Ontogenetic dietary changes in male South American fur seals Arctocephalus australis in Patagonia

Abstract

This study assessed ontogenetic dietary changes in male South American fur sealsArctocephalus australis in northern and central Patagonia (Argentina) using stable isotope ratios (δ15N and δ13C) in vibrissae and bones. Sucking pups were characterised by higher δ15N values and lower δ13C values than older specimens. Weaning was associated with a marked drop of δ15N values, both in bone and vibrissae. Such a drop was inconsistent with the consumption of local prey and may reveal movement to distant foraging grounds or physiological changes associatedwith either fasting or rapid growth. Stable isotope ratios indicated that juveniles fed more pelagically than subadults and adults, but that there were no major differences between the 2 latter age categories. As subadults and adults are rather similar in body mass and are much larger than juveniles, body mass may play a role in the ontogenetic dietary changes reported. Nevertheless, demersal benthic prey were always scarce in the diet of male fur seals, which relied primarily onArgentine shortfin squid and small pelagic fish throughout life, though adults also consumed large amounts of decapod crustaceans available at shallow depths. Vibrissae did not reveal regular oscillations of δ15N or δ13C, except in 1 individual. Thus, male fur seals from northern and central Patagonia do not appear to migrate regularly between isotopically distinct areas, although nomadic displacements cannot be ruled out.