Some aspects of the reproductive biology of three exotic cichlid species that colonize the Victoria reservoir, a deep upland reservoir in Sri Lanka
S. NATHANAEL and E.I.L. SILVA
Cichlids form the mainstay of the fishery of the Victoria reservoir, which is the largest and deepest hydropower reservoir constructed under the accelerated Mahaweli programme. Since there is a significant variation in the relative abundance of the three species of cichlids that colonize this reservoir, namely Oreochromis niloticus, 0.mossambicus and Tilapia rendalli, this investigation was carried out to determine whether there are any marked differences in their reproductive biology, which favours successful colonization of one species over the others.
The reproductive biology of the three species was analyzed using standard techniques by which the mean size at maturity, fecundity, sex ratio, monthly fluctuations in maturity stages and correlations of gonadosomatic indices with rainfall were obtained. All three cichlid species are year-round breeders having around three major spawning peaks. Among the three species O. niloticus has the highest male dominance, the latest maturation and the best correlation between fecundity and body size, which appear to be the characters favouring its establishment in the reservoir. The higher female to male sex ratio in 0. mossambicus in the commercial catches indicates that one of the reasons for the gradual decline of this species may be the elimination of females which are smaller in size than the males, and hence could be more susceptible to small meshed gill nets which are frequently used in this reservoir. The presence of larger heavily yolked eggs, lower fecundity and the mouth brooding habit of 0 mossambicus and O. niloticus make them better adapted for survival in the reservoir than T. rendalli. T. rendalli has the lowest potential for colonizing this reservoir. The survival of this species is ensured by having a high fecundity, a 1:1 sex ratio with prolonged association between breeding pairs, and an early maturation of both sexes. The results indicate that the differences in reproductive biology play an important role in the colonization success of the three cichlid species in this reservoir.