Analysis of the sources of variance in the mean size of krill consumed by Chinstrap and Adélie penguins at South Orkney Islands

The CCAMLR Ecosystem Monitoring Program aims at detecting possible impacts of the krill fishery on predator populations. Population parameters of Chinstrap and Adelie penguins are monitored at Point Martin, Laurie Island, South Orkneys Islands as part of the Program. Diet information is collected to aid in the interpretation of pop- ulation data. In the present study, the diet of both species is compared in order to detect differences between them as well as to ascertain whether the krill consumed by both species might be pooled in single estimates of prey param- eters. A nested ANOVA model was fitted to the dataset obtained in the seasons from 1997/1998 to 2005/2006. The overall variance was decomposed into variance components: within penguins (krill in stomachs), between penguins (means within each date), between dates (date means in each season), and between years (annual means). The within penguins component was similar in both species (18 % difference), while the between penguins was 64 % larger in Adelies. The within and between penguins variance com- ponents of both species showed consistent interannual changes. The annual means of both species differed signif- icantly in three seasons, but not consistently. The sources of variance are tentatively attributed to biological characteris- tics: the within penguins component would reflect searching and feeding at the individual level; the between penguins might be associated with their foraging ranges, an assu- mption to be tested tracking individuals. Given their differences, the information from both species should be viewed as complementary and not be pooled.