Digestibility, postprandial ammonia excretion and selected plasma metabolites in European sea bass (Dicentrarchus labrax) fed pelleted or extruded diets with or without wheat gluten

Abstract

The effects of diet processing technology on digestibility, postprandial ammonia excretion rates and plasma concentration of ammonia, glucose and cholesterol (CHOL) were determined in European sea bass. Four dietary treatments were compared: diet E0 (extruded basal fish meal diet); diet P0 (pelleted basal fish meal diet); diet E30 (extruded, 70% basal diet plus 30% wheat gluten) and diet P30 (pelleted, 70% basal diet plus 30% wheat gluten). Each dietary treatment was assigned to triplicate groups of fish (body weight: 200 g). Levels of ammonia-N excretion from fish fed the different diets were measured in water samples taken from culture tanks every 2 h. At the end of the experiment, plasma samples from 3 fish per diet, taken in alternative tanks every 2 h during 24 h, were assayed for ammonia, glucose and cholesterol determination. Nutrient digestibility was not affected by the diet processing methods used. Wheat gluten showed high apparent digestibility coefficient (ADC) values for protein, energy and organic matter. Postprandial ammonia excretion values showed maximum values between 4 and 10 h after the meal depending on treatment. Daily cumulative total ammonia excretion rates were lower in fish fed fish meal based diets than in those fed diets containing 30% wheat gluten. Irrespective of dietary formulation, fish fed extruded diets showed lower ammonia excretion than those fed the dry pelleted diets. Postprandial values for plasma ammonia ranged between 3 and 8 mg ml−1 and the pre-feeding values were reached 24 h after feeding in all treatment groups. Maximal levels of plasma glucose (120–180 mg ml−1) appeared within 6 to 10 h after feeding. Plasma cholesterol levels were lower in sea bass fed diets containing 30% wheat gluten than in those fed diets containing fish meal alone as the dietary protein source.