Discrimination of carbon and nitrogen isotopes from milk to serum and vibrissae in Alaska Steller sea lions (Eumetopias jubatus)

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Abstract
Knowledge of diet–tissue stable isotope discrimination is required to properly interpret stable isotope values and to identify possible diet shifts, such as might be expected from nursing through weaning. This study compared δ13C and δ15Ν οf paired serum and vibrissal roots with those of ingested milk (n = 52) from free-ranging Steller sea lion ( Eumetopias jubatus (Schreber, 1776)) pups (1–11 months) and juveniles (14–27 months) to estimate diet–tissue discrimination. Mean 15N enrichment from ingested milk to serum was 2.1‰ ± 0.6‰ and δ15Ν at the root of the vibrissae (representing current growth) were not significantly different from serum values. Milk was enriched for mean 13C by 5.0‰ ± 1.0‰ and 7.3‰ ± 1.2‰ relative to serum and vibrissal roots, respectively, which was due to the presence of 13C-depleted lipids in milk. This was confirmed by lipid extraction from a subset of milk and serum samples, resulting in a 5.8‰ ± 1.0‰ change only in milk. This study established that vibrissal roots and serum are reflective of a milk diet with approximately 2.0‰ 15N enrichment, and vibrissal roots reflect serum and lipid-extracted milk values with approximately 2.0‰ 13C enrichment. These discrimination factors are important to establish for stable isotope studies assessing diet shifts.

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