Studies on the reproductive biology of Oreochromis niloticus L.

Abstract

This study investigated the reproductive biology of Oreochromis niloticus broodstock of known age structure and spawning history with the aim of synchronising and controlling their spawning for mass fry production. Hatchery reared stock was subjected to a constant photoperiod of 12L:12D and maintained at 27 ± 1°C. All stock was fed on commercial trout pellets. The feeding frequency and protein content of the diet varied depending on fish size. Oocyte development was classified into 6 stages including that of atresia based on histology. In order to quantify ovarian maturity, three stereological methods were compared. The ovarian volume fractions of different oocyte stages estimated by the mass, graphical and intersection methods showed homogeneous results. The intersection method required less time (2.6 mins/sample) whereas the others needed 11-12 mins/sample. In addition, the numerical density technique employing the intersection method was used and yielded similar oocyte estimates to those derived from the Gilson's fluid method. Onset of sexual differentiation was influenced by the stocking densities. At 10 and 20 fry/l, 30 and 45% of those fry, respectively, were sexually differentiated by day 11 post-hatch, whereas those held at 2 fry/l were not. Gonadal development was monitored in fish of known age. Fry were randomly sampled after hatching at two week intervals until 24 weeks. Total body length and weight were recorded and gonads were fixed for maturity determination. Serum samples were analyzed for total calcium (Ca2+), testosterone (T) and oestradiol-17ß (E2). The males grew faster than the females of the same age and showed secondary sexual characteristics and attained maturity with significantly (P<0.05) higher T levels by 16 and 22 weeks, respectively. Females in comparison showed a significant (P<0.05) increase in GSI during 18-24 weeks (0.5-3.6%). The volume fraction of stage 6 oocytes, which were positively correlated to GSIs (r2=0.84; P<O.05), increased from 46.7% (20 weeks) to 71.8% by 22 weeks and then declined to 67.5% by 24 weeks. These results coincided with the mean levels of E2 whereas the Ca2+ and T levels showed high average levels through 24 weeks. These trials suggested that the females attained sexual maturity by 22 weeks. Ovarian recrudescence and average levels of Ca2+, T and E2 over 2 to 3 spawning cycles were studied. Within each spawning cycle the volume fraction of stage 6 oocytes increased from 0-15% (at day 1) to 65-72% by day 10 after spawning, which coincided with the high levels of Ca2+ and T whereas E2 levels peaked at day 5 and then decreased at day 10 after spawning. Females at day 10 post-spawning had, therefore, completed vitellogenesis and spawning occurred at the median time of 13 days. In addition, average hormonal levels, egg quality and quantity over 2 to 3 spawning cycles were monitored in eight individual females. Females were bled twice a week after their first spawning. The median of spawning cycles of these females for the first and second cycles were 13 (short cycle) and 28 days (long cycle), respectively, and their overall median spawning cycle was 15 days (short cycle). Levels of E2 were significantly (P<0.05; r2=0.79) correlated to the volume fractions of stage 6 oocytes and their peak levels were significantly correlated (P<0.05; r2=0.49) to fertilisation rates of eggs in subsequent spawns. Fecundity and fertilisation rates of eggs from those females in the second and third spawning were higher than the first spawning which indicated that the females that had spawned previously tend to ovulate more eggs than those that had spawned for the first time. The spawning history showed no effect on their fertilisation rates. The females which were selected by their external characteristics were either injected (10 to 300μg D-Ala6-Gly10-LHRH + 0.1mg pimozide/kg body weight) or implanted (fast or slow release pellets containing LHRH; 100μg/kg) with the hormones. Neither the injections nor LHRH pellets were effective in inducing the females to spawn. At day 10 after each spawning, a mixture of 100μg LHRH + 0.1mg pimozide/kg body weight was injected into the females kept under two spawning conditions. Females were held in either separated compartments (limited contact) or under normal communal spawning conditions (unlimited contact). Spawning environment affected the success of induced spawning. The females which were held in the separated compartments spawned within 2 to 6 days post-injection whereas the sham controls spawned in 7 to 8 days postinjection. In contrast, the females in the communal spawning environment did not respond to hormone induction. The timing at day ten post-spawning and the conditions of spawning were found to be the important factors affecting exogenous hormonal administration in this fish species.