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 t anks but exhibited a marked tendency to spawn soon after transfer to i ndividually partitioned aquaria. Confined conditions suppressed the le vels of serum 17 beta-oestradiol (E-2) and testosterone (T) in females , which remained low throughout the period of confinement. Levels of b oth steroids rose significantly following transfer of fish to individu al aquaria and were maintained at consistently higher levels than thos e of fish which remained confined. Proportions of stage 3 (late perinu cleolar) oocytes were significantly lower (P<0.01) in individual fish 21-30 days after transfer, coincident with significantly higher (P<0.0 1) proportions of stage 6 or 7 oocytes (late vitellogenic or maturing oocytes). However, no significant differences (P greater than or equal to 0.05) were detected between individual or confined groups of fish in the relative proportions of stage 2 (early perinucleolar), state 4 (cortical alveolar) or stage 5 (early vitellogenic) oocytes. Atresia i ncreased as the period of confinement increased. Following return of i ndividual fish to confined conditions, blood steroids fell rapidly to levels seen in fish that had remained confined throughout. It is sugge sted that the reduced sex steroid levels detected during confinement w ere insufficient to allow developing oocytes to complete oocyte growth and maturation. The detection of significantly lower proportions of s tage 3 and significantly higher proportions of stage 6/7 oocytes soon after transfer suggest that fish were preparing one batch of oocytes f or spawning (stage 6/7 oocytes) whilst recruiting from a pool of previ tellogenic oocytes (stage 3) for the ensuing spawning cycle. (C) 1998 The Fisheries Society of the British Isles.