Three-dimensional observations of swarms of Antarctic krill (Euphausia superba) made using a multi-beam echosounder

Antarctic krill (Euphausia superba) aggregate in dense swarms. Previous investigations of krill swarms have used conventional single- or split-beam echosounders that, with post-processing, provide a two-dimensional (2-D) view of the water column, leaving the third dimension to be inferred. We used a multi-beam echosounder system (SM20, 200 kHz, Kongsberg Mesotech Ltd, Canada) from an inflatable boat (length=5.5 m) to sample water-column backscatter, particularly krill swarms, directly in 2-D and, with post-processing, to provide a three dimensional (3-D) view of entire krill swarms. The study took place over six days (2–8 February 2006) in the vicinity of Livingston Island, South Shetland Islands, Antarctica (62.4°S, 60.7°W). An automatic 3-D aggregation detection algorithm resolved 1006 krill swarms from the survey data. Principal component analyses indicated that swarm morphology metrics such as length, surface area and volume accounted for the largest between swarm variance, followed by echo energy, and finally swarm geographic location. Swarms did not form basic cylindrical or spherical shapes, but had quite consistent surface area to volume ratios of 3.3 m−1. Swarms were spatially segregated, with larger sizes (mean north-south length=276 m, at least double that of two other swarm classifications), found to the northwest of the survey area. The apparent clustering of swarm types suggests that krill biomass surveys and ecosystem investigations may require stratified survey design, in response to varying 3-D swarm morphology, variation that may be driven in turn by environmental characteristics such as bathymetry.