DIGESTION AND DIGESTIBILITY IN GILTHEAD SEA BREAM (SPARUS-AURATA) - THE EFFECT OF DIET COMPOSITION AND RATION SIZE

We have studied the effects of diet composition and ration size on the apparent digestibility coefficient (ADC) of C, N, P and dry matter (D M) in gilthead sea bream weighing 10-25 g. Several methods of collecti on (intestinal dissection, stripping and sampling of voided faeces) we re used in order to calculate the ADC from several regions of the inte stine. Chromic oxide was used as a marker. Except for phosphorus, ADC values increased along the intestine. Most of the absorptive process h ad been accomplished by the time the nutrients reached the anterior re gion of the intestine for samples taken between 6 and 8 h after feedin g. For all diets, high correlations were found between N, C and DM dig estibilities, whereas P ADC showed low or no correlation with the othe r variables studied. According to correlation equations, ADC values us ually 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 f or all the elements and DM tended to be higher when the carbohydrate/p rotein ratio was lower, although such differences were usually very sm all. The effect of ration level was correlated with diet type. Increas ed daily rations produced a decrease in the ADCs in all intestinal sam ples 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 a nterior region of the intestine and did not affect ADC values for post erior 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. (C) 1998 Elsevier Science B.V. All rights reserved.