Carbohydrate in rainbow trout diets. II. Influence of carbohydrate levels on chemical composition and feed utilization of fish from different families

Groups of rainbow trout fingerlings from ten different families were sampled after 12 and 24 weeks feeding on each of three diets which were similar in protein and energy content but had different percentages of metabolizable energy present as carbohydrate: 17, 25 and 38%. Fish fed different diets had the same dressed carcass weights, expressed as percentage of body weight, but fish fed higher carbohydrate diets had heavier livers and a higher percentage of discoloured livers. Chemical analyses of fish bodies showed lower fat and energy but higher protein and ash content for those fed higher carbohydrate diets. Livers contained more fat and carbohydrate but the same dry matter, and faeces showed less dry matter, more protein and the same energy content in fish with higher dietary carbohydrate. Utilization of dietary energy and protein was poorer in fish fed high levels of carbohydrate, but health of all fish appeared good. Little variation between different fish families, and no interaction between fish family and diet was found for any of the above parameters, indicating that prospects for change through selective breeding are poor.