AGGREGATION PATTERNS OF PELAGIC PREDATORS AND THEIR PRINCIPAL PREY, ANTARCTIC KRILL, NEAR SOUTH-GEORGIA

1. We examined the spatial distributions of pelagic seabirds and tur seals near South Georgia, and asked to what extent the distributions of these predators were influenced by the spatial distribution of their principal prey, Antarctic krill Euphausia superba Dana. One novel aspect of our analysis is an explicit consideration of the separation in space between swarms of krill and aggregations of predators that feed upon krill. 2. Our data were collected in February 1986, during a systematic shipboard survey of the waters surrounding Bird Island, South Georgia. Predator abundance was estimated visually using strip transects, and krill abundance was simultaneously estimated using a hull-mounted echosounder. 3. We approached the difficult analytical problems associated with spatial distributions of organisms by using spatial autocorrelation and cross-correlation analysis, regression models with spatial terms, and randomization tests. The randomization tests involved repeated simulations of predator distributions, and subsequent estimation of spatial association between predators and prey. 4. Pelagic birds and seals were distributed in a strikingly non-random fashion at sea near South Georgia; their distributional patterns were strongly influenced by the distribution of krill swarms. 5. Differences between predators in their spatial distribution and in their response to krill swarms suggest interspecific differences in foraging strategies.