Stable carbon and nitrogen isotopic fractionation between diet and tissues of captive seals: Implications for dietary reconstructions involving marine mammals

We measured stable carbon (C-13/C-12) and nitrogen (N-15/N-14) isotope ratios in nail, whiskers, skin, hair, and blood of captive harp seals (Pagophilus groenlandicus), harbour seals (Phoca vitulina), and ringed seals (Phoca hispida) held on a constant diet of herring (Clupea harengus) for at least 2 years. In addition, isotope ratios were measured in the muscle and liver of two harp seals, and in the lung, kidney, heart, and spleen of a single harp seal that had died in captivity. Isotopic fractionation values between dietary herring (delta(13)C: -20.3 +/- 0.7 parts per thousand; delta(15)N: 13.0 +/- 0.4 parts per thousand, n = 33) and these tissues ranged for nitrogen from +1.7 to +3.1 parts per thousand for blood and liver, respectively, and for carbon from +0.6 to +3.2 parts per thousand for liver and whiskers, respectively. No differences in isotopic fractionation values among species or age groups were detected. These values will permit more accurate dietary reconstructions on the basis of isotopic analysis of the tissues of seals and other marine mammals. For two captive harp seals, carbon and nitrogen isotope values showed small variation along the length of six whiskers (range of standard deviations for delta(13)C: 0.21 to 0.57 parts per thousand, for delta(15)N: 0.27 to 0.45 parts per thousand) but showed some evidence of being inversely correlated.