Stable isotopes reveal the trophic position and mesopelagic fish diet of female southern elephant seals breeding on the Kerguelen Islands

Trophic interactions between organisms are the main drivers of ecosystem dynamics, but scant dietary information is available for wide-ranging predators during migration. We investigated feeding habits of a key consumer of the Southern Ocean, the southern elephant seal Miroungia leonina, by comparing its blood δ13C and δ15N values with those of various marine organisms, including crustaceans, squid, fishes, seabirds and fur seals. At the end of winter, δ13C values (–23.1 to –20.1‰) indicate that female elephant seals forage mainly in the vicinity of the Polar Front and in the Polar Frontal Zone. Trophic levels derived from δ15N values (trophic level = 4.6) show that the southern elephant seal is a top consumer in the pelagic ecosystem that is dominated by colossal squid. The mean δ15N value of seals (10.1 ± 0.3‰) indicates that they are not crustacean eaters, but instead feed on crustacean-eating prey. Surprisingly, most of the previously identified prey species have isotope δ13C and δ15N values that do not fit with those of potential food items. The most singular pattern to emerge from δ15N values of predators and prey is that female seals are likely to feed on myctophid fishes, not squid or Patagonian toothfish. We therefore suggest that they occupy a unique trophic niche amongst the guild of air-breathing, diving vertebrates by feeding on mesopelagic fish at great depths. In turn, this finding emphasizes the role of myctophids in oceanic waters and will help to quantify and model fluxes of matter and energy within the pelagic ecosystem of the Southern Ocean.