Interactions between salinity, dietary carbohydrate source and carbohydrate concentration on the digestibility of macronutrients and energy in rainbow trout (Oncorhynchus mykiss)

The aim of the experiment was to determine if salinity, dietary carbohydrate source, and dietary carbohydrate concentration affected the digestibility of macronutrients and energy in rainbow trout, and also to determine if there were interactions between these factors. The trout, reared in freshwater or saltwater, were fed four diets, containing moderate (100 g kg(-1)) or high (200 g kg(-1)) concentrations of glucose or maize dextrin as partial replacements for fish meal. Faecal samples were obtained by stripping and apparent digestibility coefficients (ADC) of macronutrients and energy were estimated using the indirect method with yttrium oxide, Y2O3, as an inert marker. The ADCs of organic material, fat, carbohydrate and energy were significantly higher in freshwater than in saltwater. The ADCs of organic material, carbohydrate and energy were higher for the diets with glucose than for dextrin. The diets with 100 g carbohydrate kg(-1) resulted in higher ADCs of protein and fat than diets with 200 g kg(-1). The opposite was found for the ADC of carbohydrate. This is ascribed to the fish meal used in the diets being contaminated with starch. The ADCs of fat, organic material and energy were negatively related to the concentration of indigestible carbohydrate from dextrin and starch in the diets, while that of protein was not. Significant interactions were seen between salinity and dietary carbohydrate source for the ADC of protein, and between carbohydrate source and concentration for ADCs of organic material, fat and energy. (C) 1998 Elsevier Science B.V. All rights reserved.