Natural abundance of N-15 and C-13 in fish tissues and the use of stable isotopes as dietary protein tracers in rainbow trout and gilthead sea bream
Résumé
For developing efficient diets, two sets of experiments examined whether the use and allocation of dietary protein can be traced by labelling with stable isotopes (N-15 and C-13) in two culture fish (Oncorhynchus mykiss and Sparus aurata). In the first experiment, natural abundance and tissue distribution of these isotopes were determined, by measuring the delta C-13 and delta N-15 values by isotopic ratio mass spectrometry, in fingerlings (14-17 g) adapted to diets differing in the percentage of fish meal replacement by plant protein sources. For both species, delta N-15 and delta C-13 were greater in tissues with higher protein and lower lipid content. Delta N-15 of diets and tissues decreased as replacement increased, suggesting delta N-15 can be used as a marker for dietary protein origin. The N-15 fractionation (delta N-15 fish - delta N-15 diet) differed between groups, and could thus be used to indicate protein catabolism. In the second experiment, fish (75-90 g) of each species ingested a diet enriched with N-15-protein (10 g kg(-1) diet) and C-13-protein (30 g kg(-1) diet). These proportions were suitable for determining that the delta values of tissue components were high enough above natural levels to allow protein allocation to be traced at 11 and 24 h after feeding, and revealed clear metabolic differences between species.