Variability in the Southern Ocean is frequently reflected in changes in the abundance of Antarctic krill Euphausia superba and subsequent effects on dependent predators. However, the nature and consequences of changes in krill population dynamics that accompany l1uctuations in its abundance are essentially unknown. A conceptual model, developed froll1 quantitative measures of krill length ill the diet of predators at South Georgia from 1991-1997, allowed predictions to be made about the abundance and population structure of krill in 1998 and the consequences for predators. Consistent with model predictions, in 1998 there was a serial change in krill population structure, low krill biomass and low predator reproductive performance. The change in modal size of krill, from 56 mm in December to 42 mm in March was apparently a result of the transport of krill into the region. This is the first occasion when the future status and structure of the krill population at South Georgia has been successfully predicted. By representing local krill population dynamics, which may also reflect large-scale physical and biological processes, predators have a potential key role as indicators of Southern Ocean environmental variation at a range of spatial scales.