate experiments.
Even though it is true that the scarcity of fry and feed are the major constraints to Clarias culture, and high yields have been obtained in experiments, it should also be pointed out that standard methods of producing marketable fish have not yet been developed. Once the main constraints are overcome, it will be necessary to evolve appropriate production techniques, including pond management and health protection measures.
(i) Brood fish selection and rearing; standardization of induced breeding techniques;(ii) development of hatchery methods, including mass rearing of larvae using special artificial feeds and natural foods;
(iii) nutritional studies and the preparation and testing of feeds for larvae, fry and adults, based on locally available ingredients;
(iv) studies to determine the possible value of Clarias as a predator in tilapia ponds;
(v) production trials to determine optimum stocking rates in mono- and polyculture, growth rates, yields and economies of operations;
(vi) determination of diseases or health hazards in intensive culture systems.
As the first step in the development of an effective breeding and seed production programme, a system of brood fish selection and rearing of males and females in appropriate numbers has to be established. It is necessary to delineate more clearly the breeding behaviour and seasons, so as to schedule breeding operations for best results. Comparative experiments will be needed to determine the most efficient hormones to be used for induced breeding, as well as the dosages and procedures to be adopted.
A hatchery operation, including rearing of hatchlings up to the fry stage, has to be developed. Indoor hatchery jars and rearing tanks can be used for the purpose, but success would largely depend on the efficiency of the larval feed used. So priority has to be given to the formulation, preparation and testing of a suitable feed. Initially the whole-egg larval feed suggested for tilapia larvae should be tried. In comparative experiments zooplanktonic organisms collected from nature or raised in the farm should be used. Based on the results of initial tests, methods of large-scale production of larval food have to be developed.
Observations seem to indicate that adequate aeration and protection from direct sunlight are of importance for successful larval rearing. This has to be experimentally verified, and if found to be true, necessary modifications and additions should be made in the hatchery design. In indoor hatcheries it should not be difficult to incorporate the necessary provisions.
As for nutritional studies and the development of feeds, the approach should essentially be the same as suggested for tilapia. The observation that Clarias may need only feeds with low protein content, should be taken into account when formulating test feeds. It may also be necessary to incorporate some attractant in feeds to induce more ready feeding. As an alternative, feeds of higher water stability have to be developed so that loss of feeds will be minimized. Comparative studies on feeding methods should also be conducted.
Comparative fry and fingerling rearing experiments should be carried out, both indoors and in outdoor earthen ponds, to determine the most efficient methods, and to see whether direct sunlight has any unfavourable influence. Methods of reducing or avoiding cannibalism should be determined experimentally, as for example, by means of adequate artificial feeding, induced turbidity, or size manipulation of fry.
Factoriallv designed production experiments in monoculture as well as polyculture with T. nilotica should be conducted, incorporating all the significant variables. Possible losses due to fish escaping over dikes during rainy seasons should be prevented by fencing or other devices. Observations should be made on the extent of cannibalism among adult fish and culture experiments should be conducted with fingerlings of uniform size range and grading of fish at regular intervals, to prevent the occurrence of undersized fish (and consequent cannibalistic tendencies in the stock). The economics of Clarias culture should be determined by the evaluation of all operating costs and price of production obtained.
Experimental work needed to determine the efficiency of Clarias as a predator in tilapia ponds has been described earlier. In a polyculture system where a high production of both Clarias and Tilapia are aimed at, optimum stocking rates, feeding regimes and the influence of feeding on predation have to be determined through comparative experiments. Through cost-benefit assessments the relative advantages of polyculture should be determined.
Although diseases do not seem to present a major problem at present, it is very likely that health hazards would occur in intensive farming with high stocking rates and intensive feeding. It will therefore be necessary to monitor the health of the fish along with water quality conditions.
