Discrimination of stable isotopes in fin whale tissues and application to diet assessment in cetaceans

RATIONALE In stable isotope research, the use of accurate, species-specific diet-tissue discrimination factors (i.e., ?13C and ?15N) 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 (d13C and d15N values) in muscle from 65 aged fin whales remained constant through age classes (465 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 ?15N values between krill and whale tissues ranged from 2.04 in bone protein to 4.27 parts per thousand in brain, and those of ?13C ranged from 1.28 in skin to 3.11 parts per thousand 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. Copyright (c) 2012 John Wiley & Sons, Ltd.