| This study was conducted to determine the optimal dietary carbohydrate/lipid ratios from two aspects of growth and immunity of juvenile yellow catfish, and further explore the protein-sparing effect induced by dietary carbohydrate/lipid ratios. The results may provide theoretical basis for optimizing the formula feed for juvenile yellow catfish. This study mainly includes three parts presented as follows:Series 1 Effects of dietary carbohydrate/lipid ratios on growth performance, body composition and metabolism of juvenile yellow catfish (Pelteobagrus fulvidraco)This study aimed to investigate the effects of dietary carbohydrate/lipid (CHO:L) ratios on growth performance, body composition, carbohydrate metabolism and lipid metabolism of juvenile yellow catfish(Pelteobagrus fulvidraco) (average initial weight: 0.90±0.02 g). Fish were fed by six isonitrogenous and isoenergetic diets (the CHO:L ratios of 1.11,1.67,2.45,3.63,5.58,9.50) in cages for 8 weeks. The results indicated that the best weight gain, specific growth rate, protein efficiency ratio and nitrogen and energy retention efficiency were all observed in fish fed the diet with a CHO:L ratio of 3.63. Hepatosomatic index, viscerosomatic index, whole-body moisture contents, and liver and muscle glycogen contents all increased significantly (P<0.05) as dietary CHO:L ratios increased, while lipid contents of whole-body, liver and muscle all decreased significantly (P<0.05) with increasing CHO:L ratios. Contrary to lipase activities, stomach and intestine amylase activities increased significantly (P<0.05) as dietary CHO:L ratios increased. The stomach protease activity increased significantly (P<0.05) as dietary CHO:L ratios ranged from 1.11 to 3.63, but decreased significantly with the further increasing CHO:L ratios. In addition, the activities of pyruvate kinase, malate dehydrogenase and glucose-6-phosphate dehydrogenase all increased significantly (P<0.05) as dietary CHO:L ratios increased. Plasma lipid contents, lipoproteinlipase and hepatic lipase all decreased significantly as dietary CHO:L ratios increased (P<0.05). Based on the second-order regression analysis of weight growth ratio, the optimal dietary carbohydrate and lipid contents for yellow catfish were 29.16% and 7.94%, respectively, with a corresponding dietary CHO:L ratio of 3.67.Series 2 Effects of dietary carbohydrate/lipid ratios on non-specific immune responses, oxidative status and liver histology of juvenile yellow catfish (Pelteobagrus fulvidraco)The experimental design was the same as study one. The results indicated that the leucocyte count, plasma globulin content and lysozyme activity offish fed the 5.58 CHO:L diet were significantly (P>0.05) higher than those of fish fed 1.11 and 1.67 CHO:L diets. A similar trend was observed in plasma alternative complement (ACH50) pathway activity, while the erythrocyte count decreased significantly (P<0.05) as the dietary CHO:L ratios increased from 1.11 to 2.45. Contrary to reduced glutathione contents, catalase, glutathione peroxidase activities and malondialdehyde contents in the liver decreased significantly (P<0.05) as dietary CHO:L ratios increased froml.11 to 5.58. A similar trend was observed in total superoxide dismutase (P<0.05). Activities of plasma aspartate aminotransferase and alanineaminotransferase and alkalinephosphatase decreased significantly (P<0.05) as dietary CHO:L ratios increased from 1.11 to 5.58. Liver histology was normal in fish fed 2.45,3.63 and 5.58 CHO:L diets. However, hepatic steatosis scores and hepatocyte apoptosis rates of fish fed 1.11 and 1.67 CHO:L diets were significantly (P<0.05) higher than those of fish fed with other diets. On the whole, the diets with CHO:L ratios of 2.45-5.58 (approximately 24.5-33.5% carbohydrate and 6-10% lipid) are optimal for physiological condition and health status of juvenile yellow catfish.Series 3 Effects of dietary carbohydrate/lipid ratios and protein levels on growth performance, body composition and digestive enzymes activities of juvenile yellow catfish (Pelteobagrus fulvidraco)This study was conducted to investigate the effects of dietary carbohydrate/lipid (2.65, 3.63,5.58) and protein levels (37%,40%), and their interaction on growth performance, body composition, and digestive enzymes activities of juvenile yellow catfish Pelteobagrus fulvidraco(average initial weight:0.78±0.03 g). Fish were randomly distributed into 18 aquariums and fed three times daily for 8 weeks.The results indicated that final weight, weight gain, specific growth rate, protein efficiency ratio and energy retention all increased significantly (P<0.05) as dietary protein levels increased. The best final weight, weight gain and specific growth rate were all observed in P40/R3.63 group, but showed little difference (P>0.05) with the group of P37/R3.63. Feed conversion ratio, hepatosomatic index and nitrogen retention decreased significantly (P<0.05) as dietary protein levels increased. Whole-body and dressed carcass moisture contents increased significantly (P<0.05) as dietary CHO:L ratios increased, while lipid contents of whole-body and dressed carcass all decreased significantly (P<0.05) with increasing CHO:L ratios. Contrary to stomach amylase activities, stomach and intestine lipase activities decreased significantly (P<0.05) as dietary CHO:L ratios increased. The stomach protease activities increased significantly (P<0.05) as dietary protein levels increased. In addition, no interaction was observed between carbohydrate/lipid and protein levels of all above indexes. In all, an optimal carbohydrate/lipid (3.63) could spare protein in yellow catfish. |
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