EFFECT OF DIETARY CARBOHYDRATE-LEVELS ON GROWTH, BODY-COMPOSITION ANDGLYCEMIA IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS, REARED IN SEAWATER

Three experimental diets were fed to near-satiation twice a day for 9 weeks to triplicate groups of 40 rainbow trout (mean initial weight 23 0 g) reared in seawater (36.5 ppt) at 9-degrees-C. The diets were form ulated to have the same levels of nitrogen (crude protein 43% DM) and energy (digestible energy: around 15 kJ/g DM) but they contained diffe rent amounts of digestible carbohydrate (8-24.4%) and fat (5.7-11.1%). Digestibility was measured using chromic oxide as a marker. Plasma gl ucose concentrations were measured in 48-h fasted trout and at selecte d intervals after a single meal. Digestibility of starch and energy wa s lowest in trout fed the diet containing the highest level of crude s tarch. 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 com position were not affected significantly (P> 0.05) by the non-protein energy sources of the diets. However, the amounts of digestible protei n and digestible energy required per kilogram production were higher f or 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 direc t relationship to the dietary level of digestible carbohydrate. The hi ghest percentage of muscle lipid was found in fish fed the diet contai ning 25% carbohydrate and 10% lipid as non-protein energy sources. Gly caemia was increased by elevated digestible carbohydrate intake. In co nclusion, 230 g rainbow trout were able to utilize carbohydrate effici ently 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 a t a similar level of digestible energy intake. High dietary levels of digestible carbohydrate induced lipogenesis and enhanced lipid deposit ion in the liver.