Distributions and predator-prey interactions of macaroni penguins, Antarctic fur seals, and Antarctic krill near Bird Island, South Georgia

Abstract

Vol. 86: 15-30, 1992 l MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published September 3 I Distributions and predator-prey interactions of macaroni penguins, Antarctic fur seals, and Antarctic krill near Bird Island, South Georgia George L. Hunt, ~ r ' Dennis ~einemann'.', Inigo ~ v e r s o n ~ 'Department of Ecology and Evolutionary Biology, University of California. Irvine, California 92717, USA 'British Antarctic Survey, Natural Environment Research Council, High Cross Madingley Road, Cambridge CB3 OET, United Kingdom ABSTRACT: We studied the distributions, abundances and interactions of macaroni penguins Eudyptes chrysolophus, Antarchc fur seals Arctocephalus gazella, and their zooplankton prey, m particular Antarctic krill Euphausia superba, near Bird Island, South Georgia, South Atlantic Ocean, in February 1986. Simultaneous surveys of marine birds, Antarctic fur seals and Antarctic krill were conducted along a series of transects radiating from the breeding colonies of the vertebrate predators. We examined the relationships between the distributions of predators and their prey with respect to the abundance of krill in the water column and marine habitats near the colonies. Antarctic fur seals and macaroni penguins showed positive correlations with Antarctic krill density across a wide range of spatial scales. Because krill was abundant close to the colony and predator densities decreased with distance due to geometry, distance from colony was a confounding variable. When the influences of distance and direction on predator abundance were factored out, we were able to demonstrate an additional influence of Antarctic krill abundance at measurement scales between 10 and 100 km for Antarctic fur seals and for macaroni penguins at the scale of 70 to 100 km. Water depth was an important correlate of Antarctic krill and Antarctic fur seal abundances but not of the abundance of macaroni penguins. We found no evidence that the fur seals or macaroni penguins were concentrating their foraging for krill in the vicinity of the shelf-break. INTRODUCTION Students of marine birds and mammals have sought to describe and understand the at-sea distributions and abundances of these organisms (e.g. Laws 1977, Brown 1980, Hunt & Schneider 1987, Hunt 1990). In early studies, the measurement of prey abundance at appropriate scales was not feasible, and investigators relied on the measurement of physical features of the ocean to describe the marine habitats used by birds and mammals. The implicit assumptions in these studies were that prey availability varied between the habitats described, and that birds or mammals selected those habitats where prey were most profitably acquired. The possibility of making continuous meas- urements of prey distributions and abundances now ' Present address: Manomet Bird Observatory, PO Box 936, Manomet, Massachusetts 02345, USA O Inter-Research 1992 permits the study of the relationships between predator and prey abundance over a variety of spatial scales. Quantitative studies of the relationships between the at-sea dstribution and abundance of marine mammals and the distribution and abundance of their prey are few, possibly due to the generally low at-sea densities of marine mammals. At the scale of tens to hundreds of km, there is evidence that marine mammals concen- trate where their prey is predictably abundant (e.g. Brodie et al. 1978, Nerini 1984, Oliver et al. 1984, Au & Pitman 1986). In the Antarctic, there have been no quantitative studies of the joint pelagic distributions of marine mammals and their prey. Inferences concerning the coincidence of the distributions of the great whales and Antarctic knll Euphausia superba have been attempted (Harmer 1931, Hardy & Gunther 1935, Mac- kintosh 1965, Laws 1977, Everson 1984), but these studies lacked simultaneous determination of prey resources (see also Plotz et al. 1991, Ribic et al. 1991).