The Foraging Ecology of Female Emperor Penguins in Winter

The foraging location, diving behavior, dietary composition, and feeding rates of female Emperor Penguins (Aptenodytes forsteri) from the Auster and Taylor Glacier colonies in Antarctica were investigated during the 1993 austral winter. The study was conducted between late May and early August, when male emperors remain at the colonies to incubate eggs and females forage at sea for self-maintenance. During winter, two satellite-tracked penguins from Auster foraged ≈100 km northeast of the colony in open water 200–500 m deep, over the outer continental shelf and shelf slope. Ten Auster and four Taylor Glacier penguins that carried time–depth recorders took ≈8 d to reach the ice edge, spent 50–60 d at sea foraging, and took 4 d to return across the fast ice to the colony. The females occasionally huddled together to minimize heat loss while in transit to the ice edge and between foraging days. The penguins foraged on 93.2% of their days at sea and rested for the remainder. On each foraging day, penguins usually entered the water just after dawn and averaged 4.71 h in the water before exiting at dusk. The hourly dive rate was constant throughout winter, but the daily dive rate increased as day length increased, suggesting that day length is a primary determinant of hunting effort. Penguins exhibited behavior indicative of foraging on 47% of their dives, the remainder being travel or search dives. Penguins made, on average, 26 foraging dives/d. Females from Auster targeted prey at water depths of 20–70 m and 100–150 m, whereas Taylor Glacier birds targeted prey at 10–70 m, 250–300 m, and 330–400 m, suggesting between-colony differences in prey distribution. The stomach contents of 17 females returning to Auster to brood their chicks were dominated by pelagic prey species: Antarctic krill (Euphausia superba, 70% by mass) and Antarctic silverfish (Pleuragramma antarcticum, 13% by mass). Food consumption rates during winter for five penguins from both colonies were similar and averaged 62.6 ± 5.8 g·kg−1·d−1 (1.8 ± 0.1 kg/d for a 28.8 kg female), which equated to a metabolizable energy intake of 236.6 ± 22.0 kJ/kg for each day they foraged (all values are mean ± 1 sd). This enabled the birds to gain ≈6.1 kg for a trip spanning ≈70 d. Based on mean prey masses and the penguins' dive rates, the penguins consumed ≈115 × 0.6 g krill or 16 × 4.3 g Antarctic silverfish per foraging dive, or some combination of both. In their winter trip, each breeding female consumed ≈100 kg of prey. The female populations at Auster and Taylor Glacier consumed an estimated 1350 Mg and 250 Mg of prey, respectively, in winter.