The correct interpretation of ecology studies based on stable-isotope analysis (SIA) require of good estimations of discrimination factors (13C and 15N). This study was conducted to provide discrimination factors in young of the year Atlantic Bluefin tuna, Thunnus thynnus, fed Sardinella sp. for three months. The influence of the fat content on carbon and nitrogen isotope ratio (13C and 15N) values, as well as prey-predator discrimination factors were assessed in white-muscle samples of bluefin tuna. Lipid extraction resulted in significantly increased 13C values. Four 13C and 15N estimates were obtained using all possible combinations of bulk and lipid-extracted muscle data from predator and prey. We suggest that, where specific experimental data are not available, the values proposed here for 13C (0.32‰) and 15N (1.46‰) be used in future ecology works based on SIA
