Effects of dietary protein levels on gonad maturation, size and age at first maturity, fecundity and growth of Nile tilapia

Six-week-old Nile tilapia (Oreochromis niloticus) fry with an average weigh t (SD) of 0.51(0.2) g were reared for 140 days on five formulated, isocalor ic diets of different protein levels (25, 30, 40 and 45% by dry weight). Fi sh fed diets of higher protein levels (40 and 45%) showed better growth and feed conversion ratio than those on lower protein levels. Fast-growing fis h matured earlier. Maturation rate was affected by the dietary protein leve ls. Males matured earlier than females: the first mature males were recorde d when they were 14 weeks old, whereas the females matured after 18 weeks. In both sexes, mean percentage of mature fish rose with increasing dietary protein level, the percentage of mature males being higher than that of the females. Similarly, the percentage of mature fish rose with the increasing age of fish, with more than 50% males and females mature at the age of 22 and 24 weeks, respectively, the exception being the 25% protein diet fed fi sh, where the percentage of mature fish was below 50%. In all treatments, s pawning was initiated when the fish were 22 weeks old. Smallest size at spa wning of males and females was 9.2 cm (13.1 g) and 8.1 cm (8.9 g), respecti vely, and dietary protein levels influenced the size of fish at first matur ity. For both sexes, no difference was found in the gonado-somatic index (G SI) among different treatments. Fecundity increased with increasing dietary protein levels, but significant differences were found only between 40-45% and 25-35% dietary protein levels. The relative fecundity (eggs g(-)1 fema le) was higher at the lower dietary protein levels (25-35%) than at the hig her dietary protein levels (40-45%). The dietary protein levels did not hav e any significant influence on the size and weight of mature eggs. The chem ical composition of fish and mature ovaries was significantly influenced by the dietary protein level.