This study investigated several important aspects of reproductive physiology in the Nile tilapia (Oreochromis niloticus). Although tilapias have been cultured for several decades, there has been a rapid increase in production by aquaculture over recent years. Tilapia have thus become one of the main teleosts contributing significantly to world aquaculture. However, significant problems exist that constrain the efficient management of reproduction in these fish. These include low fecundity, and the asynchronous, and hence unpredictable, nature of spawning cycles. Manipulation of photoperiod has proved to be a powerful tool in the control of reproductive cycles in various other fish species, and has become an established aspect of the culture of certain species. We know very little about the ways in which photoperiod might be involved in the control of reproduction in tilapiine species, especially the hormonal rhythms associated with this regulatory mechanism. In addition, manipulation of environmental parameters such as photoperiod has often been reported to influence fertilization rates in some teleosts. Spawning induction using the application of exogenous hormones, a technique often used in aquaculture to produce predictable spawning patterns, is also associated with problems concerning fertilization. It is clear from the literature that there is very little information available concerning the precise mechanisms involved with fertilization and egg activation in fish. Thus, the research described in this Thesis falls into two main sections. The first section investigates how photoperiod may impart a regulatory role over reproduction in O. niloticus broodstock, how circulating levels of the hormone melatonin vary in this species, and describes the isolation and partial characterization of a melatonin receptor from this species. In the second part of this Thesis, I have made a preliminary investigation of the precise mechanisms that might be involved at egg activation in fish using tilapia as a research model; these studies were then extended to three other commercially-important fish.
Tilapia are now a major aquaculture species with production levels of over a million tonnes annually. The hatchery production of fry however, remains very inefficient due to relatively low fecundity and lack of spawning synchrony. Any methodology that enables farmers to synchronise the reproductive cycles of their broodstock would have immense practical advantages. Light is already known to play an important role in the initiation of gonad maturation in other fish species. In this investigation the reproductive performance of 32 sibling Nile tilapia was evaluated under four different photoperiods: short daylength (6L:18D), normal daylength (12L:12D), long daylength (18L:6D), and continuous illumination (24L:0D). Significantly larger eggs (P