Spatial-temporal relationships between two euphausiid species in the Ross Sea

This paper summarizes the results obtained during the Italian marine Antarctic expeditions to the Ross Sea in November and December 1994, December 1997, and January 2000. The distribution and abundance of the two krill species (Euphausia superba and Euphausia crystallorophias) are shown in relation to the degree of ice cover, and a potential mechanism is presented that allows the coexistence of the two similar species in the same region. The surveys indicate that the northern and north-western areas of the Ross Sea are largely dominated by Euphausia superba, while the southern and southwestern areas are dominated by Euphausia crystallorophias. The two krill populations show some overlap in the central part of the Ross Sea, where the distribution of the two species appears related to the ice cover. The biomass centroids of the two krill species seem to move northward associated with the ice edge retreat in summer. The centroid of the Euphausia superba biomass moves much faster and further than that of Euphausia crystallorophias and reaches ice-free waters at the edge of the Ross Sea in summer. This suggests that the spatial extent of the Euphausia superba population may extend beyond the Ross Sea, and a portion of this population may spread into the ocean waters starting in early January. In contrast, the Euphausia crystallorophias population appears to be restricted to the Ross Sea.