Seasonal variation in the diet composition of non-breeding and breeding gentoo penguins (Pygoscelis papua) at Stranger Point/Cabo Funes, South Shetland Islands, Antarctica

Understanding predator-prey relationships is essential for monitoring Antarctic marine ecosystem structure and function, assessing changes in predator abundance, and implementing effective conservation measures. Despite this, diet studies of Antarctic predators using traditional methods are limited outside the breeding period due mainly to constraints associated with the life-history strategies of the predators and the logistics and operability of sampling. At Stranger Point/Cabo Funes (25 de Mayo/King George Island), we characterized the diet composition of gentoo penguins (Pygoscelis papua) during four stages of the annual cycle (i.e. non-breeding and pre-breeding stages of 2022, and guard and crèche stages of the 2022/23 breeding season) from the analysis of stomach contents obtained using the water-offloading technique. Antarctic krill (Euphausia superba) dominated the diet of gentoo penguins in terms of frequency of occurrence and percentage in mass throughout the annual cycle, followed by fish and amphipods. However, the diet composition changed seasonally, with a decrease in the contribution of fish and amphipods as the year progressed. Furthermore, higher individual variability was observed outside the breeding period than during the chick-rearing period, since the krill was the prey with the highest contribution by mass in 100% of the samples collected during the breeding period. A change was also observed in the krill sizes consumed by gentoo penguins, which was higher during the breeding period. Although gentoo penguins are opportunistic feeders, these results suggest that krill was an important fraction of the non-breeding and breeding diet. Moreover, outside the breeding period, the mean krill size consumed was smaller and the proportion of juvenile krill was higher than that recorded in the chick-rearing period.