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 (Mirounga leonina) 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.