Effect of dietary carbohydrate levels on growth, body composition and glycaemia in rainbow trout, Oncorhynchus mykiss, reared in seawater

Abstract

Three experimental diets were fed to near-satiation twice a day for 9 weeks to triplicate groups of 40 rainbow trout (mean initial weight 230 g) reared in seawater (36.5 ppt) at 9°C. The diets were formulated to have the same levels of nitrogen (crude protein 43% DM) and energy (digestible energy: around 15 kJ/g DM) but they contained different amounts of digestible carbohydrate (8–24.4%) and fat (5.7–11.1%). Digestibility was measured using chromic oxide as a marker. Plasma glucose concentrations were measured in 48-h fasted trout and at selected intervals after a single meal. Digestibility of starch and energy was lowest in trout fed the diet containing the highest level of crude starch. Apparent digestibility values of protein (86.0–87.4%) and fat (76.8–81.5%) were similar for the three diets. Fish growth and body composition were not affected significantly ( P > 0.05) by the non-protein energy sources of the diets. However, the amounts of digestible protein and digestible energy required per kilogram production were higher for fish fed the high-carbohydrate diet (475 ± 7 g DP and 18.8 ± 0.3 MJ DE) than for fish fed the high-fat diet (419 ± 21 g DP and 16.0 ± 0.8 MJ DE). Liver size and liver lipid content increased in direct relationship to the dietary level of digestible carbohydrate. The highest percentage of muscle lipid was found in fish fed the diet containing 25% carbohydrate and 10% lipid as non-protein energy sources. Glycaemia was increased by elevated digestible carbohy-drate intake. In conclusion, 230 g rainbow trout were able to utilize carbohydrate efficiently for growth in seawater (up to 25% of digestible carbohydrate in diet) under the conditions of this study. However, dietary lipid had a slightly better protein-sparing effect than digestible carbohydrate at a similar level of digestible energy intake. High dietary levels of digestible carbohydrate induced lipogenesis and enhanced lipid deposition in the liver.