EVIDENCE OF ENVIRONMENTAL-EFFECTS ON REPRODUCTIVE CHARACTERISTICS OF NILE TILAPIA (OREOCHROMIS-NILOTICUS) POPULATIONS FROM MAN-MADE LAKES OF IVORY-COAST

Large differences in reproductive traits were observed between populat ions of Oreochromis niloticus sampled in 9 manmade lakes of Ivory Coas t. In order to assess whether these variations in reproductive charact eristics resulted from short term adaptation or from a longer evolutio nary process, living specimens were caught in the two most differentia ted populations in term of life history traits, and placed in a common environment in culture conditions. Genetic analysis of fish from thes e two populations were performed using four microsatellite markers and revealed that both descended originally from the same strain (Bouake station) which was constituted from broodfish initially caught in the Nile and Volta basins. Fish from the two populations were subjected to a common environment (pond and aquariums) for five months. Then, thei r reproductive characteristics were analysed and no significant differ ences were found in fecundity, egg size and spawning frequency. These results indicate that reproductive differences between the two populat ions, originally observed in the two reservoirs, mostly reflect the ph enotypic plasticity of the species in facing different environmental c onditions. (C) Ifremer/Elsevier, Paris.