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

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 delta C-13 and delta N-15 of 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 N-15 enrichment from ingested milk to serum was 2.1 parts per thousand +/- 0.6 parts per thousand and delta N-15 at the root of the vibrissae (representing current growth) were not significantly different from serum values. Milk was enriched for mean C-13 by 5.0 parts per thousand +/- 1.0 parts per thousand and 7.3 parts per thousand +/- 1.2 parts per thousand relative to serum and vibrissal roots, respectively, which was due to the presence of C-13-depleted lipids in milk. This was confirmed by lipid extraction from a subset of milk and serum samples, resulting in a 5.8 parts per thousand +/- 1.0 parts per thousand change only in milk. This study established that vibrissal roots and serum are reflective of a milk diet with approximately 2.0 parts per thousand N-15 enrichment, and vibrissal roots reflect serum and lipid-extracted milk values with approximately 2.0 parts per thousand C-13 enrichment. These discrimination factors are important to establish for stable isotope studies assessing diet shifts.