Effects of sea ice cover on the swarming behaviour of Antarctic krill, Euphausia superba

Much of the distribution range of Antarctic krill, Euphausia superba, is co vered by permanent or seasonal sea ice. Sea ice extent has been implicated as a major factor affecting reproductive success of krill and krill dispers al, but little is known of the way in which ice cover may influence krill b ehaviour. This is largely because the under-ice environment is difficult to study. Ship-borne echosounders have, however, detected krill aggregations in midwater in ice-covered regions. We used 120-kHz echograms collected und erway during three cruises that crossed ice-covered and adjacent open water s in the Bellingshausen, Weddell, and Scotia seas to compare morphological and next-neighbour characteristics of krill swarms within and without ice c over. No significant differences were detected between the horizontal and v ertical extent of swarms or swarm next-neighbour distance in ice-covered or open waters. Distributions of swarm mid-depths did, however, differ signif icantly between ice-covered and open areas in all three seas, although the direction of difference was not the same in each instance: swarms in the We ddell and Scotia seas were generally shallower under ice than in open water , whereas in the Bellingshausen Sea the opposite prevailed.