To examine the effect of lactation and post‐weaning fast on stable isotope ratios in GS and NES, blood and hair were sampled from 21 GS mother‐pup pairs on the Isle of May and on 22 weaned NES pups at Año Nuevo State Reserve during their respective breeding seasons. Milk samples were also collected from GS mothers. Stable isotope measurements were performed with an isotope ratio mass spectrometer coupled to an N‐C elemental analyser.
Changes in stable isotope ratios in blood components during fasting were similar and weak between GS and NES mothers especially in blood cells (GS: Δ15N = 0.05‰, Δ13C = 0.02‰; NES: Δ15N = 0.1‰, Δ13C = 0.1‰). GS showed a 15N discrimination factor between maternal and pup blood cells and milk, but not for 13C. The strongest relationship between the isotopic compositions of the mother and the pup was observed in the blood cells.
Isotopic consequences of lactation, fasting, and growth seem limited in NES and GS, especially in medium‐term integrator tissues of feeding activity such as blood cells. Stable isotope ratios in the blood of pups and mothers are correlated. We observed a subtle mother‐to‐pup fractionation factor. Our results suggest that pup blood cells are mostly relevant for exploring the ecology of female seals.