Foraging Behavior of Chinstrap Penguins (Pygoscelis antarctica) at the South Orkney Islands (Antarctica) during their Breeding Season

Abstract

Chinstrap penguins (Pygoscelis antarctica) are central place foragers during their breeding season in the austral summer, which restricts the distance and duration they can forage away from their chicks. Recent studies indicate a regional decline in chinstrap penguin populations across their distribution range. During breeding, they almost exclusively feed on Antarctic krill (Euphausia superba), a prey resource that is unevenly distributed and undergoes diel vertical migration to evade predators like penguins. Using acoustic krill survey data from the South Orkney Islands and four-dimensional tracking data from 35 individuals across Monroe and Powell Islands from the austral summer seasons of 2022 and 2023, this study links predator and prey dynamics. I demonstrate that chinstrap penguins exhibit a pronounced diel foraging pattern, utilizing different areas and dive types. The findings suggest that chinstrap penguins target shallowly distributed krill at the shelf areas north on the South Orkneys – an area known for high krill concentrations – at nighttime, where the reduced energy expenditure from shallow diving offsets the increased travel distance. In contrast, when krill is located deeper during the daytime, chinstrap penguins tend to remain closer to their colonies and dive deeper. Furthermore, the results indicate that chinstrap penguins target lower krill densities than those found at peak concentrations, suggesting that suitable prey is determined more by encounter rates than sheer density. The presence of a large multispecies aggregation, including whales, seals, and seabirds, within the penguins’ home range in 2022 likely influenced the observed krill depth distribution, reflecting varying top-down pressures from different krill predators. However, due to the lack of synoptic data, the latter observations should be viewed as broad patterns that warrant further investigation through studies with temporally overlapping and appropriately scaled data. In the face of climate change, understanding the intricate biotic interactions among recovering krill predators, such as baleen whales, and penguins alongside the effects of krill fisheries, is essential for gaining a comprehensive insight into the dynamics of Southern Ocean ecosystems.