Seasonal change in the foraging ecology of emperor penguins on the Mawson Coast, Antarctica

We investigated the foraging location, diving behaviour, dietary composition, feeding rates and foraging trip durations of emperor penguins Aptenodytes forsteri raising chicks at the Auster and Taylor Glacier colonies on the Mawson Coast of Antarctica in the winter, spring and early summer of 1993, to examine seasonal changes in the penguins' foraging ecology. As day-length increased after winter, the penguins' daily swimming time increased from 7.83 L 1.50 h in August to 12.23 t 1.25 h in September and 12.95 ? 1 24 h in October. Accordingly, the penguins' dive rate increased from 92.7 r 28.5 to 149.4 i 23.4 and 161.6 r 19.3 dives d.' in the respective months. The birds targeted prey in the vicinity of the continental slope mainly at depths 200 m, and the maximum depth achieved was 438 m. Antarctic krill Euphausia superba were the most common prey taken overall, 4 1 % ~ of the diet by mass, and dominated the diets between August and October. The contribution of Antarctic krill to the diet reduced over time from 68% in August to 1 % in early December. In November, the glacier squid Psychroteuthis glacialis dominated the diet (47 to 63 %), and in early December the diet comprised various species of fish. Trematomus species (2704,), Pagothenia borchgrevinki (24%), and Pleuragramma antarcticum (8%), and squid, P. glacialis (13%) and Alluroteuthis antarcticus (9%). The birds' prey consumption rates more than doubled between late winter and early summer, from 4.0 + 1.0 to 8.7 + 1.7 kg d-l spent foraging; these values are equivalent to metabolisable energy intakes of 628 r 134 and 1422 * 308 kJ kg-' d-', respectively. During brood~ng (late winter to early spring), females spent less time at sea than males (8.7 k 2.7 vs 17.7 i 3.8 d); thereafter trip durations of both sexes were similar and declined from 15-19 d in spring to c10 d in early summer. Between hatching and about 1 wk prior to fledging each parent fed its chick 7 or 8 times. To raise a chick, females and males consumed approximately 410 and 470 kg of prey respectively, or 880 kg for each breeding pair. Seasonal variations in the penguins' foraging were probably influenced by fluctuating sea-ice conditions, differences in the prey types available, changes in day-length toward summer, and increasing demands of the growing chicks.