Lipid normalization and stable isotope discrimination in Pacific walrus tissues

Analysis of stable carbon and nitrogen isotope values (delta C-13 and delta N-15) of animal tissues can provide important information about diet, physiology, and movements. Interpretation of delta C-13 and delta N-15 values, however, is influenced by factors such as sample lipid content, tissue-specific isotope discrimination, and tissue turnover rates, which are typically species-and tissue-specific. In this study, we generated lipid normalization models for delta C-13 and investigated the effects of chemical lipid extractions on delta C-13 and delta N-15 in Pacific walrus (Odobenus rosmarus divergens) muscle, liver, and skin. We also evaluated tissue-specific isotope discrimination in walrus muscle, liver, skin, and bone collagen. Mean delta C-13(lipid)-free of skin and bone collagen were similar, as were mean delta N-15 of muscle and liver. All other tissues differed significantly for both isotopes. Differences in delta C-13(lipid)-free and delta N-15 among tissues agreed with published estimates of marine mammal tissue-specific isotope discrimination factors, with the exception of skin. The results of this work will allow researchers to gain a clearer understanding of walrus diet and the structure of Arctic food webs, while also making it possible to directly compare the results of contemporary walrus isotope research with those of historic and paleoecological studies.