Linking oceanographic conditions to foraging behaviour of southern elephant seals by characterising mid-trophic levels with an animal-borne echosounder

Changes in Southern Ocean thermohaline conditions could drive changes in phytoplankton community composition propagating into upper-trophic levels, from the abundance and composition of mid-trophic level communities to foraging behaviour of top marine predators. Studies exploring the ecological consequences of such processes are largely limited by our in-situ observation capacity. The simultaneous deployment of a miniature sonar tag, combining active acoustics and movement sensors, with oceanographic tags on 4 adult female southern elephant seals from the Kerguelen Islands provided in-situ measurements of thermohaline conditions, mid-trophic level abundance and seal-foraging behaviour. In addition, chlorophyll-a concentration and relative abundance of different phytoplankton size classes were assessed from satellite ocean colour data. This study reveals that east of the Kerguelen Islands, the subsurface scatterer abundance (related to mid-trophic level organisms) assessed by the sonar tag was partly positively linked to surface satellite-derived microphytoplankton biomass, especially in colder waters and frontal zones. Female elephant seals were found to dive shallower with higher subsurface scatterer abundance, which indicated a better prey accessibility, and were found to catch more prey when foraging in shallow and/or frontal zones. This study highlights the usefulness of the sonar tag, deployed on deep-diving elephant seals, to link thermohaline conditions and phytoplankton community composition in mid-trophic levels. Our result suggests that in addition to phytoplankton biomass, mid-trophic level abundance and distribution might be key factors in influencing top-marine predator foraging performances. • Biologging data and satellite phytoplankton data unveil trophic and biophysical interactions. • The sonar tag allows characterisation of scatterer abundance and distribution in the water column. • Functional oceanographic domains approach reveals SES foraging patterns in mesoscale fronts. • Surface microphytoplankton biomass increases subsurface scatterer abundance. • Scatterer abundance varies by water mass and in fronts, impacting SES prey accessibility.