Effect of dietary protein-energy levels and fish initial sizes on growth rate, development and production of Nile tilapia, Oreochromis niloticus L.

The effects of dietary protein-energy levels on the growth rate, proximate composition and production were examined in Nile tilapia, Oreochromis niloticus, at two starting weights (22.9 and 39.8 g) reared in concrete ponds for 180 days. The highest weight gain (183.1 g) was obtained with fish fed a 30\% protein and 10.5 kJ g(-1) diet for the small initial size and 180.2 g for a diet containing 25\% protein and 12.6 kJ g(-1) for the large initial size. Dressed yields (edible mass) and fillets increased to 56.9\% and 52.5\% in fish fed diet with 25\% protein and 10.5 kJ g(-1) at the initial size of 22.9 g, while fish started at 39.8 g exhibited the best values (56.5\% and 52.1\%) when fed the 30\% protein and 10.5 kJ g(-1) diet. Proximate composition of soft tissue (wet weight basis) in small fish was significantly influenced by dietary protein-energy levels. Protein was 26.1 +/- 0.3\% in fish fed the high protein (30\%) and low energy (10.5 kJ g(-1) diet), while lipid content was 6.4 +/- 0.3\% at diet containing 20\% protein and 14.7 kJ g(-1) diet. Large initial size fish fed the diet with 25\% protein and 14.7 kJ g(-1) had the highest body protein (32.0 +/- 0.4\%) and lowest lipid content (2.2 +/- 0.3\%). Feed conversion ratio (FCR) and protein efficiency ratio varied with different dietary protein-energy levels and initial fish sizes. Feed conversion ratio increased with increasing protein and decreasing energy level in the diet, and values in small fish were higher than values in large fish. Protein efficiency ratio decreased with increasing dietary protein level and decreasing energy level. The maximum total production (7.6 tons feddan(-1)) was with dietary high protein (30\%) and low energy (10.5 kJ g(-1)) for small-sized fish, while large initial fish had the highest production (3.7 tons feddan(-1)) when fed the 25\% protein and 12.6 kJ g(-1) diet energy. Starting with 22.9 g fish was more advantageous than the initial size of 39.8 g for rearing Nile tilapia. Small fish required a high-protein and low-energy diet, whereas large fish required a low-protein and high-energy diet to achieve highest production.