Spatial and temporal variability in the structure of aggregations of Antarctic krill (Euphausia superba) around South Georgia, 1997-1999

Antarctic krill are important in the South Georgia (548 degrees S 358 degrees W) marine ecosystem. They form aggregations that vary widely in packing density (<1 to 1000 s of individuals m(-3)), length (tens to thousands of metres), and height (tens of metres). Acoustic surveys are often used to estimate krill biomass and provide data that give insight into aggregation structure. Using dual-frequency (38 and 120 kHz) acoustic data collected during six surveys conducted around South Georgia during the 1997, 1998, and 1999 austral summers, we isolated 2990 aggregations by applying the Shoal Analysis and Patch Estimation System algorithm in Echoview and a krill-length-dependent acoustic identifier (Delta Sv120-38). Multivariate cluster (partition) analysis was applied to metrics from each of the aggregations, resulting in three aggregation types with an overall proportional split of 0.28 : 0.28 : 0.44. Types 1 and 3 had low mean densities (<2 g m(-3)), whereas Type 2 had a mean density of 94 g m(-3). Intersurvey differences were found between the effort-corrected numbers of aggregation types (p 2.5e(-6)), and between on-and off-continental shelf areas (p = 1.5e(-7)), with a greater number of Type 2 aggregations being found on-shelf. The findings suggest intersurvey variation in krill catchability, with krill being more likely to be caught on-shelf.