Variability in the Southern Ocean is often characterised by fluctuations in the distribution and abundance of a single dominant zooplankton species, Antarctic krill Euphausia superba. At South Georgia interannual variability in the krill-based marine ecosystem has historically been most frequently detected in the response of upper-trophic level predators. Analysis of the changes in the population size structure of krill in the diet of Antarctic fur seals Arctocephalus gazella, particularly associated with periods of low krill biomass, has shown that the population dynamics of krill is a major factor in driving the inter-annual variability in its abundance. The ability to sample krill in the diet of predators at temporal scales not available using conventional (i.e. ship based) sampling methods has provided the basis for a re-evaluation of the role of recruitment variability and high rates of growth and mortality in generating the high amplitude of variability in krill abundance at South Georgia. Furthermore the production of a consistent index of krill population size composition from the diet of predators at Bird Island over the past decade has provided evidence for a relationship between sea-surface temperature and the level of krill recruitment. Krill-dependent predators not only show distinct behavioural responses to changes in krill abundance but also provide critical dietary data with which to elucidate the mechanisms underlying those changes. Using predators as samplers of commercial prey species can provide information on the key life-history parameters of prey species that are fundamental to reducing uncertainty in fisheries management models.