Behavioural, reproductive and growth studies on Oreochromis mossambicus (Peters 1852)

Abstract

A major obstacle facing the successful creation of an African aquaculture industry, based upon Oreochromids, is the irregular supply of good quality fish seed. There are several causative biological processes behind its irregular supply. The aim of this research was therefore to determine the basic requirements for the establishment and maintenance of a small breeding facility, for O. mossambicus. The goal was to make a unit that was simple and which could be easily replicated in rural, satellite aquaculture seed stations. The results obtained illustrate that a small reproduction unit can produce large quantities of healthy 90-day fry. Asynchronous hatching of the eggs and spawning asynchrony in female Oreochromis mossambicus are two elements which negatively affect uniformity in the fry produced. Typical fish seed production uses large ponds partitioned into breeding allotments or a series of breeding pools. While the earthen ponds provide a substrate in which a nest can be excavated, its presence is not required for mating success in the closely related O. niloticus (Linneaus 1758). Female mate choice, as well as apparent fecundity, according to nest size has been clearly recorded in related cichlids but no investigations have been made as to nest size and spawning synchrony in O. mossambicus. The main focus of this investigation was to ascertain whether O. mossambicus would accept artificial nest substitutes in preference to their own constructed ones and secondly, whether different alternatives would elicit different levels of acceptance. The observed results indicate a ready acceptance for artificial nest alternatives, with nest lip height being prioritised by the fish . The implications thereof are discussed in relation to the potential for optimization of breeding arenas for O. mossambicus by the provision of artificial nests whose dimensions satisfy both male and female preferences. In established communities, Oreochromis mossambicus display various complex and ritualised behaviours during stable and disruptive events. The aim of this research was primarily to produce a glossary of behaviours defining these interactions, particularly with reference to male-male behaviours. Three males and six females were allowed to acclimatise over one month, with various social groupings being established within the first few days. Results from this study illustrated not only a dynamic social structure, signaled via various chemosensory and visual methods, but also supported recent findings in apparent male-male courtship and the underlying ii causes. Furthermore, the observed male-male activity of the nestholder malesfirmly corroborate the current practice in aquaculture whereby only one male is allocated per breeding arena. The use of artificial incubation of Oreochromis spp. eggs has become widespread in high intensity fish seed production. Various types of incubator exist, and their selection is dependent on the specific attributes of the egg to be incubated. Currently available incubators are typically of a funnel (up-flow) or round bottomed (down-flow) design. Neither permits easy access to the eggs, which is particularly important when dealing with poor quality water as is typically found in rural areas. The aim of this study was to devise and test an easy-to-use incubator, applicable to rural seed production projects, which offers advantages over currently available incubator types. The final design, WETNURSE Type II, offered improved hatching rates over Type I, with a mean hatching success of 75%. While falling short of the desired 80% success rate (Rana 1986), the various other benefits provided by the design justify further optimization and testing. Three distinct populations of O. mossambicus, representing populations of inbred, randomly mated and genetically unknown (wild-caught) pedigree were analysed according to their food conversion efficiency (FCE). The intra- and inter-sample crosses were done with single males in order to produce half-sib progeny batches which allowed for the assessment of sire influences on the FCE of the progeny batches. The results show that the population of unknown pedigree is comparable to that of the randomly mating population, indicating the presence of sufficient genetic variation to permit further selection; the genetic contribution of the males to their respective progeny was insignificant in relation to that made by the female.