Predicting changes in the Antarctic krill, Euphausia superba, population at South Georgia

Variability in the Southern Ocean is frequently reflected in changes in the abundance of Antarctic krill Euphausia superba and subsequent effects on d ependent predators. However, the nature and consequences of changes in kril l population dynamics that accompany fluctuations in its abundance are esse ntially unknown. A conceptual model, developed from quantitative measures o f krill length in the diet of predators at South Georgia from 1991 to 1997, allowed predictions to be made about the abundance and population structur e of krill in 1998 and the consequences for predators. Consistent with mode l predictions, in 1998 there was a serial change in krill population struct ure, low krill biomass and low reproductive performance of predators. The c hange in the 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 popul ation at South Georgia has been successfully predicted. By representing loc al krill population dynamics, which may also reflect large-scale physical a nd biological processes, predators have a potential key role as indicators of environmental variation in the Southern Ocean at a range of spatial scal es.