| Summary: | RATIONALE In stable isotope research, the use of accurate, species‐specific diet‐tissue discrimination factors (i.e., Δ13C and Δ15N) is central to the estimation of trophic position relative to primary consumers and to the identification of the dietary sources of an individual. Previous research suggested that the diet of fin whales from the waters off northwestern Spain is overwhelmingly based on krill, thus permitting reliable calculation of discrimination values in this wild population. METHODS After confirming that the stable isotope ratios (δ13C and δ15N values) in muscle from 65 aged fin whales remained constant through age classes (4-65 years), the signatures were determined in muscle, bone protein, skin, liver, kidney, baleen plates and brain, as well as food (krill), from a subset of individuals to calculate discrimination factors. Signatures were determined by means of elemental analysis isotope ratio mass spectrometry (EA‐IRMS) using a ThermoFinnigan Flash 1112. RESULTS The isotopic values remained constant regardless of age. The mean Δ15N values between krill and whale tissues ranged from 2.04 in bone protein to 4.27¿ in brain, and those of Δ13C ranged from 1.28 in skin to 3.11¿ in bone protein. This variation was consistent with that found in other groups of mammals, and is attributed to variation in tissue composition and physiology. CONCLUSIONS Because discrimination factors are relatively constant between taxonomically close species, the results here obtained may be reliably extrapolated to other cetaceans to improve dietary reconstructions. The skin discrimination factors are of particular relevance to monitoring diet through biopsies or other non‐destructive sampling methods. The large difference in bone protein discrimination factors from those of other tissues should be taken into consideration when bone collagen is used to determine trophic level or to assess diet in paleodietary isotopic reconstructions.
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