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With a production of 150 to 200 kg/ha/an, capture fisheries in West African coastal lagoons no longer meet the local demand for fish. This situation is aggravated by the increasing pressures of urban expansion in the lagoon areas. The Ebrié lagoon in Côte d'Ivoire is affected by the development of the city of Abidjan and provides a good example of this expansion. Improved production for lagoon areas is consequently drawing increasing interest. The adoption of acadja-enclos and pen production systems in the Ivorian lagoons reflect this development. Pen culture is essentially based on the intensive culture of catfish and requires private investment, but acadja-enclos systems are extensive, village-based farming systems. The problems of competition (for space and resources) between the acadja-enclos culture system and capture fisheries are discussed, as well as the introduction of acadja-enclos into villages and their economic effiency. (Résumé d'auteur)

The adaptation of fish to a new feed was monitored for the first week of experiments by measuring variations in the apparent digestibility coefficient (ADC) of dry matter, nitrogen and organic matter. Three series of experiments were conducted on nine different feeds and one reference feed. Each feed was distributed to two batches of animals. Each batch comprised 15 fish (87 more or less 4 g) for the first experiment, and 20 fish (177 more or less 3 g) for the second and third experiments. Feces were collected every 12 hours (0800 and 1700) before each distribution of feed. The influence of the day/night cycle on digestibility was measured by comparing feces excreted by day with those excreted by night. ADC values obtained during the first day of distribution were significantly lower than those obtained over the following days. There was no difference between day and night in the ADCs of nitrogen and organic matter, except for the first meal. However, a slight difference between day and night in the ADC of dry matter does persist beyond the second day. To determine the ADC of a feed, the first collection of feces must occur at least 24 hours after the first distribution of feed. If fish are fed several times per day, it is best to use the aggregate of feces excreted in each 24-hour period to estimate a practical value for the different ADCs. The ADCs of dry matter, proteins (nitrogen) and organic matter were determined for several by-products available in Côte d'Ivoire : fish meal, maize meal, cassava meal, copra cake, soy cake and cotton cake. (Résumé d'auteur)

The study of constraints affecting the development of fish culture in rural Côte d'Ivoire has shown that it is impossible to use farming methods that require expensive inputs such as supplementary feed. In contrast, farmers are willing to devote much of their time to fish culture if their work is adequately compensated. When available, most inputs really accesible to farmers have a poor nutritional and/or fertilizing value. The efficient use of the limited trophic resources can be done through : (1) the qualitative trophic and/or quantitative improvement of the flow of substances in the different levels of the pond trophic web (direct feeding, autotrophic productivity and heterotrophic microbial productivity) and (2) improvement of the accessibility to trophic resources by the fish. The present study is based on on-farm and on-station experiments focusing on : (1) the improvement of culture environment (treatments based on use of rice bran to which green manure may or may not be added, and on combined fish and rabbit culture) and (2) the use of a substratum made of bamboo or branches (acadja) to improve fish accessibility to primary production (attempts to substitute commercial feed by acadja system in lagoon pens have already been giving promising results). The results from this study confirm the importance of adding substrate for primary producers, of combining fish and rabbit culture and of using green manure for the improvement of pond production. New approaches for research and R&D in low input fish culture are suggested. (Résumé d'auteur)

The size at first sexual maturity, absolute fecundity and egg and spawn weight were compared in females of #Sarotherodon melanotheron$ (Rüppel, 1852) collected in the wild (Ebrié Lagoon) and reared in different lagoon culture conditions : intensive farming in pens and extensive farming in "acadja-enclos" (pens fitted with branches or bamboo poles). In pens, females reach maturity at a smaller size and produce smaller eggs and in larger quantities than in the wild. In "acadja-enclos" where conditions are intermediate between intensive culture and natural conditions, an intermediate situation is also observed in fecundity and in the size of eggs produced. However, the size at first maturity is similar to that observed in the wild. In contrast, the relationships between spawn weight and female weight hardly vary with the different growth environments. These results suggest that the quantity of matter produced during a breeding cycle (based on the spawn weight) is a specific constant that may be genetically determined. Environmental conditions have an effect on the gonadal material division process and on the breeding strategy : many small eggs or large ones but few. (Résumé d'auteur)

The year-round, spontaneous and continued reproduction of tilapia in captivity was initially regarded as an unexpected asset for African aquaculture development, but it was quickly found to be a serious handicap when farming the fish in ponds due to the resulting population. Various techniques have overcome this obstacle. The reproductive efficiency of tilapia has advantages and disadvantages for the supply of fry to fish farmers. Among the advantages is the possibility for fish farmers to produce their own fry ; among the disadvantages is the difficulty to control the broodstock quality. The low number of eggs in each spawning requires the use of a large number of broodstock and the asynchrony of spawnings makes the simultaneous production of large quantities of graded fry difficult. Finally, the control of undesirable reproduction in farming involves the use of monosex populations. These issues are analyzed and discussed in relation to development strategies, microeconomics and the identification of research themes that can be developed to improve the different options. The data for three species of tilapia (#Oreochromis niloticus$, #Sarotherodon melanotheron$ and #Tilapia guineensis$) are compared to the data for two species of African catfish of the family #Clariidae$ (#Claris gariepinus$ and #Heterobranchus longifilis$) that require induced reproduction techniques, but are far more fertile. (Résumé d'auteur)

In #Sarotherodon melanotheron$, mouthbrooding efficiency and spawning frequency are two important parameters to consider in the practical management of broodfish reared in captivity. In this species, where males practice mouthbrooding, the number of eggs or fry incubated is positively correlated to male body weight (study made on a population reared in lagoon pens). Mouthbrooding efficiency is discussed in relation to female fecundity, the capacity of the males' mouth, the development stage of the incubated fry and the size ratio between males and females during pairing. Individual spawning frequency was studied in concrete tanks with isolated pairs and families of different sex ratios. While sex ratios shifted toward the males do not increase spawning frequency significantly, sex ratios shifted toward the females decrease it considerably. The most suitable sex ratio for the production of fry of #S. melanotheron$ captivity is 1:1. Using this sex ratio, mean individual spawning intervals are generally 10-16 days. Annual egg production of this species is estimated, based on long periods of observation (174-587 days) using isolated pairs. (Résumé d'auteur)

Enzyme protein polymorphism was studied in 12 stocks of #Tilapia guineensis$ (238 individuals) and seven populations of #Sarotherodon melanotheron$ (221 individuals) from Côte d'Ivoire, Senegal and Gambia. Of the 28 loci coding 18 enzyme systems, 17 were found to be polymorphic in #T. guineensis$ and 11 in #S. melanotheron$. In both species, each population is clearly distinguishable. The high polymorphism observed in stocks of both species warrants their use in programs of genetic improvement through selection or interspecific crossbreeding. (Résumé d'auteur)

Although tilapia culture in the Senegal river basin was promising at its beginnings, it now faces major difficulties (production of less than 50 tonnes). In spite of the many positive elements such as a vast water potential, abundance of by-products from agricultural and animal farming, and a deficit of about 21,000 t in fish supply from the river watershed, tilapia culture has not been successful. Its development has been affected by many constraints : (1) environmental - irregularity in the natural water system, poor mineral content in the rivers, high turbidity during the rainy season and low water temperatures from November to March ; (2) technical - poor choice of sites, insufficient depth of the fishponds and inadequate dike construction, poor water quality control and poor fry quality ; (3) administrative - poor definition of responsibilities, insufficient management support services and constant confusion in the experimental and extension objectivs; and (4) socioeconomic - farmers' perception of fish farming as a secondary activity, high costs of construction and management, and competition with the fisheries sector. However, the construction of the Diama and Manantali dams seems to offer better prospects. (Résumé d'auteur)