Foraging strategies of female elephant seals from Península Valdés, Patagonia, inferred from whisker stable isotope signatures of their pups

The foraging strategies of gestating female elephant seals (<em>Mirounga leonina</em>) from Península Valdés, Patagonia, were assessed by analyzing the values of stable isotopes of carbon (C) and nitrogen (N) from whiskers of 60 weanlings as a proxy for maternal spatial niche utilization. The data were combined with isotopic values and at-sea satellite locations of juvenile seals and adult female satellite tracks to provide classifications of the likely foraging strategies of the mothers of the studied pups. Based on at-sea locations during the austral summer, females foraged in oceanic waters while juveniles foraged both in neritic and in oceanic habitats. Weanling isotopic values (n = 60 pups) ranged from -19.9 to -14.8‰ for C and from 10.6 to 18.9‰ for N. The degree of variation of spatial niche distribution exhibited individual patterns of habitat use over time and revealed significant intra-population differences. Ten percent of the individuals exhibited neritic maternal foraging (δ13C = -15.6 ± 0.5‰, δ15N = 17.3 ± 1.1‰) and high consistency, thus suggesting specialization (%CV δ13C values = 0.3 to 2.2), while 90% of the individuals exhibited oceanic maternal foraging (δ13C = -17.9 ± 0.7‰, δ15N = 12.4 ± 0.5). Additionally, oceanic maternal foraging could be further classified to distinguish broader individual variability: 58% were specialists (%CV = 0.03 to 2.2), 30% were intermediate specialists-generalists (%CV = 2.5 to 4.5), and 12% were generalists (%CV = 5.0 to 7.3). The prevailing strategy for females was oceanic foraging as exhibited by location at sea and the greater extent of oceanic habitats (88%) potentially available for foraging. At the population level, the existence of both alternate foraging strategies and high individual variability exhibited by gestating females in a high-quality foraging area such as the oceanic environment of the Argentine Basin may confer an ecological edge to these females to succeed in a less predictable (although fairly rich) environment, thus influencing population trends.