Stable isotopes document winter trophic ecology and maternal investment of adult female southern elephant seals (Mirounga leonina) breeding at the Kerguelen Islands

Individual specialisation is widespread and can affect a population's ecological and evolutionary dynamics. Whether intra-specific niche differences can influence reproductive investment was examined in a marine mammal, the southern elephant seal (Mirounga leonina), whose females were known to forage in two different areas during the austral winter. The study was conducted at Kerguelen Islands (49 degrees 21'S, 70 degrees 18'E), southern Indian Ocean, in late winter-early spring 2006. Pups were used as proxies of their mothers' biology and combined information on their weaning mass (a proxy of females' foraging success and short-term fitness) together with their blood delta C-13 value (a proxy of female foraging zone). First, the use of isotopic signature of pups was validated to study the female foraging ecology during their pre-breeding trip by demonstrating that delta C-13 and delta N-15 values of pups and their mothers were positively and linearly correlated. Then, blood samples were taken from a large number of newly-weaned pups, which were also weighed, to provide information at the population level. Estimated delta C-13 values of female seals encompassed a large range of values (from -23.7 to -19.1 parts per thousand) with an unimodal frequency distribution, suggesting no contrasted foraging areas within the population. No significant relationship was found between pup weaning mass and their carbon signature, indicating no link between female foraging areas and maternal foraging success and investment. Finally, blood delta C-13 and delta N-15 values gave new insights into southern elephant seal ecology, suggesting that females mainly foraged north of the Polar Front where they preyed upon myctophid fish in late winter.