Oceanographic variability and changes in Antarctic krill (Euphausia superba) abundance at South Georgia P. N. TRATHAN, 1, * A. S. BRIERLEY, 2 M. A. BRANDON, 3 D. G. BONE, 1 C. GOSS, 1 S. A. GRANT, 1 E. J. MURPHY 1 AND J. L. WATKINS 1 1 British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK 2 Gatty Marine Laboratory, University of St. Andrews, Fife KY16 8LB, UK 3 The Open University, Earth Sciences, Walton Hall, Milton Keynes MK7 6AA, UK ABSTRACT Oceanographic data collected to the north of South Georgia were examined for three consecutive summers (1996/97, 1997/98, 1998/99). The results show the existence of a shelf break front during each period. The most reliable means of defining the front was the potential density anomaly at the near-surface potential temperature minimum. In each year, off-shelf waters were separated from on-shelf waters by water with a potential density anomaly between 27.22 and 27.29 kg m )3 . During 1997/98, the near-surface potential temperature minimum was much colder and much shallower than in other years and was consistent with waters originating from much further south than South Georgia; these differences were further evident at a single deep off-shelf station. The oceanogra- phic changes during 1997/98 were consistent with a mesoscale or large-scale movement of the southern Antarctic Circumpolar Current front. Acoustically determined densities of Antarctic krill, Euphausia superba, at South Georgia showed consistent patterns between years. Densities were substantially higher over the shelf compared with off-shelf, with the highest densities at the shelf edge; densities were also higher to the east of the island. During 1997/98, acoustic den- sities of krill were substantially higher than in other years. The coincidence of the elevated acoustic den- sity and the cooler oceanographic conditions was explored. When data from all years were combined and analysed by Generalized Additive Model, an in- verse relationship between acoustic density and tem- perature was apparent. Historical data were also examined and it was noted that the only other occurrence of such a high estimate of krill density at South Georgia, was when oceanographic conditions were also colder. Key words: acoustic density, Antarctic Circumpolar Current, Antarctic krill, Euphausia superba, inter- annual variability, oceanography, Scotia Sea, South Georgia, southern Antarctic Circumpolar Current boundary, southern Antarctic Circumpolar Current front, Southern Ocean INTRODUCTION Variability in the distribution and abundance of Antarctic krill (Euphausia superba Dana) close to the sub-Antarctic island of South Georgia has been well documented (Heywood et al., 1985; Priddle et al., 1988; Brierley et al., 1997, 1999a). However, the de- tailed mechanisms that drive this natural variability are not well understood. Determining the causes that underlie the variability is important for at least two reasons. First, a variety of marine predators at South Georgia are dependent upon krill, and variability in their breeding success is linked to levels of prey abundance (Croxall et al., 1988). Secondly, the com- mercial fishery for krill at South Georgia (Everson and Goss, 1991; Murphy et al., 1997; Trathan et al., 1998) potentially competes with these predators. Manage- ment objectives in the Southern Ocean attempt to minimize this potential overlap between natural predators and the commercial fishery (Anon., 1992) and, as such, require a detailed understanding of the factors that drive variability in the krill stock. Efforts to understand the mechanisms driving nat- ural variability stretch back as far as the early part of the last century when the Discovery Expeditions were initiated in association with the shore-based whaling industry at South Georgia. Early analyses indicated that substantial levels of biological variability were linked with changes in mean temperature (Harmer, *Correspondence. e-mail: p.trathan@bas.ac.uk Received 15 October 2001 Revised version accepted 14 November 2002 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 12:6, 569–583, 2003 Ó 2003 Blackwell Publishing Ltd. 569