Tissue-Specific Stable Isotope Fractionation in Subarctic Pinniped Species

Tissue-Specific Stable Isotope Fractionation in Subarctic Pinniped Species

Alvin Sherman Library

Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios are widely used to evaluate marine mammal diet, trophic position, and nutrient assimilation. However, interpretation is often confounded by tissue-specific isotopic discrimination, which alters isotope ratios during nutrient assimilation and tissue synthesis. Quantifying these offsets is essential for ensuring accurate trophic reconstructions across multiple tissues with different biochemical compositions. This study measured δ¹³ and δ¹⁵N values in muscle, blood, blubber, brain, collagen, kidney, liver, skin, fur, and vibrissae of Steller sea lions (Eumentopias jubatus), harbor seals (Phoca vitulina), and northern fur seals (Callorhinus ursinus). Samples from each species were collected from subsistence harvests in regions of the Bering and Gulf of Alaska between 1975 and 1997. Mean δ¹⁵N values varied less than 1% across all tissues and species, indicating minimal nitrogen fractionation and suggesting that δ¹⁵N values from most tissues provide comparable information for trophic-level estimates. In contrast, δ¹³C fractionation exhibited pronounced tissue and species-specific patterns. Lipid-rich tissues, particularly blubber, showed substantial δ¹³C depletion relative to muscle, consistent with the biochemical discrimination against ¹³C during lipid synthesis. Enrichment gradients from blubber toward keratinized tissues were evident in Steller sea lions, harbor seals, and northern fur seals. These results highlight the necessity of incorporating tissue-specific δ¹³C discrimination factors when interpreting isotope data. Accounting for these offsets will improve the reliability of trophic analyses, ecological reconstructions, and physiological assessments derived from stable isotope measurements.