New insight into the trophic biology of age-0 Atlantic bluefin tuna in the western Mediterranean using stomach content and stable isotope analyses

The trophic biology of age-0 Thunnus thynnus (ABFT) was investigated at two locations off the Iberian Mediterranean coast (Cambrils and Valencia) between 2012 and 2014 using stomach content analysis (SCA), and stable isotope analyses of bulk-tissue (BSIA) and individual amino acids (AA-CSIA). Interannual and geographic differences were observed in diet and isotopic compositions. Although the diet was diverse, the major food resources were by far anchovy (%W = 77.04 and 47.76 for Cambrils and Valencia, respectively) and clupeids (%W = 18.97 and 25.31, respectively). SCA and BSIA results suggested a wider trophic niche for ABFT from Valencia, where the proportions of prey items were more homogeneous. Beside differences in diet composition, bulk and amino acid (AA) isotopic spatial and temporal variations may also reflect shifts in the isotopic baselines. δ13C values of essential AAs may be good tracers of carbon sources, though clear patterns relative to primary consumers were not found. Patterns of δ15N values of trophic relative to source AAs reflected differential fractionation rates, underpinning their usefulness as trophic position (TP) indicators. TPs estimated from the three analyses lay about 4. TP values calculated from muscle samples collected in 2013 using the AA pair glutamic acid and phenylalanine (3.63 and 4.03 for Cambrils and Valencia, respectively) were close to, though lower than, TP estimates derived from SCA (4 and 4.05, respectively) and BSIA (4.39 and 4.63, respectively).