Under the sea ice: Exploring the relationship between sea ice and the foraging behaviour of southern elephant seals in East Antarctica

International audienceInvestigating ecological relationships between predators and their environment is essential to understandthe response of marine ecosystems to climate variability and change. This is particularly true inpolar regions, where sea ice (a sensitive climate variable) plays a crucial yet highly dynamic and variablerole in how it influences the whole marine ecosystem, from phytoplankton to top predators. For mesopredatorssuch as seals, sea ice both supports a rich (under-ice) food resource, access to which dependson local to regional coverage and conditions. Here, we investigate sex-specific relationships between theforaging strategies of southern elephant seals (Mirounga leonina) in winter and spatio-temporal variabilityin sea ice concentration (SIC) and coverage in East Antarctica. We satellite-tracked 46 individualsundertaking post-moult trips in winter from Kerguelen Islands to the peri-Antarctic shelf between2004 and 2014. These data indicate distinct general patterns of sea ice usage: while females tended tofollow the sea ice edge as it extended northward, the males remained on the continental shelf despiteincreasing sea ice. Seal hunting time, a proxy of foraging activity inferred from the diving behaviour,was longer for females in late autumn in the outer part of the pack ice, 150–370 km south of the iceedge. Within persistent regions of compact sea ice, females had a longer foraging activity (i) in the highestsea ice concentration at their position, but (ii) their foraging activity was longer when there were morepatches of low concentration sea ice around their position (either in time or in space; 30 days & 50 km).The high spatio-temporal variability of sea ice around female positions is probably a key factor allowingthem to exploit these concentrated patches. Despite lack of information on prey availability, females mayexploit mesopelagic finfishes and squids that concentrate near the ice-water interface or within the watercolumn (from diurnal vertical migration) in the pack ice region, likely attracted by an ice algal autumnbloom that sustains an under-ice ecosystem. In contrast, male foraging effort increased when theyremained deep within the sea ice (420–960 km from the ice edge) over the shelf. Males had a longer foragingactivity (i) in the lowest sea ice concentration at their position, and (ii) when there were morepatches of low concentration sea ice around their position (either in time or in space; 30 days &50 km) presumably in polynyas or flaw leads between land fast and pack ice. This provides access tozones of enhanced resources in autumn or in early spring such as polynyas, the Antarctic shelf and slope.Our results suggest that some seals utilized a highly sea ice covered environment, which is key for theirforaging effort, sustaining or concentrating resources during winter