Interannual variability in krill abundance at South Georgia

Interannual variability within the pelagic marine environment around South Georgia is a well-recognised phenomenon. A key aspect of this variability is interannual fluctuation in the abundance of Antarctic krill Euphausia superba. Here we describe a new acoustic survey programme to monitor krill abundance in the South Georgia region. We present biomass estimates for 2 survey boxes, located over the shelf-break to the northeast and northwest of the island, derived from the first of these surveys conducted in January 1996. We contrast these with the most recent previous estimates for the region obtained in January 1994. Weighted mean krill density (and weighted variances) estimates for the 1996 surveys were 40.57 g m-2 (13.37) and 26.48 g m-2 (54.30) for the eastern and western boxes respectively. These are high compared with those obtained in January 1994, when estimates for similar areas were 1.87 g m-2 (0.14) and 7.43 g m-2 (1.33) respectively. The greater than 20-fold difference between surveys reveals a very large interannual variability in krill abundance at South Georgia. In 1994 the low abundance resulted in greatly reduced breeding success in most habitual krill predator species there. In the 1996 season, however, breeding success of these species was normal. Instantaneous estimates of krill abundance using acoustic techniques are therefore consistent with measurements from predators, whose breeding performances provide a longer-term indication of prey abundance in the surrounding pelagic ecosystem in a particular season.