Evidence of demersal foraging from stable isotope analysis of juvenile elephant seals from Patagonia

Abstract

Diet information of elephant seals from Patagonia is poor, especially in juveniles. Juveniles use the same oceanic foraging grounds of adults, but some remain extensively in shallow waters of the Patagonian Shelf (PS), where demersal resources are abundant. We measured d13C and d15N on vibrissae of juveniles from Patagonia (Argentina), that were satellite tracked at sea, to see if differences in the spatial pattern are reflected in the trophic preferences. Isotopic data of ~65 marine species from foraging grounds were used to analyze trophic association. A first analysis with 5 preys reported in the diet, showed that demersal neritic fish Dissostichus eleginoides was highly important in the diet of seals that foraged exclusively on the PS (22-92%). A principal component analysis contrasted a group of ?shelf user? seals and neritic organisms (mainly fish) characterized by high isotopic values and trophic level, with a group of seals foraging in open ocean (high and mid latitude) and oceanic organisms of low trophic level, mainly invertebrates and small pelagic fish (Principal Factor: 91% variance, eigenvalue:2.7). We found trophic strategies involving different foraging areas and potentially different prey types, which could be a way to deal with intraspecific competence within a vulnerable age class.