4.1 Background
4.2 Research Needs
In contrast to the indigenous fish species which require considerable time to study the biology as related to aquaculture and to establish practical techniques for their culture, much of the biology and culture techniques of the non-indigenous fishes are known. Although time is required for adopting these techniques to Latin American conditions many could quickly be advanced to pilot-scale or commercial trials saving several years in promoting aquaculture.
On the other hand, such fish may possibly affect the ecological equilibrium in the natural waters if they were to escape into the wild, and they could also expose the indigenous fish species to new diseases. Direct and conclusive evidence that such environmental upsets have occurred in the past is lacking. Experimental evidence shows that some exotic species may compete for food or otherwise hamper development of the indigenous species if they breed freely in their new environment. However, conditions in the open waters do not always favour the development and breeding of the fish and survival may be low; for example Nomura (1977) reports that in Brazil carps were eliminated in natural waters by the local carnivorous fish species. Even so, the relative value to the fisheries gained from the introduced fish should be evaluated against the possible loss to fisheries of the damaged species.
Several exotic fishes have already been introduced to Latin America, some more extensively than others. The introduction of exotic species in some cases has given favourable results, as reported by Nomura and Castagnolli (1977) on Tilapia rendalli introduced into reservoirs in Brazil, and by Granados (1977) on the introduction of T. nilotica 1/ to the major reservoirs of Mexico. Some of these species warrant further study and the techniques for their culture need to be determined for Latin American conditions before attempting widespread introduction of other exotic fishes. If, however, a new species is to be introduced, even for experimentation, more care should be given than in the past. The Centre in Pirassununga can thus play an important role in such introductions, providing quarantine facilities and a safeguard against introduction of disease pathogens, and for the conduct of preliminary studies on possible effects on the ecology of natural waters.
1/ It has been suggested that mouth-breeding species like T. nilotica, T. aurea, T. mossambica and T. hornorum be classified under the generic name Sarotherodon, while the non-mouth-breeding species such as T. rendalli would keep the original genus
Among the warm-water exotic fish which have been introduced to Latin America and about which some knowledge has been gained, two have shown promising results: tilapia and carp, and it is proposed that these species be tested further.
Five species of tilapia have been introduced to Latin America in recent years:
(i) Tilapia mossambica is an omnivorous fish, feeds on plankton, detritus and bottom fauna, and reaches a size of 500-600 g and over. As it reproduces when young and small, further growth is hampered. Considerable time is taken to reach marketable size, and even then only a few reach this size as others are stunted by the production of too many young. Growth rate, even when conditions are good, is lower than that of other tilapia species introduced to Latin America (Bowman, 1977). T. mossambica has already been introduced to Ecuador (Departamento de Limnología, 1977), El Salvador (Bowman, 1977), Mexico (Lizárraga, 1977; Granados, 1977) and Colombia (Acero-Sánchez, 1977), but further introductions should be discouraged.(ii) Tilapia rendalli (= T. melanopleura) feeds on higher vegetation and has shown good growth rate and yields in the Latin American countries to which it has been introduced, such as Brazil (Nomura and Castagnolli, 1977), Colombia (Ramos, 1973; Acero-Sánchez, 1977), Mexico (Granados, 1977), Paraguay (División Caza, Pesca y Piscicultura, 1977) and Bolivia (1977).
(iii) Tilapia aurea feeds on detritus and small animals living on the upper layer of the bottom of the water body (the organic ooze). It has shown very good rates of growth and yields per unit of pond area in both Puerto Rico (Pagan-Font, 1977) and El Salvador (García-Ramirios, 1977).
(iv) Tilapia nilotica has been introduced to Brazil (Lovshin, Da Silva and Fernandez, 1977), Mexico (Granados, 1977; Lizárraga, 1977), Paraguay (División Caza, Pesca y Piscicultura, 1977) and Costa Rica (FAO/UNDP, 1976). It feeds on plankton, both phyto- and zooplankton, grows well and a high production can be obtained from pond culture of this fish.
(v) Tilapia hornorum is a small fish, dark in colour, used mainly to produce monosex hybrids when crossed with Tilapia nilotica. It was introduced to Brazil (Lovshin, da Silva and Fernandez, 1977) and to Puerto Rico (Pagan-Font, 1977).
Culture of tilapia as a cash crop has two basic options: (i) regular culture without sex separation, and (ii) the culture of monosex male populations.
(i) Regular culture without sex separation has often failed in the past because of the "wild spawning" of the tilapia that produces a large number of fry which stunt the entire population. Theoretically this can be overcome in two ways:(a) management practices that produce fish as large as possible in as short a period of time as possible overcome the adverse effects of "wild spawning" if it occurs when the fish have almost reached market size. Thus fry do not grow large enough to be a major feed competitor. To follow such a practice, the pond must be completely drained after each cropping, the bottom dried and the remaining fish destroyed so no adult fish is left to spawn during the next growth period. Increasing the growth rate requires selection of fast growing strains, preferably coupled with late spawning tendencies (all tilapias spawn young, but even a month's delay in spawning can be most beneficial). T. aurea is the most suitable of the five mentioned above for this purpose. However, this option requires the availability of proteinous feed and management practices that will encourage fast growth together with low-stocking densities which usually involve some loss of potential yield per unit of area;(b) one variation of this option is the addition of a predator species to control the young fry. Some experiments in culturing tilapia with Cichlasoma nigrofasciatum have been relatively successful in controlling the young fry (García-Ramirios, 1977). Cichla ocellaris may also be a suitable fish in such polycultures (Nión, 1977). However, this practice requires a supply of fry of the predator species, and the infrastructure to produce it, and only C. ocellaris can be reproduced with existing technologies. However, this fish is cannibalistic and requires much pond space, so a relatively large pond area will be required. Tilapia-predator stocking ratios and size ratios also have to be studied and determined; the equilibrium between the predator and tilapia is quite delicate, and any disturbance in it may result in poor growth and yield leading to a loss of revenue. Maintaining this equilibrium requires skilled aquaculturists; training them may take some time.
(ii) Culture of monosex male tilapia resolves the problem of "wild spawning" in rearing ponds. Since the males also have higher growth rates than the females, additional advantages of larger fish and higher yields are gained. Monosex male tilapia can be produced in one of three ways:
(a) manual sexing requires skilled personnel; even the most experienced people encounter an error of at least 2-5 percent. If the error is over that, a detrimental effect on the productivity of the pond can result. Both sexes must be reared to 50-80 g before sexing is possible, thus resulting in a loss of about 50 percent of the production up to this size due to the destruction of the females;(b) monosex male hybrids can be produced by the crossing of some tilapia species. The genetics of sex determination in these hybrids is quite complicated, and not all crosses will produce 100 percent males. The cross between T. nilotica males and T. aurea females in Israel produced only 80-90 percent male hybrids; the cross between T. mossambica males and T. nilotica in Mexico produced only 80 percent male offspring (González, 1977), whereas the cross between T. hornorum males and T. nilotica females usually gives an all-male offspring. Some females, however, may be produced if the brood stock is not of pure lines. Difficulties have been encountered in Israel with T. nilotica imported from Uganda that were of a diverse nature. It should also be borne in mind that the F1 is fertile and may backcross with the female parent fish to produce fry of both sexes. Constraints encountered in supplying hybrids are therefore the difficulties in maintaining the purity of brood stocks and the limited fecundity that restricts fry supply. Mortalities sometimes occur in heavily populated nursing ponds and further reduce fry production. Clearly, the handling of fry production is beyond the means of most individual fish farmers. Large-scale production units are needed for the supply of hybrid fry with special facilities for keeping pure brood stocks, nurse fry, and control diseases, if this option is to be pursued;
(c) sex-reversal technologies are promising. Since size of tilapia fry is determined between the second and sixth week after hatching, feeding sex hormones (testosterones) during this period will reverse the females into males. These techniques have been successful on a laboratory scale, but have yet to be proven on a commercial scale. This procedure requires young fry being held in non-earthen ponds (to exclude natural food) for about six weeks with hormone-fortified feed; thus space can become a problem. Production of the special feed can be troublesome, especially where a licence is required for the use of hormones. Of these various techniques for preventing reproduction in the ponds, the last two seem most promising for the development of tilapia culture in Latin America. Experiments to date on the culture of hybrids are encouraging (Bowman, 1977; Lovshin, da Silva and Fernandez, 1977) and can be quite profitable (Greenfield, Lira and Jensen, 1977). These techniques should therefore be considered for further development at the Centre, although the sex reversal techniques may prove to have constraints in the immediate future.
In view of the relatively wide regional distribution that tilapia has already gained in Latin America, its acceptability by the consumers and the promising results obtained hereto, it is proposed by the Task Force that high priority be placed on the modification and improvement of the techniques for its culture. It is believed that this could be achieved sooner than the development of techniques for the culture of indigenous fishes. As explained above, it would seem that the approach of culturing monosex male tilapia has the greatest possibility. Since the Latin American market demands fish of at least 200 g in weight, and prices will presumably be much higher for larger fish, procedures should be developed to grow the fish to this size. This will mean, firstly, dropping the Tilapia mossambica which has a smaller chance of reaching this size. The hybrid T. hornorum x T. nilotica is the only combination that up to now has given monosex male offspring. However, it is dark in colour and in some place has less market appeal than the lighter coloured T. aurea. If sex reversal is accomplished this seems to be the most suitable species. Secondly, the possibility of achieving intermediate or even high intensification levels should be studied using fertilizers, organic manure, or feed with a view to obtaining larger fish and higher yields. It should be borne in mind that tilapia utilizes cereal grains such as sorghum poorly, but may utilize vegetable protein such as soybean or any other oil meal cake well.
It is proposed that research on tilapia culture involve the following experimentation:
(i) Breeding of tilapia hybrids(a) crosses between two species cannot usually be obtained in small aquaria and require ponds or at least large tanks, and the fecundity of hybrid tilapia is small. Therefore more brood fish are required, and the main emphasis of the breeding scheme should be production of pure brood stocks within the hatchery for breeding outdoors. Careful monitoring of the purity of the line should be made and brood stock must be marked properly;(b) experiments should be carried out to increase the number of fry obtained by cross-breeding;
(c) studies are needed on improved nursing methods, density of stocking for different sizes of fry, feed of fry, and diseases that may cause loss of fry in the nursing stage;
(d) other possible species which give 100 percent male offspring may also be studied.
(ii) Sex reversal of tilapia (T. aurea or other species)
(a) a laboratory scale study is needed first;
(b) a pilot-scale study should follow if the early results warrant it.(iii) Feeding studies
These will involve mainly a comparison of the relative utilization of the various feeds available rather than basic research on nutritional requirements of the fish:
(a) the acceptability of various local feeds by tilapia;(b) digestibility tests of the various feeds. These studies are quite complicated and must have the facilities of a chemical laboratory with the possible use of inert tracers (Cr2O3) or even radio-active tracers. In the last case it may require the cooperation of a nearby university with the proper equipment;
(c) growth studies in running water tanks for comparison of feedstuffs.
(iv) Culture studies
Since there is a strong interaction between some of the factors involved in culture, such as the degree of intensification, the amount of feed, the composition of the feed and the rate of stocking, these experiments should have a factorial design, studying the following parameters:
(a) stocking density;
(b) fertilizers, fertilizer amounts and fertilization;
(c) manuring, manure amounts, forms, manuring frequency;
(d) feeding, feedstuffs, feeding levels, methods of feeding.These independent variables which Can be combined in the factorial experiment should be followed by the study of some dependent variables which may affect the growth and yield. These may change according to the experiment and include:
(a) temperature;
(b) dissolved oxygen;
(c) pH;
(d) primary production;
(e) phytoplankton;
(f) zooplankton;
(g) benthos;
(h) the feeding habits of the fish (gut analysis).The experimental fish should be sampled frequently (every 10-14 days) in order to determine their growth rate, state of health and the feeding level for the next period.
(v) Pilot-scale yield trials should be undertaken as soon as feasible. Emphasis in these trials should be on costing production so that economic projections for the system can be made.
(vi) studies on product quality, marketability and the preparation of new products are also needed early in the investigation of the system.
