Temporal variability and evaluation of methods used to infer diet of a Southern Ocean predator, the Adélie penguin Pygoscelis adeliae

Predicting ecosystem response to change and ensuring long-term sustainable management of Southern Ocean marine living resources is reliant upon ecosystem monitoring programmes that will provide data on key physical and biological components of the ecosystem and the functional relationships between these components. Integral to such monitoring programmes is accurate and reliable information on the diet of predators. In this study, I examined the long-term variability in the diet of Adelie penguins Pygoscelis adeliae, and their dependence on Antarctic krill Euphausia superba, the target of a large commercial fishery, to evaluate their effectiveness as an indicator species monitored to detect the effect of anthropogenic disturbance on the Southern Ocean ecosystem. Krill and fish were the dominant prey items in the diet of Adelie penguins from Bechervaise Island, however there was substantial inter- and intra-annual variation, as well as differences between sexes, in meal mass and diet composition. In years of low amounts of krill in the diet, reproductive performance declined, indicating Adelie penguins from this region are dependent on krill and could be considered an effective indicator species. However the large year-to-year variability naturally present in Adelie penguin diet limits the power to detect change due to an impact over short time periods (i.e. <20-years), unless one is willing to relax Type I error levels above the traditional 0.05 level. Diet of Adelie penguins has traditionally been inferred from stomach samples, however execution of this technique is restricted to when birds are accessible and have full stomachs. Hence, diet data is biased towards the chick-rearing period when adults bring food ashore to feed chicks. Therefore I evaluated two alternate, indirect techniques - stable-isotope analysis (SIA) and fatty acid signature analysis (FASA) - that may complement or enhance our knowledge of Adelie penguin diet. Diet inferred from the analysis of stable carbon (13C) and stable nitrogen (15N) isotopes in penguin blood and feather samples, and from fatty acids in blood samples, was similar to that determined from stomach contents. Blood and feather samples analyzed by SIA or FASA can integrate diet over different time periods. Therefore I examined intraand inter-annual variation in the diet of adult and chick Adelie penguins. Although diet did not differ between age classes, it did vary between breeding stages and between the two years of study. I also developed an in situ method to calibrate blood FA profiles with stomach contents, which offers a simple and effective alternative to more complex calibration techniques developed elsewhere. I conclude that SIA and FASA are useful for monitoring Adelie penguin diet at broad taxonomic resolutions, and, combined with stomach content analysis, provide a more comprehensive picture of Adelie penguin foraging ecology. Additionally, and most importantly, these techniques extend the temporal window for obtaining diet information, including those periods when it is difficult to use conventional sampling techniques, although penguins may be vulnerable to impacts such as commercial fishing during these periods as well.