Seasonal variability in the distribution of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula

Abstract

A unique Antarctic data set from four multidisciplinary cruises (spring, Nov 1991; summer, Jan–Feb 1993; fall, Mar–May 1993; winter, Aug–Sept 1993) was analyzed to provide a description of seasonal variability in the distribution and abundance of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula. Analyses of acoustic observations revealed distinct seasonal variations in estimates of krill abundance, the dimensional parameters used to characterize individual krill aggregations, the geographic area over which krill were acoustically detected, and the depth distribution of krill biomass. Spatially averaged estimates of krill biomass were an order of magnitude higher in spring (32 g m -2) and summer (95 g m -2) compared to fall (12 g m -2) and winter (8 g m -2). In summer, krill were detected throughout the region and most of the krill biomass was associated with aggregations of small cross-sectional area (<2000 m 2) and high mean biomass (>150 g m -3), which were positioned in the upper 50 m of the water column. Winter observations, in contrast, were characterized by the absence of krill throughout most of the region and aggregations occuring deeper than 100 m with large cross-sectional area (>10000 m 2) and low mean biomass (<10 g m -3). Two features common to all seasons were the presence of higher krill biomass (>50 g m -2) at selected locations on the inner shelf and the disproportionate contribution, in terms of total krill biomass (>80%), by a small number of aggregations (<20%). The length frequency distributions of krill collected by nets showed an across-shelf pattern of larger (>40 mm) krill positioned offshore of smaller krill (32–38 mm) in all seasons except winter. Synthesis of these data and historical observations suggest that there is a seasonal shift in the primary habitat of krill and that changes in krill behavior are an important factor affecting variations in krill distributions.