INTERACTIONS BETWEEN SALINITY, DIETARY CARBOHYDRATE SOURCE AND CARBOHYDRATE CONCENTRATION ON THE DIGESTIBILITY OF MACRONUTRIENTS AND ENERGYIN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The aim of the experiment was to determine if salinity, dietary carboh ydrate source, and dietary carbohydrate concentration affected the dig estibility 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 mo derate (100 g kg(-1)) or high (200 g kg(-1)) concentrations of glucose or maize dextrin as partial replacements for fish meal. Faecal sample s were obtained by stripping and apparent digestibility coefficients ( ADC) of macronutrients and energy were estimated using the indirect me thod with yttrium oxide, Y2O3, as an inert marker. The ADCs of organic material, fat, carbohydrate and energy were significantly higher in f reshwater than in saltwater. The ADCs of organic material, carbohydrat e 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 pr otein 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 th e diets being contaminated with starch. The ADCs of fat, organic mater ial and energy were negatively related to the concentration of indiges tible 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 ca rbohydrate source and concentration for ADCs of organic material, fat and energy. (C) 1998 Elsevier Science B.V. All rights reserved.