Stable isotopes in southern rockhopper penguins: foraging areas and sexual diVerences in the non-breeding period

Southern rockhopper penguins (Eudyptes chrysocome chrysocome) have experienced severe population declines across their distribution area, potentially in response to bottom-up effects following elevated sea surface temperatures, changes in the food web and prey availability. We conducted stable isotope analysis to compare trophic levels and distribution patterns in the non-breeding period over three consecutive years, and between males and females, using egg membranes, blood cells and feathers of parent birds. Tissues representing the non-breeding season had lower delta C-13 values than prey sampled around the Falklands and red blood cells from breeding rockhopper penguins. In contrast, delta N-15 values were higher in red blood cells from the end of winter compared to those from the breeding season and compared to feathers. This indicated that rockhopper penguins left the Falkland Island area in the non-breeding season and foraged either around Burdwood Bank further south, or over the Patagonian Shelf. In winter, only males took more prey of higher trophic level than females. Inter-annual differences in isotopic values partly correlated with sea surface temperatures. However, as prey isotope samples were collected only in 1 year, inter-annual differences in penguin isotopic values may result from different foraging sites, different prey choice or different isotopic baseline values. Our study highlights the potential for stable isotope analyses to detect seasonal and gender-specific differences in foraging areas and trophic levels, while stressing the need for more sampling of isotopic baseline data.