Digestion and digestibility in gilthead sea bream ( Sparus aurata): the effect of diet composition and ration size

Abstract

We have studied the effects of diet composition and ration size on the apparent digestibility coefficient (ADC) of C, N, P and dry matter (DM) in gilthead sea bream weighing 10–25 g. Several methods of collection (intestinal dissection, stripping and sampling of voided faeces) were used in order to calculate the ADC from several regions of the intestine. Chromic oxide was used as a marker. Except for phosphorus, ADC values increased along the intestine. Most of the absorptive process had been accomplished by the time the nutrients reached the anterior region of the intestine for samples taken between 6 and 8 h after feeding. For all diets, high correlations were found between N, C and DM digestibilities, whereas P ADC showed low or no correlation with the other variables studied. According to correlation equations, ADC values usually ranked in the order N>C>DM>P. Diets made with brown whole fish and trash fish meal (IND diets) gave lower ADC values than diets made with capelin meal (NOR diets) and for both diet types, the ADCs for all the elements and DM tended to be higher when the carbohydrate/protein ratio was lower, although such differences were usually very small. The effect of ration level was correlated with diet type. Increased daily rations produced a decrease in the ADCs in all intestinal samples in the case of IND diets, mainly in those with higher C/N ratios; but in the case of NOR diets, such a decrease was restricted to the anterior region of the intestine and did not affect ADC values for posterior intestine or faeces even in the case of NOR diets with high C/N ratios and a high content (up to 21.4%) of gelatinized corn meal.