New insight into the trophic biology of age-0 Atlantic blue¿n 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 base-lines. d 13 C values of essential AAs may be good tracers of carbon sources, though clear patterns relative to primary consumers were not found. Patterns of d 15 N 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)

Determining natal origin for improved management of Atlantic bluefin tuna

Effective sustainable management of marine fisheries requires that assessed management units (that is, fish stocks) correspond to biological populations. This issue has long been discussed in the context of Atlantic bluefin tuna (ABFT, Thunnus thynnus) management, which currently considers two unmixed stocks but does not take into account how individuals born in each of the two main spawning grounds (Gulf of Mexico and Mediterranean Sea) mix in feeding aggregations throughout the Atlantic Ocean. Using thousands of genome-wide molecular markers obtained from larvae and young of the year collected at the species’ main spawning grounds, we provide what is, to the best of our knowledge, the first direct genetic evidence for “natal homing” in ABFT. This has facilitated the development of an accurate, cost-effective, and non-invasive tool for tracing the genetic origin of ABFT that allows for the assignment of catches to their population of origin, which is crucial for ensuring that ABFT management is based on biologically meaningful stock units rather than simply on catch location.S