A thesis submitted in partial fulfillment of the requirement for the degree of Master of Science

Thesis (M.Sc.) - Asian Institute of Technology, 1996

An experiment (experiment 1) compared performance of yearling (> 1<2) Nile tilapia broodfish of two strains (GIFT and AIT) showed no differences. Fish (density 30 fish/hapa) were allowed to spawn naturally in triplicate 20 m2 hapas suspended in the same earthen pond. Seed was harvested every 5 days over a period of 12 weeks by checking individual female fish before returning them to the same hapa. Fecundity and relative fecundity were not significantly different (P<0.05), despite GIFT fish being larger throughout the experiment due to different pre-experimental history. Total seed harvested and frequency of spawning declined throughout the experiment despite water quality being maintained at suitable levels. Samples of seed retained for estimation of quality indicated no differences in viability, hatching and survival to the yolk-sac absorbed stage (5 day old). A further two trials compared the reproductive performance of known age (5 months) GIFT and AIT broodfish (experiment 2) and known age (5 months) GIFT, AIT and Swansea broodfish (experiment 3). Both experiments were run over the same period, allowing broodfish to spawn in single pair in 1 m2 hapas suspended in the same pond. Around 50 % of all three strains were mature within 150 days but no spawning occurred within this time. The lack of spawning, despite suitable early morning dissolved oxygen, may have been linked to high levels of silt in the pond water.