Seasonal steroid hormone profiles in plasma and gonads of the tilapia

Syferkuil Dam is situated 8 km NW of the University of the North and comprises a series of eight interconnected rectangular dams, having cement sides and mud bottoms. Throughout the experimental period, male and female adult specimens of the mouthbrooding tilapia, Oreochromis mossambicus were collected for further analysis. Aspects of the reproductive physiology of O. mossambicus that were investigated included reproductive hormone profiles in plasma and gonadal supernatant. This study has shown that three distinct stages breeding season, resting season and a gonadal recrudescent season may be distinguished in both male and female O. mossambicus. During the female resting season there are generally low levels of steroid hormones. However, during ovarian recrudescence, there is an increase in trophic and steroid hormones. This relates to a period of endogenous vitellogenesis that occurs during winter. After winter, the photoperiod begins to lengthen leading the breeding cycle into a state of exogenous vitellogenesis, wherein a second steroid (predominantly estradiol) surge is observed. In males, testosterone peaks prior to the gonadotropins because the former hormone is more important for sperm development. The results imply a close interaction between environmental cues and endocrine control of reproduction. The endocrine control cannot continue without the appropriate environmental cues required to stimulate reproduction. This may be used as a starting point for breeding in controlled laboratory conditions when attempting to manipulate artificial breeding by hormonal intervention.