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


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Robertson, GG 1994 , 'The foraging ecology of emperor penguins (Aptenodytes forsteri) at two Mawson coast colonies, Antarctica', PhD thesis, University of Tasmania.

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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
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 d-1 for brooded chicks to 44 for pre-departure fledglings,
which were fed a sub-maintenance ration following attainment of asymptotic mass.
Mass-specific field metabolic rates declined from 583 -1 for chicks
weighing about 1 kg to 323 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
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 -1 (2.3 kg.d-1) in
winter when chicks were < 5 % of adult mass, to 259 (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 d-1 (11.4 MJ.d-1) and 1,380 (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
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.

Item Type: Thesis - PhD
Authors/Creators:Robertson, GG
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