Inhibition of spawning and associated suppression of sex steroid levels during confinement in the substrate-spawning Tilapia zillii

Substrate-spawning Tilapia zillii failed to spawn in crowded holding tanks but exhibited a marked tendency to spawn soon after transfer to individually partitioned aquaria. Confined conditions suppressed the levels of serum 17 β -oestradiol (E₂) and testosterone (T) in females, which remained low throughout the period of confinement. Levels of both steroids rose significantly following transfer of fish to individual aquaria and were maintained at consistently higher levels than those of fish which remained confined. Proportions of stage 3 (late perinucleolar) oocytes were significantly lower ( P <0.001) in individual fish 21–30 days after transfer, coincident with significantly higher ( P <0.01) proportions of stage 6 or 7 oocytes (late vitellogenic or maturing oocytes). However, no significant differences ( P ≥0.05) were detected between individual or confined groups of fish in the relative proportions of stage 2 (early perinucleolar), stage 4 (cortical alveolar) or stage 5 (early vitellogenic) oocytes. Atresia increased as the period of confinement increased. Following return of individual fish to confined conditions, blood steroids fell rapidly to levels seen in fish that had remained confined throughout. It is suggested that the reduced sex steroid levels detected during confinement were insufficient to allow developing oocytes to complete oocyte growth and maturation. The detection of significantly lower proportions of stage 3 and significantly higher proportions of stage 6/7 oocytes soon after transfer suggest that fish were preparing one batch of oocytes for spawning (stage 6/7 oocytes) whilst recruiting from a pool of previtellogenic oocytes (stage 3) for the ensuing spawning cycle.