Internal structure of Antarctic krill Euphausia superba swarms based on acoustic observations

Ping-by-ping acoustic data from more than 3000 Antarctic krill Euphausia superba aggregations were analyzed and patterns in density distribution within the swarms examined. It was observed that krill were neither randomly nor evenly distributed within the swarms, particularly in larger swarms (extending for more than about 35 m along track), where there was significant evidence of krill concentration in the centre. Linear trends in density across swarms were also common. In swarms of intermediate size (15 to 35 m along track), these linear effects were predominantly negative, perhaps indicating reaction of the swarms to the approaching ship. Significant linear effects were present in the larger swarms as well, but in these, positive gradients were as common as negative ones. The effects of beam width on estimates of swarm density and internal structure made by echosounder are examined, and a method of correcting for these effects described. It was found that if no beam width correction is applied, artefacts are introduced into the apparent density structures, while the mean swarm density can be underestimated by as much as 50 %.