Histological classification of oocyte growth and the dynamics of ovarian recrudescence in Tilapia zillii

Serum levels of 17β-oestradiol and testosterone peaked and fell within 6 days of spawning in Tilapia zillii suggesting that vitellogenic growth began as early as day 2 or 3 postspawning. As early as day 8, stage 6/7 (late nitellogenic and maturing) oocytes occupied 60-70% of the ovary. From day 8 onwards the proportion of stage 6/7 oocytes changed little, though I(G) increased (as oocytes grew) to reach maximal levels of ~5.5% by day 14. I(G) was correlated significantly to the proportion of stage 6/7 oocytes. Postovulatory follicles were observed immediately following spawning occupying up to ~7% of the ovary but were not present from day 3 onwards. Atretic oocytes were found throughout the time period monitored (generally occupying <2% of the ovary) but were more prevalent from day 18 onwards. Data suggest that previtellogenic oocytes are recruited into vitellogenic growth immediately after spawning and can complete vitellogenesis as early as day 8 postspawning. Knowledge of this timing is likely to he important in the development of spawning induction programmes in T. zillii and other related species.