الفهرس 
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Abstract
This study was conducted at the Aquaculture Department, Faculty of Aquaculture, Aquatic and Fisheries Sciences, Kafrelsheikh University. The present study investigated the effect of different dietary carbohydrate: protein ratios without or with biotin supplementation on the growth performance, blood biochemical, intestinal morphology and expression of some growth and fat metabolism related genes in Nile tilapia fingerling.
Two hundred and seventy Nile Tilapia fingerlings (an average body weight 1.9 g) 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 obtained results are summarized in the following:
• Different dietary CHO: CP ratios, biotin supplementation and the interaction between them (P ≤ 0.05) significantly affected the final BW, WG, FI, FCR, PER, EEU throughout the first 6 weeks of experiment.
• Feeding of fish on high CHO: CP ratio (57:21) containing diet significantly reduced the fish BW, WG and FI (P ≤ 0.05), while improved the feed efficiency parameters including FCR, PER, EEU from the second period (2-4 weeks) when compared to medium ratio (47:32) and low CHO: CP ratio (36:42) containing diets during the period 0-6w.
• Fish fed on the low CHO:CP ratio diet (36:42) showed no significant difference in the previously mentioned parameters when compared with those fed the medium ratio (P ≤ 0.05).
• Biotin addition to the high or the medium ratios containing diets resulted in lower growth performance (BW and WG), increased FI and deteriorated the feed efficiency parameters when compared to their reference groups fed on the biotin free diets (P ≤ 0.05), while had no significant effect when added to the low ratio (high CP/ low CHO) containing diet.
• Fish fed on the low ratio (high CP/ low CHO) containing diets without or with biotin supplementation showed the highest BW obtained when compared with other groups.
• Serum total protein and globulin concentration were significantly increased in fish fed on high CHO: CP ratio (57:21) containing diet supplemented with biotin compared with the other experimental groups.
• Feeding fish on the high CHO: CP ratio (57:21) containing diet without or with biotin showed significantly lower serum glucose level when compared with those fed the medium ratio.
• Serum TG and VLDL were significantly increased (P≤ 0.05) in fish fed on the high CHO: CP ratio (57:21) containing diet, while were numerically reduced in those fed the low ratio (36:42) containing diet compared to the medium ratio (47:32) (P≥ 0.05).
• Biotin addition resulted in lower serum TG and VLDL concentrations (P≤ 0.05) in fish fed the high ratio diet compared to the same diet without biotin supplementation.
• Feeding fish on the high CHO:CP ratio diet (57:21) significantly increased activities of GPT and GOT enzymes (P≤ 0.05) compared to medium CHO:CP ratio diet (47:32) or the low CHO: CP ratio (36:42).
• Biotin supplementation to the high CHO:CP ratio diet (57:21) resulted in lower activities of GPT and GOT enzymes (P≤ 0.05), while increased them when added to the low CHO: CP ratio (36:42).
• Fish fed on the high CHO:CP ratio diet (57:21) containing diet without or with biotin showed lower moisture content when compared with the other experimental groups.
• Whole body CP content showed no significant difference among groups however, it was increased with the high (57:21) and low ratios (36:42) supplemented without or with biotin when compared with the medium CHO:CP ratio diet
• Ash content in fish body was significantly increased in fish fed on the high CHO:CP ratio (57:21) containing diet when compared with the medium CHO:CP ratio (47:32) or the low ratio (36:42).
• EE content was significantly increased in fish fed on the high CHO:CP ratio (57:21) supplemented with biotin when compared with the medium CHO:CP ratio (47:32) or the low ratio (36:42) containing diets (P≤ 0.05) supplemented with biotin.
• The whole-body composition of nutrients showed no difference in response to biotin supplementation; however, CP and EE and ash contents were non-significantly reduced in the biotin supplemented fish compared to their reference fish except for fish fed the high CHO diet had higher EE content.
• Villus height was significantly increased in fish fed on the high CHO: CP ratio (57:21) containing diet when compared to the medium ratio or the low ratios .
• Biotin supplementation significantly reduced the villus height in the medium and high CHO: CP ratios containing diets, while increased it in the low CHO: CP ratio containing diets when compared to their reference groups without biotin addition.
• The morphology in the middle part of intestine, non-significant difference was observed in the different morphometric parameters (villus height, villus width, crypt depth and VH/CD ratio) among the different groups (P≤ 0.05).
• Villus height and width in the posterior part of intestine were significantly increased in fish fed the high (57:21) or low CHO: CP (36:42) containing diets either biotin supplemented or not compared to their medium ratio containing groups (P≤ 0.05).
• The relative mRNA expression of GHR and IGF was significantly upregulated in fish fed on the low (high CP) and the high ratios (high CHO) containing diets respectively when compared with those fed the medium ratio (P≤ 0.05).
• A significant upregulation of FABP and CD36 expression was observed in fish fed on the medium ratio supplemented with biotin when compared with other groups.
• Higher expression of CD36 was observed in fish fed on low or high ratios containing diets respectively than the medium ratio diet with the highest expression level obtained in those fed the low ratio (high CP diet) (P≤ 0.05).