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    Diets of sympatrically breeding Adélie, gentoo and chinstrap penguins from Admiralty Bay, South Shetland Islands, Antarctica, 1981 to 2000

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    W.Z. Trivelpiece (USA), K. Salwicka (Poland) and S.G. Trivelpiece (USA)
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    Given the importance and interdependence of diet studies to monitoring work, we compared data on the stomach contents and food load masses among three species of Pygoscelis penguins during chick rearing over the 20-year period from 1981 to 2000. All three penguin species were largely dependent on Antarctic krill (Euphausia superba), which accounted for over 93% of each species’ diet by frequency of occurrence and mass. Gentoo penguins ate significantly more fish than either congener, specifically the benthic Nototheniid species, while Adélie and chinstrap penguins largely ate two species of pelagic fishes, Pleurogramma antarcticum and Electrona antarctica. All species exhibited significant inter-annual variability in mean food load sizes during chick rearing and there was a high degree of coherence among the three species in the years of high versus low food loads. Adélie and chinstrap penguin adults experienced significant declines in body weight during the chick provisioning period in several of the latter years of the study, suggesting food availability may have declined significantly between the earlier 1980s and the more recent 1990s periods. This conclusion is supported by annual US Antarctic Marine Living Resources (AMLR) marine surveys in the area. Finally, we examined the digested portion of the stomach contents and noted a significant increase in the proportion of digested versus fresh stomach contents as the season progressed and as chick food demands increased. We propose that the digested contents of a penguin’s food load has more than twice the caloric value of a comparable mass of fresh krill in the same bird’s stomach. This allows Pygoscelis penguins to significantly increase the caloric value of food brought to their chick per foraging trip. We discuss the energetic consequences of this hypothesis to the energy balance of the birds provisioning chicks and we point out the implications of this finding for past and future studies of penguin energetics using the Doubly-labeled water technique.