Comparative swarming dynamics of Antarctic krill and ice krill in the Ross Sea Region Marine Protected Area.

Antarctic krill (Euphausia superba) and ice krill (Euphausia crystallorophias) are vital to the Southern Ocean ecosystem, yet their swarming behavior remains poorly understood. This study applied a novel approach in the Ross Sea by directly linking acoustically detected swarms with net-captured krill, enabling precise identification of species and body size-key traits often unavailable in previous studies. From 14 Antarctic krill and 55 ice krill swarms, key properties-depth, length, height, area, and packing concentration-were extracted. Hierarchical clustering grouped swarms into two Antarctic krill groups and three ice krill groups with significant differences (P < 0.01). Antarctic krill swarms were often shallower, in warmer, less saline waters away from the coastline and ice-edge, compared to ice krill. Larger Antarctic krill (47.5 mm) formed swarms about 40 times smaller in area than smaller ones (40.1 mm). The smallest ice krill (11.4 mm) appeared in Terra Nova Bay, at greater depths, forming dense swarms. Despite similar body sizes and environmental conditions, some ice krill swarms were up to 15 times larger than others. Body length was a key factor influencing swarm properties. These findings provide new insights into swarming strategies and behavioral ecology of both species in the Ross Sea.