Krill biomass and aggregation structure in relation to tidal cycle in a penguin foraging region off the Western Antarctic Peninsula

Antarctic krill are a key component of the diet of Adlie penguins inhabiting the Western Antarctic Peninsula (WAP), yet our understanding of the variability of krill distribution patterns within nearshore penguin feeding grounds is limited. A recent study of the foraging patterns of penguins breeding in the northern WAP suggests that tidal phase plays a role in foraging distance. We used acoustics to examine biomass and aggregation structure of krill in the penguin foraging grounds off Palmer Station during diurnal and semi-diurnal tides. Nearshore, integrated krill biomass during diurnal tides was significantly higher than during semi-diurnal tides. Krill aggregations were also shallower, closer together, and larger in dimension during diurnal tides. Conversely, krill aggregations had higher volumetric biomass and abundance during semi-diurnal tides. Further offshore, at the head of the Palmer Deep canyon, krill aggregations were similar to those observed nearshore during diurnal tides (i.e. shallow, close together, and large in dimension). Since krill aggregation structure strongly influences availability as a potential prey source, we suggest that foraging behavior of Adlie penguins in this region is strongly linked to the variability in nearshore krill aggregation structure as well as biomass.