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

Abstract

We measured stable carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 14 N) 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 (δ 13 C: -20.3 ± 0.7‰; δ 15 N: 13.0 ± 0.4‰, n = 33) and these tissues ranged for nitrogen from +1.7 to +3.1‰ for blood and liver, respectively, and for carbon from +0.6 to +3.2‰ 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 δ 13 C: 0.21 to 0.57‰, for δ 15 N: 0.27 to 0.45‰) but showed some evidence of being inversely correlated. Resume : Nous avons mesure des rapports d'isotopes stables pour le carbone ( 13 C/ 12 C) et l'azote ( 15 N/ 14 N) dans les griffes, les moustaches, la peau, le poil et le sang de phoques captifs, soit des phoques du Groenland (Pagophilus groenlandicus ), des phoques communs (Phoca vitulina) et des phoques annele (Phoca hispida), soumis a un regime alimentaire constant de hareng (Clupea harengus) pendant au moins 2 ans. De plus, on a mesure les rapports isotopiques dans le muscle et le foie de deux phoques du Groenland, ainsi que dans les poumons, les reins, le coeur et la rate d'un phoque du Groenland mort en captivite. Les plages des valeurs de separation isotopique observees pour le hareng du regime alimentaire (δ 13 C : -20,3 ± 0,7‰; δ 15 N : 13,0 ± 0,4‰, n = 33) et pour ces tissus etaient comprises entre +1,7 et 3,1‰ (sang et foie, respectivement),