The main characteristics of the reproductive biology of Tilapia guineensis and Sarotherodon melanotheron have been studied in Ebrié lagoon (Ivory Coast), with a comparison between natural and cultured populations. In this lagoon, the two species breed throughout the year without interruption. However, seasonal variations in the intensity of the sexual activity have been observed and are more pronounced at the Layo aquaculture station, where the hydroclimate is more unstable than in other lagoon sectors studied. The proportion of mature individuals, the mean GSI, the relative fecundity and the spawning frequency are higher during the dry season than during the rainy season. In rearing enclosures, both species reach sexual maturity at a smaller size, and produce smaller but more numerous oocytes than in the wild. On the other hand, the relationship between the spawn weight and the female body weight are remarkably similar under both situations. For S. melanotheron, which is a male mouthbreeder, the number of brooded eggs or fry is positively related to the male body weight. In 2 m3 concrete tanks, the mean elapsed time between two successive spawnings is about 2 weeks for S. melanotheron and about 3 weeks for T. guineensis.

In intensive culture, the two species of brackish water tilapia, T. guineensis and S. melanotheron, have proved disappointing. Even under most favourable conditions, i. e. the monosex male culture of S. melanotheron, a poor growth rate (about 0.5 g.d−1) and a mediocre feed conversion do not presage a profitable aquaculture exploitation. On the other hand, extensive culture trials using the original "acadja-enclosure" technique have given very encouraging results. Annual fish yields of more than 8 t/ha, including 70 to 80% of S. melanotheron of marketable size, are possible without any artificial feeding.

Tilapia species are generally characterized by a large tolerance to salinity; however, such capacity to adapt to brackish or seawater may be modulated by environmental factors. The major osmoregulatory mechanisms involved in salinity adaptation are presented. Most of the available data concern the role of gills in salt or water exchange. The importance of different factors (environmental or endogenous) in such adaptation is also discussed. The second part presents a survey of the endocrine control of osmoregulation in tilapia. Both fast-acting (e. g. glucagon, urotensins, catecholamines) and long-acting (e.g. prolactin, cortisol) hormones have been studied. In conclusion, several areas of osmoregulatory physiology potentially interesting for aquaculture are discussed.

Reliable data on tilapia nutrition and feeding are scarce. In the best documented field, that of protein requirement, current data show good agreement with available information on coldwater and warmwater fishes. Various feedstuffs have been used to satisfy the protein needs, but the effects of variations of their quality have not been sufficiently studied. The main problem lies in the assessment of the effects of natural food in open rearing structures; thus there are some discrepancies with the results obtained in laboratory. More insight on in situ nutrition of pond fish, as well as the management of the feedstuffs and feeding practices is needed.

In confined environments, the precocious reproductive efficiency of "tilapias" (various cichlid species belonging to the genera Oreochromis, Sarotherodon and Tilapia) can rapidly lead to overcrowding and nanism. Therefore, the artificial control of reproduction in these species is very important for success in their culture. Such a control may consist either in completely preventing fry production in a population, in order to improve growth, or in favouring synchronous massive productions of alevins, in order to set up cultures with homogeneous populations.

At present, some of the most intensive techniques are based on the separate rearing of male fish, either after manual sorting or following hormonal inversion treatments leading to monosex populations. New ways of obtaining a male monosex progeny are suggested, based on the viability and fertility of a new genotype YY. Recent data on the sex differentiation of tilapia gonad suggest the possible use of steroids which have been identified in vitro during the early stages of development of the testis and which exhibit high masculinizing potentialities; these studies also suggest the tentative use of specific inhibitors of the activity of some of the enzymes involved in the synthesis of the above mentioned steroids.

The occurrence of parental care is an important factor in the reproductive efficiency of tilapias. Recent data show close relationships between the chronology of the mouth-brooding behaviour in Oreochromis, and that of the ovarian cycle. A better knowledge of the underlying physiological factors may facilitate the development of new techniques for the inhibition and/or the synchronization of reproduction.

The pathology of farmed tilapias is reviewed from presently available published data. The effects of intensive production systems on the aspects and importance of pathology are outlined. The main pathogenic organisms are recorded. For the moment, these pathogens seem to represent a potential threat, rather than being primary causes of losses. Conversely, environmental factors together with practical errors resulting from a lack of biological and zootechnical data on tilapias, seem to be responsible for most of the difficulties which are encountered. In the short term, intensive research in genetics, nutrition and zootechny should result in significant health improvements for tilapias.