The effect of different methodologies used in penguin diet studies at three US AMLR predator research sites: Admiralty Bay, Palmer Station and Cape Shirreff

Diet studies are a key component of the CCAMLR predator monitoring program as they provide direct assessments of the prey types and amounts of import to predators, which, in turn, are hypothesized to influence variability in related parameters such as breeding success, foraging trip durations and chick fledging weights. Given the importance and interdependence of diet studies to monitoring work, we compared data on stomach weights and prey types among three US AMLR land-based predator study sites in the Antarctic Peninsula region where different methodologies were used to select, pump and sort the diet samples. At Palmer Station, Adélie penguins selected for diet sampling were not confirmed to be breeding, while all Adélie and chinstrap penguins selected for diets sampling at Admiralty Bay and Cape Shirreff were confirmed breeders. Adélie penguins at Palmer Station had significantly smaller mean stomach weights (349 vs. 550 g) and a significantly higher proportion of birds with small < 200 g) stomach weights (15% vs. 1 %), compared to Admiralty Bay. In contrast, no significant differences in chinstrap penguin stomach weights were found between Admiralty Bay and Cape Shirreff (593 vs. 610 g, respectively), where all birds selected were confirmed breeders. We found a significant positive correlation between Adélie penguin stomach weights and chick fledging weights at Admiralty Bay; no such relationship was found in the Palmer Station data. The significantly lower stomach weights of Adélie penguins at Palmer Station were likely the result of including non-breeding birds in the diet samples. This significantly reduced the mean meal size reported and masked any correlation among food load sizes, provisioning rates and chick fledging weights.