SEASONAL CHANGE IN THE FORAGING ECOLOGY OF EMPEROR PENGUINS ON THE MAWSON COAST, ANTARCTICA

We investigated the foraging location, diving behaviour, dietary compo sition, feeding rates and foraging trip durations of emperor penguins Aptenodytes forsteri raising chicks at the Auster and Taylor Glacier c olonies on the Mawson Coast of Antarctica in the winter, spring and ea rly summer of 1993, to examine seasonal changes in the penguins' forag ing ecology. As day-length increased after winter, the penguins' daily swimming time increased from 7.83 +/- 1.50 h in August to 12.23 +/- 1 .25 h in September and 12.95 +/- 1.24 h in October. Accordingly, the p enguins' dive rate increased from 92.7 +/- 28.5 to 149.4 +/- 23.4 and 161.6 +/- 19.3 dives d(-1) in the respective months. The birds targete d prey in the vicinity of the continental slope mainly at depths <100 m, although some individuals frequently hunted at depths >200 m, and t he maximum depth achieved was 438 m. Antarctic krill Euphausia superba were the most common prey taken overall, 41% of the diet by mass, and dominated the diets between August and October. The contribution of A ntarctic krill to the diet reduced over time from 68% in August to 1% in early December. In November, the glacier squid Psychroteuthis glaci alis dominated the diet (47 to 63%), and in early December the diet co mprised various species of fish, Trematomus species (27%), Pagothenia borchgrevinki (24%), and Pleuragramma antarcticum (8%), and squid, P. glacialis (13%) and Alluroteuthis antarcticus (9%). The birds' prey co nsumption rates more than doubled between late winter and early summer , from 4.0 +/- 1.0 to 8.7 +/- 1.7 kg d(-1) spent foraging; these value s are equivalent to metabolisable energy intakes of 628 +/- 134 and 14 22 +/- 308 kJ kg(-1) d(-1), respectively. During brooding (late winter to early spring), females spent less time at sea than males (8.7 +/- 2.7 vs 17.7 +/- 3.8 d); thereafter trip durations of both sexes were s imilar and declined from 15-19 d in spring to <10 d in early summer. B etween hatching and about 1 wk prior to fledging each parent fed its c hick 7 or 8 times. To raise a chick, females and males consumed approx imately 410 and 470 kg of prey respectively, or 880 kg for each breedi ng pair. Seasonal variations in the penguins' foraging were probably i nfluenced by fluctuating sea-ice conditions, differences in the prey t ypes available, changes in day-length toward summer, and increasing de mands of the growing chicks.