Abstract
The current experiment evaluated the effects of different dietary carbohydrate: protein ratios (CHO: CP ratio) without or with biotin supplementation on the growth performance, body composition, serum biochemical parameters and intestinal morphology in Nile tilapia. Fingerlings were assigned to 6 treatments arranged as following; Group 1 (G1) fed the basal diet (BD) with a medium CHO: CP ratio (47:32); G2 with high CHO: CP ratio (57:21); G3 with low CHO: CP ratio (36:42); while G4, G5 and G6 had the same previous experimental design of the first 3 groups but with biotin supplementation (0.06 mg/kg diet). The growth performance was only assessed for the first 6 weeks, then the experiment continued to 4 months and ended by sampling for other parameters determined. Fish feeding on high CHO: CP ratio (57:21) significantly reduced the fish body weight, gain and feed intake (FI) (P ≤ 0.05), while improved the feed efficiency compared to the medium and low ratios. Biotin addition to the high or the medium ratios containing diets resulted in lower growth, increased FI and deteriorated the feed efficiency when compared to their control groups without biotin addition (P≤ 0.05). Different dietary CHO: CP ratios significantly altered tilapia body composition of nutrients (moisture, EE, and ash), serum parameters (glucose, triglyceride, LDL and VLDL) and the liver function enzymes (GPT and GOT) (P≤ 0.05). The relative mRNA expression of GHR and IGF was significantly upregulated in fish fed on the low and the high ratios containing diets respectively. In conclusion, feeding of the early fingerling of tilapia on the high CHO: CP ratio during the first 6 weeks of life resulted in lower growth, however, it showed a protein sparing effect. Supplementation of biotin (extra over the diet) was not recommended as it showed adverse effects on tilapia growth especially during the first periods of life, however it showed some beneficial effects on alleviating the high-dietary carbohydrate-induced effects. Feeding on medium CHO: CP ratio (47:32) or low ratio (36:42) would be recommended during the fry and early fingerling stage of tilapia while the high ratio (57:21) would be beneficial in the diet of late fingerlings or adult tilapia.

Key words: Nile tilapia, carbohydrate: protein ratio, biotin, performance