The CCAMLR Ecosystem Monitoring Programme has two aims: to detect changes in critical components of the antarctic ecosystem, and to distinguish between changes due to the harvesting of commercial species and changes due to environmental variability. Data from Seal Island are used to construct a multivariate model which relates chinstrap penguin breeding success, krill abundance and sea ice conditions, which effectively predicts chinstrap success given sea-ice data with an R2 of 0.914. This model is then used to propose a method of distinguishing between effects of environmental variation and harvesting on chinstrap breeding success. Other Antarctic Peninsula predator parameters are also briefly analysed. The results demonstrate that this methodology could be applied to other CEMP sites, even with the relatively short time series now available, and provides a way forward in the analysis and interpretation of the CEMP indices.