MEPS 370:239-247 (2008)  -  DOI: https://doi.org/10.3354/meps07673

ABSTRACT: 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.

KEY WORDS: Cephalopod · Mesopelagic fish · Myctophid · Pelagic ecosystem · Southern Ocean · Squid