Krill of the Ross Sea: distribution, abundance and demography of Euphausia superba and Euphausia crystallorophias during the Italian Antarctic expedition (January–February 2000)

Net samples data from the 15th Italian Antarctic Oceanographic Cruise (Jan-Feb 2000) were analysed to obtain a general picture of the summer distribution pattern, abundance and demography of krill in the western Ross Sea (Antarctica). A midwater sampler-trawl (Hamburg Plankton Net) was used to collect zooplankton and fish larvae. Mean relative biomass of Antarctic krill Euphausia superba, in the area north of Continental Shelf, was 9.3 g/1000m3 of filtered water with a mean density of 10.9 individuals per 1000m3. Ice krill Euphausia crystallorophias replaced the Antarctic krill in dominance in the High Antarctic Zone (south of 74°), with a mean relative biomass of 3.0 g/1000m3 and mean density of 19.1 ind/1000m3. The present data have demonstrated that in the Ross Sea during the summer period, the two species of euphausiid inhabited different areas. Oceanographic data indicate that both euphausiid species were found in surface waters, Euphausia crystallorophias in proper Ross Sea water, and Euphausia superba in Antarctic Surface Water. The catch data of Euphausia superba were characterized by the complete absence of larval stages, scarce occurrence of juveniles and composed primarily of large adult stages, whereas the overall length frequency distribution of Euphausia crystallorophias was characterized by a first mode of juvenile individuals and a second mode consisting of sub-adults and adults.