HISTOLOGICAL CLASSIFICATION OF OOCYTE GROWTH AND THE DYNAMICS OF OVARIAN RECRUDESCENCE IN TILAPIA-ZILLII

Serum levels of 17 beta-oestradiol and testosterone peaked and fell wi thin 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 chan ged little, though I-G increased (as oocytes grew) to reach maximal le vels of similar to 5.5% by day 14. I-G was correlated significantly to the proportion of stage 6/7 oocytes. Postovulatory follicles were obs erved immediately following spawning occupying up to similar to 7% of the ovary but were not present from day 3 onwards. Atretic oocytes wer e found throughout the time period monitored (generally occupying <2% of the ovary) but were more prevalent from day Is onwards. Data sugges t 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 be importan t in the development of spawning induction programmes in T. zillii and other related species. (C) 1998 The Fisheries Society of the British Isles.