Foraging strategies of chinstrap penguins at Signy Island, Antarctica: importance of benthic feeding on Antarctic krill

Chinstrap penguins Pygoscelis antarctica are one of the major consumers of Antarctic krill Euphausia superba in the Southern Ocean. To examine their foraging strategy, we studied foraging trip patterns and diving behaviour of chinstrap penguins breeding at Signy Island, Antarctica, using time-depth recorders. Foraging trips of penguins could be divided into 2 groups, short diurnal (7.8 h) and longer overnight (19.9 h) trips, with diurnal trips (74%) being dominant in number (263 out of 355 trips). The diving depths of our study birds were much deeper (to 179 m) than previous studies on this species, with modal maximum dive depth at around 90 to 100 m. Diving patterns and profiles included typical pelagic dives, but also included series of consecutive square-wave shaped dives reaching similar maximum depth, the typical characteristics of benthic dives. These benthic-type dives were more abundant in diurnal foraging trips than overnight trips. Analysis of stomach contents showed that penguins on both types of trip fed almost exclusively on Antarctic krill. There was a positive relationship between indices of the proportion of benthic feeding and of foraging efficiency (stomach content mass divided by foraging trip duration). These results highlight the potential importance of benthic feeding on Antarctic krill, the first such recorded instance for chinstrap penguins. This previously undescribed foraging strategy by one of the major avian consumers of Antarctic krill provides a new insight into the predator-prey interactions of the Antarctic coastal marine ecosystem.