INTERNAL STRUCTURE OF ANTARCTIC KRILL EUPHAUSIA-SUPERBA SWARMS BASED ON ACOUSTIC OBSERVATIONS

Ping-by-ping acoustic data from more than 3000 Antarctic krill Euphaus ia superba aggregations were analyzed and patterns in density distribu tion within the swarms examined. It was observed that krill were neith er 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. L inear trends in density across swarms were also common. In swarms of i ntermediate size (15 to 35 m along track), these linear effects were p redominantly negative, perhaps indicating reaction of the swarms to th e approaching ship. Significant linear effects were present in the lar ger swarms as well, but in these, positive gradients were as common as negative ones. The effects of beam width on estimates of swarm densit y and internal structure made by echosounder are examined, and a metho d of correcting for these effects described. It was found that if no b eam width correction is applied, artefacts are introduced into the app arent density structures, while the mean swarm density can be underest imated by as much as 50%.