Multiple acoustic estimates of krill density at South Georgia during 2000/2001 reveal significant intra-annual and spatial variability

Three separate acoustic surveys of Antarctic krill (Euphausia superba) were conducted around South Georgia in the 2000/2001 season: in October 2000 (early-season); during late December / early January 2000/2001 (mid-season), and in March 2001 (late-season). The surveys were the first in a newly-planned five-year series of observations designed to complement and extend an existing time series maintained by the British Antarctic Survey regularly since 1996 (and on a more ad hoc basis since the early1980s). We hoped that conducting several surveys in one season would provide information on short-term variability that could be used to set data from more restricted “snap-shot” cruises in a broader context. The early- and late-season surveys were associated with logistic support voyages to South Georgia and were restricted to four transects within a box to the north-west of South Georgia. The dedicated mid-season survey covered that box in more detail (twice as many transects) and, in addition, examined boxes to the north, north-east and south-west of the island. Together these surveys provided temporally and spatially extensive coverage around South Georgia. Krill density in the western box in the early-season survey was very low (3.5 g m-2 ) but rose significantly (P = 0.048) by mid-season (to 34.7 g m-2). In a pattern that is consistent with observations from previous years, krill density in the western box mid-season was less than that in the eastern box (80.4 g m-2). Analysis of mid-season western survey box transect data revealed no significant difference between the mean krill density derived from only those 4 transects surveyed early- and late-season or from the full 8 transects. Our first occupation of a survey box off the central north coast of South Georgia mid-season revealed a krill density of 47.2 g m-2 that was intermediate between the eastern and western areas. The size structure of the krill in the central region also reflected a mix of those to the east (generally small) and west (generally large). Krill density to the south-west of South Georgia was 32.1 g m-2 mid-season. By March, krill density in the western survey area had fallen significantly (P = 0.04) from the mid-season high to 7.8 g m-2. Our multiple surveys at South Georgia have revealed major intra-annual changes in krill density at the island and have shown that the timing of the acoustic survey can significantly effect the estimate of krill density. The multiple estimates of krill density will allow indices of reproductive performance of top level predators to be compared to prey availability at time scales more appropriate than have previous single “snap-shot” acoustic survey data. This is a crucial step in the elucidation of response functions of dependent species to changes in krill abundance, and could be a useful contribution to ecosystem management.