Summer distribution, abundance and structure of krill populations (Euphausia superba and Euphausia crystallorophias) sampled by plankton net in the western Ross Sea (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 at 63 stations. The net was deployed normally as a standard double oblique tow. Mean relative biomass of Antarctic krill Euphausia superba, in the area north of Continental Shelf, was 9.4 (±18%) g/l000m3 of filtered water with a mean density of 11.0 (±16%) indviduals per 1000m3. The high abundance of Euphausia superba (1510 g/1000m3) found in a single haul on the Mawson Bank Area (73°S 173°E) was remarkable. 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 (±26%) g/l000m3 and mean density of 16.8 (±10%) individuals per 1000m3. The present data have demonstrated that in the Ross Sea during the summer period, the two species of euphausiid inhabit different areas. Lengths of Ice krill and length, sex and reproductive stage of Antarctic krill occurring in discrete aggregations were analysed. This indicated that the composition of the aggregations in terms of the mean length, sex ratio and maturity stage were very variable. The catch data of Euphausia superba was characterized by the complete absence of larval stages, scarce occurrence of juveniles and composed primarily of large adult stages. From the analysis of the krill population structure it emerged that in mid summer, Euphausia superba of age 3+ and 4+ were clearly predominant. While the overall length frequency distribution of Euphausia crystallorophias is characterized by a first mode of juvenile individuals (Age class 1 +) and a second mode consisting of sub-adults and adults (Age groups 2+ and 3+).