Breeding behavior and the associated GtH and steroid levels in female Nile tilapia, Oreochromis niloticus

Breeding families of Oreochromis niloticus were maintained in aquaria, each comprising of one male and 7-10 females. Fish in such families establish a social hierarchy in which the females are situated according to their breeding status. Seven distinct and successive phases of breeding behavior are defined in the females tilapia starting with the initiation of courtship and culminating in actual mouth-brooding. Females from such families were bled at each of these phases and circulating levels of tilapia gonadotropic hormone (taGtH), estradiol-17B (E₂) and testosterone (T) were determined by respective radioiminunoassays (RIAs). Levels of taGtH were low and stable throughout the breeding cycle (ca. 10-14 ng cm^-3) except a five fold peak (76 ± 29 ng cm^-3) which occurred during actual spawning. The levels of E₂ were relatively low (ca. 19-26 ng cm^-3) during the first three phases but peaked to 145 ± 62 ng cm^-3 upon acquiring the nuptial coloration and remained elevated when the female attended the nest. T levels increased from < 10 to 26 ± 8 ng cm^-3 during these two phases. The levels of the steroids dropped at actual spawning and mouthbrooding. The peak of taGtH, which occurred only around the time of actual spawning, may indicate its physiological similarity to the maturational gonadotropin (GtH-II) as defined in salmonids. The increase in the level of estradiol and testosterone during early phases of the cycle may be attributed to the effect of another GtH (probably GtH-I) which could not be measured by the RIA employed.