The foraging ecology of emperor penguins (Aptenodytes forsteri) at two Mawson coast colonies, Antarctica

1.The foraging ecology of emperor penguins was examined at two colonies on the Mawson Coast of Antarctica during the winter, spring and summer of 1988. The study sought to quantify the penguin's reproductive performances and chick mortality schedules, chick diet composition, energy intake and food consumption of adults and chicks, and adult hunting performance during the period of chick care. 2. Several techniques commonly used for conducting ecological research on penguins were modified for use on emperors. These included techniques for: capture and restraint, measuring body masses, marking individuals for identification, sampling stomach contents, preventing liquid radio-isotopes from freezing in sub zero temperatures, handling adults during incubation and brooding, withdrawing blood samples and attaching dive recorders. The techniques are of particular relevance to field research conducted during the Antarctic winter. 3. The population size and breeding success of emperor penguins at the Auster and Taylor Glacier colonies was estimated from winter photographs of incubating males and collections of abandoned eggs and dead chicks. At Auster a total of 10,963 pairs produced about 6,350 fledglings for a breeding success of 58%. At Taylor Glacier about 2,900 pairs raised 1,774 fledglings for a breeding success of 61 %. Fledglings left Taylor Glacier over a period of 33 days at a mean mass of 10.56 kg. 4. A validation study was conducted on the tritiated water (HTO) and sodium-22 (22Na) turnover methods as estimators of dietary water and sodium intake by freeranging emperor penguins. Penguins assimilated 76.2% and 81.8% of available energy in squid and fish diets, respectively. Both isotopes had equilibrated with body water and exchangeable sodium pools by 2 h after intramuscular injection. The tritium method yielded reliable results after blood isotope levels had declined by 35%. On average the tritium method underestimated water intake by 2.9%, with a range of -10.3% to +11.1%. The 22Na method underestimated Na intake on average by 15.9% with the errors among individuals ranging from -37.2% to -1.8%. Discrepancies with 22Na turnover were significantly greater with the squid diet than the fish diet. The results confirm the reliability of the tritium method as an estimator of food consumption by free-living emperor penguins (provided sea water and fresh water ingestion is known) and support the adoption of the 22Na method v to derive an approximation of sea water intake by tritiated emperor penguin chicks, and by tritiated adults on foraging trips of short duration. 5. The diet composition of emperor penguin chicks was examined at Auster and Taylor Glacier colonies by water-offloading adults serially between hatching in midwinter and fledging in mid-summer. Chicks at both colonies consumed a similar suite of prey species. Crustaceans occurred in 82% of stomach samples at Auster and 87% of stomachs at Taylor Glacier and were heavily digested; their contribution to food mass could not be quantified. Fish, primarily bentho-pelagic species, accounted for 52% by number and 55% by mass of chick diet at Auster and squid formed the remainder. At Taylor Glacier the corresponding values were 27% by number and 31% by mass of fish and 73% by number and 69% by mass of squid. Of the 33 species or taxa identified the fish Trematomus eulepidotus and the squid Psychroteuthis glacialis and Alluroteuthis antarcticus accounted for 64% and 74% of the diets by mass at Auster and Taylor Glacier respectively. The sizes of fish varied temporally but not in a linear manner from winter to summer. Adult penguins captured fish ranging from 60 nun (Pleuragramma antarcticum) to 250 nun (T. eulepidotus) in length and squid (P. glacialis) from 19 nun to 280 mm mantle length. The length-frequency distribution of P. glacialis showed seasonal variation with the size of squid increasing from winter to summer. The energy density of chick diet mix increased significantly prior to fledging. 6. The energy requirements and food consumption of emperor penguin chicks during their 150-day development was studied at Auster and Taylor Glacier colonies by means of doubly labelled water (3H2180)-derived estimates of field metabolism and measurement of tissue energy accumulation during growth. Growth rates to asymptotic mass ranged from 19 g.d-1 for brooded chicks to 74 g.d-1 for chicks mid-way through their development. Tritiated water derived feeding rates ranged from 199 g.kg-1 d-1 for brooded chicks to 44 g.kg-1.d-1 for pre-departure fledglings, which were fed a sub-maintenance ration following attainment of asymptotic mass. Mass-specific field metabolic rates declined from 583 kJ.kg-1.d -1 for chicks weighing about 1 kg to 323 kJ.kg-1.d-1 for chicks near to asymptotic mass (c 12 kg). Field metabolism scaled on body mass was best described by FM = 0.446 MO.91 (r2 = 0.74; n = 32), where FM is field metabolism (MJ.d-1) and M is chick mass (kg). The relationship was statistically indistinguishable from tritiated water-derived estimates of FM for chicks fed a ration suitable for maintenance (growth excluded) only. An 'average' chick expended a total of 444 MJ metabolized energy during development, which consisted of 125 MJ accumulated in new tissue and 319 MJ VI expended for maintenance. Food intake to satisfy total metabolized energy was 84 kg, which represents the amount of food required to produce an emperor penguin chick from hatching to independence. Based on this estimate and knowledge of chick diet composition, chicks at Auster and Taylor Glacier colonies consumed an estimated 648 tonnes and 290 tonnes of food, respectively, during the 1988 breeding season. 7. The food and energy requirements of adult emperor penguins parenting chicks were examined at Auster (about 11,000 pairs) and Taylor Glacier (about 2,900 pairs) colonies during the winter, spring and summer of 1988. Tritiated waterderived estimates of adult food consumption trebled during the five-month period of chick care. For birds at sea, estimates ranged from 89 g.kg-1.d -1 (2.3 kg.d-1) in winter when chicks were < 5 % of adult mass, to 259 g.kg-1.d-1 (6.3 kg.d-1) in summer, when chicks represented 40 - 50% of adult mass. These food consumption rates were equivalent to the acquisition of 440 kJ.kg-1 d-1 (11.4 MJ.d-1) and 1,380 kJ.kg-1.d-1 (33.4 MJ.d-1) metabolizable energy in winter and summer respectively. Chick provisioning was not accompanied by a major increase in food consumption by adults. Adults assimilated 85-92% of their daily food intake themselves and retained the remainder for the chick. The food ration of chicks for the three seasons (648 tonnes at Auster) constituted only about 6% of the total food intake by the adults during the same period. In spite of this, adults appeared to be operating near their maximum food gathering capacity when raising chicks. Adults consumed an estimated 482 kg of food (including the ration for the chick), which amounts to about 10,615 tonnes and 2,800 tonnes of fish and squid consumed by the breeding populations at Auster and Taylor Glacier, respectively. 8. The diving performance of adult emperor penguins was examined in winter, early spring, late spring and summer. These seasons coincided with the brood, creche, mid-growth and pre-fledging stages of chick development, respectively. The distribution in the water column of modal depths did not appear to be affected by season. Mean maximum dive depth (53-66 metres) was similar for all four seasons, as was mean maximum dive duration (2.8-4.1 minutes). Maximum depths and durations of individuals in the four groups ranged from 130-460 metres and from 6.2-22.0 minutes, respectively. Diving was restricted to daylight and twilight, and dive rate increased as day length increased, from 64.1 ± 26.9 dives/ day in winter to 131 dives/day in summer (one individual). Deep dives occurred at any time of the day, not only during periods of peak light intensity. The large proportion of ben tho-pelagic prey in the emperors' diet, combined with knowledge of their dive vii patterns, suggests the emperors' hunted prey in the meso-pelagic region of the water column, including on the sea bottom of the continental shelf. The penguins' prey composition, prey habitat, marine topography, seasonal changes in the fast-ice and the birds' diving performances are used to make some basic estimates of the foraging locations and prey capture rates of emperor penguins.