by
S. N. SEMAKULA and J. T. MAKORO
Fisheries Department
Entebbe, Uganda
Abstract
Several species of Tilapia have been raised in ponds in Uganda since 1953. Because of their prolific breeding habits, the ponds soon become overcrowded. The size of fish produced is unacceptable to the consumers and the total yield from ponds in low. Little success has been obtained with predators to control the populations. Mono-sex culture through hybridization seems to be the answer to this over-breeding and low yield problem.
L'ELEVAGE DE TILAPIA SPP. EN OUGANDA
Résumé
En Ouganda plusieurs espèces de Tilapia sont élevées en étang depuis 1953. En raison de la tendance de ces espèces à proliférer les étangs deviennent rapidement surpeuplés. Les poissons sont de taille insuffisante pour satisfaire les consommateurs, et le rendement total des étangs est faible. L'emploi de prédateurs pour lutter contre le surpeuplement n'a guère donné de résultats jusqu'ici. Il semble que les problèmes posés par la prolifération et le faible rendement puissent être résolus par l'élevage de Tilapia hybrides d'un seul sexe.
EL CULTIVO DE TILAPIA SPP. EN UGANDA
Extracto
Desde 1953 se han criado varias especies de Tilapia en estanques de Uganda. Dada su proliferación, los estanques resultan superpoblados enseguida. La talla de los ejemplares obtenidos no tiene aceptación entre los consumidores, y el rendimiento total de los estanques es bajo. Poco se ha conseguido con los depredadores para reducir las poblaciones. La solución de este problema de superpoblación y bajo rendimiento parece estar en el cultivo monosexual por hibridación.
Fish farming started in Uganda in 1953 with the culture of the indigenous tilapia species in small family ponds. The species of tilapia used were Tilapia nilotica, Tilapia zillii and Tilapia leucosticta. These were raised in small ponds constructed on peasants' farms and which rarely exceeded one tenth of an acre (0.04 ha) in area. Prior to the spread of the fish farming idea to the peasant farmers Tilapia spp. had been successfully raised at a government experimental farm at Kijansi near Kampala. However, most of the ponds at the experimental farm were much bigger (about half of an acre - 0.2 ha), well-maintained and the fish were regularly fed. In these respects they greatly differed from the peasant farmers' ponds which were on the whole too small, un-maintained and the fish were only occasionally fed. Although the Fisheries Department had tried its best to advise on the siting, construction and maintenance of ponds, the demand for ponds from the farmers was so great that the department, with its limited staff, could not cope with the supervision of the thousands of ponds that were being constructed.
At Uganda temperatures (15° to 36°C), various species of Tilapia reach sexual maturity in ponds at the age of about six months when about 200 g weight. The fish then begin to breed and breeding is prolific and at very frequent intervals. Populations build up to overcrowding levels, the food supply becomes insufficient and growth rates of all sizes of fish are slowed down appreciably. Each succeeding generation reaches maturity at a smaller size and the ponds become overcrowded with stunted fish. Such small bony fish are entirely unacceptable to the farmers who prefer a few but good-sized table fish (around 400 g). The fish farming program was therefore immediately faced with the problem of producing the size of fish that was acceptable to the farmers. As this stunting was due to the lack of food in the ponds for the rapidly multiplying population, there seemed to be two lines of approach to this problem. One was to increase the food supply in the ponds, and the other to reduce the number of fish so that the few remaining could get enough to eat and achieve better growth rates. Both these methods were tried.
The pond fish were fed regularly with choppings of elephant grass (Pennisetum purpureum), sweet potatoe leaves (Ipomaea batatas) and any other soft vegetation which the fish could take. This type of feeding alone, however, was not sufficient since not all the species of Tilapia that were being used ate higher plant material. T. leucosticta feeds on phytoplankton and bottom deposits formed largely from the decomposition of phytoplankton from the upper layers. Therefore, in ponds lacking phytoplankton T. leucosticta rarely grew to a desired size. T. nilotica feeds mainly on phytoplankton either in suspension or from the bottom deposits. It also feeds on insects and crustaceans and under certain conditions will digest blue-green algae. T. zillii on the other hand feeds on leaves and stems of rooted aquatic plants and their associated epiphytic algae, and this species could therefore be fed on the higher plants as described above.
Of the three species of tilapia used, Tilapia leucosticta was soon eliminated as being unsuitable for culture because of its feeding habits. In addition this species is a prolific breeder and in all ponds where it had been stocked, overcrowding and stunting had occurred much to the discouragement of the fish farmers. Tilapia nilotica and Tilapia zillii were therefore favoured, and of these two, T. zillii seemed to be the most suitable. However, in practice, farmers rarely fed their pond fish and because of under-feeding, T. zillii in those ponds were very small in size but bred prolifically. Farmers were therefore discouraged from using this species unless it was grown in combination with T. nilotica.
Because the different tilapia species used were not all vegetation feeders, fertilization experiments were carried out to encourage the growth of the organisms utilized by the other species. Most Uganda waters have a low pH, and there is a general lack of calcium, phosphates and sulphates. Organic fertilizers (manure, cowdung, and poultry droppings) were first tried. These could be obtained by most of the farmers at no cost, unlike the industrial fertilizers which were on the whole very expensive. These organic fertilizers, however, did not seem to be useful, for although they encouraged the growth of some algae like Spirogyra and Microcystis, they were on the whole not utilized by the tilapia. At the experimental farm superphosphates were therefore tried and the chemical results obtained after fertilization of one pond were as follows:-
Total organic nitrogen increased from 3.36 mg/l to 6.5 mg/l; nitrates increased from nil to 0.1 mg/l; phosphate from nil to 2 mg/l; ammonia from 0.03 mg/l to 1.76 mg/l; and albuminoid nitrogen from 0.8 mg/l to 1.2 mg/l. (FAO/UN, 1961).
Because of the low pH in most areas in Uganda, to obtain better results with these superphosphate fertilizers it was first necessary to lime the ponds. But, although these increases in some of the chemical substances in the water were often accompanied by increases in the phytoplankton and zooplankton, these were so short-lived that the cost of obtaining an increase in the total production of fish from these ponds by this method would have been prohibitive.
Green manure, such as nitrogen fixing plants, was also used in Kajansi. Ponds were planted with beans (Phaseolus spp.) and when they were beginning to flower these were cut down and the ponds flooded. This too, did not prove satisfactory for due to the rapid decomposition at the bottom of the pond, there was in most cases de-oxygenation and the fish died.
Combinations of T. nilotica and T. zillii when regularly fed in fertilized ponds usually produced good results in the first generations. However, uncontrolled breeding soon led to overcrowding in the succeeding generations with resulting stunted populations. It was not possible to produce enough food in the ponds, and even in those ponds where the food seemed to be adequate the tilapia, due to their specialized feeding habits, did not fully utilize all the available resources. Population control measures were therefore necessary. Two approaches to this problem were attempted. One was to use a predator, and the other one to use mono-sex cultures.
Two predators were tried in mixed cultures with tilapia: the largemouth black bass (Micropterus salmoides) and the Nile perch (Lates niloticus). The black bass used in these experiments were obtained from Kenya and had probably been acclimatized to East African conditions. They grew well at Kajansi Fish Farm, but did not spawn. They were not effective in controlling a breeding population of tilapia. The experimental ponds had a very high population of tadpoles and it is possible, as some other workers have shown (Lewis and Helms, 1964) that these formed an easier prey than tilapia fry. The Nile perch on the other hand proved to be a more efficient predator, and in the experimental ponds with Nile perch, stocked at a rate of 1 : 30 (sizes adjusted), the tilapia populations were kept at a minimum and attained better growth rates. Although the Nile perch bred once in Kajansi, it was not found possible to repeat the breeding to obtain Nile perch fingerlings in sufficient numbers for experimentation in the farm or elsewhere. This proved a handicap to the experiments and they had to be abandoned.
Efforts were now directed to mono-sex culture. The first experiments were made by Wurtz in 1959–60 (FAO/UN, 1961). Wurtz tried mono-sex cultures of T. nilotica and T. zillii based on visual identification of the males and females of tilapia between 2.5 cm and 5 cm in length. Because of the difficulty in sexing these species at this age, they did not prove very successful as breeding often occurred in what had been thought to be a mono-sex culture. In these two species of tilapia, unlike T. nigra, the sexes do not grow at greatly different rates, and the sexes could therefore not be separated by mechanical devices.
Hickling (1960) working with Tilapia sp. in Malacca, experienced similar overcrowding problems, and although he could easily sex the fish, the job was tedious and wasteful in that if only males were to be used, the females were wasted. Hickling therefore resorted to hybridization, and as is now well known, he found that the cross between female Malayanacclimatized T. mossambica and male Tilapia hornorum from Zanzibar produced 100 percent male offspring. These could be used for stocking without danger of breeding. (The reverse cross produced 75 percent males).
Several tilapia crosses have now been tried at Kajansi, with the assistance of Mr. Y. Pruginin, FAO/TA Fish Culturist. One of these, T. mossambica (male) and T. nilotica (female), has produced results similar to those obtained by Hickling - almost 100 percent males. The hybrids obtained from these crosses are fertile and reach maturity at about five to six months. In the aquarium tanks where they have been observed they develop breeding colours and periodically dig nests. A back cross is also possible. They have fast growth rates and some individuals have grown up to 150 g in one month. In ponds the hybrids have given higher yields than have hitherto been obtained with wild populations. Yields of up to 1,000 kg per ha per annum have been obtained in ponds with little or no supplementary feeding as compared with 300 kg originally obtained with wild tilapia populations in well fed ponds. The fish population in the ponds can be controlled at the required density.
In conclusion, mono-sex cultures of tilapia through hybridization seem to be the answer to the overbreeding problem that occurs in populations of tilapia in ponds. The technique is receiving great attention in Uganda. Hybrids are easy to obtain and give a higher total yield per ha of water. The method is recommended for fish culturists in tropical areas using tilapia. It is also hoped that through hybridization it will be possible to obtain a tilapia that will usefully utilize all the available food resources in ponds.
REFERENCES
FAO/UN, 1961 Report to the government of Uganda on an experimental fish culture project in Uganda, 1959–60, based on the work of A.G. Wurtz, FAO/TA inland fisheries biologist. Rep.FAO/EPTA, (1387):32 p.
Hickling, C.F., 1960 The Malacca tilapia hybrids. J.Genet., 57(1):1–10
Lewis, W.M. and D.R.Helms, 1964 Vulnerability of forage organisms to largemouth black bass. Trans.Amer.Fish.Soc., 93(3):315–8
