This book began its evolution in 1999 when the British Antarctic Survey, where I worked at the time, began a new research programme on the management of marine ecosystems. This programme concentrated upon the krill-based ecosystem at South Georgia which has been the subject of almost continuous study since the Discovery Expeditions in the 1920s. Latterly, international efforts to understand the dynamics of this ecosystem and the wider Southern Ocean have been coordinated by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). The daunting task of describing ecosystem dynamics over such a large oceanic area with relatively limited resources led to the establishment of the CCAMLR Ecosystem Monitoring Programme, an internationally coordinated effort at data collection. Among other things, this contained a major component of monitoring the seal and seabird populations in the region. The logic for their inclusion was that they foraged over most of the regions of interest but returned to breed at very well defined locations. By undertaking a series of measurements of these predators at these locations, it was then argued that aspects of the ecosystem dynamics should be reflected by variability in the measurements of the predators. It was hoped that appropriate choices of the predators and measurement variables would provide indicators of the dynamics of their prey at different spatial and temporal scales.