Investigating the use of stable isotope analysis of milk to infer seasonal trends in the diets and foraging habitats of female Antarctic fur seals

Abstract

The use of intrinsic biomarkers to infer the foraging ecology of marine predators has become a common alternative to traditional methods of estimating diets and foraging behavior. In this study we examined the ability of the stable isotope analysis (δ 15N and δ 13C) of milk to infer seasonal variations in the diets and foraging habitats of female Antarctic fur seals ( Arctocephalus gazella) breeding at Cape Shirreff, Livingston Island, Antarctica. We found that the stable nitrogen values of lipid-free milk were correlated with seasonal changes in female diet composition during the lactation period identified from scat collections. While we could not fully quantifying female diets using isotopic analysis, evidence from both scat collections and stable isotope analysis suggest that Antarctic krill ( Euphausia superba) remained a major dietary item throughout lactation. In addition, these two methods independently highlighted the increasing importance of fish and squid in the diets of females as the season progresses. Furthermore, the isotopic values of milk collected during the perinatal fast suggest that females may be foraging in high productivity areas north of the South Shetland Islands and consume a larger proportion of fish and/or squid prior to parturition. While more confirmatory studies are needed to refine these methods, our results suggests that the δ 15N and δ 13C values of lipid-free milk can be used as a proxy to describe the foraging ecology of female Antarctic fur seals prior to and across the lactation period.