Krill demography and large-scale distribution in the southwest Atlantic during January/February 2000

This paper summarizes the results of krill demographic studies from the Commission for the Conservation of Antarctic Marine Living Resources 2000 Survey—a large-scale krill survey across the Scotia Sea conducted in January/February 2000. Standard Rectangular Midwater Trawl net tows were carried out at midnight and midday stations between 20° and 70°W south of the Polar Front. The overall mean numerical density and biomass of krill estimated from nets (247 krill per 1000 m3 and 18.7 g m−2, respectively) were similar to lower values reported previously for the Southwest Atlantic, and comparable with high values for other regions of the Southern Ocean. Krill varied in mean size across the survey area, with cluster analyses showing three distinct groups of length-frequency distribution. Small juvenile and immature krill occurred east of the South Orkney Islands. Adult krill <50 mm dominated the shelf areas of the Antarctic Peninsula and to the north of the juvenile stock across the Scotia Sea. Adult krill >50 mm were mostly restricted to the west of the South Orkney Islands. Maturity stage composition indicated that peak spawning occurred before early February. Distribution of the spawning stock showed two hotspots, the first between the South Shetland and South Orkney Islands, and the second around the South Sandwich Islands. Reproductive krill were largely absent in the central Scotia Sea and around South Georgia. Krill larvae concentrations occurred slightly to the east of the spawning stock. Mean density of larvae in the western Scotia Sea was 2044 m−2, but only scattered aggregations of larvae were recorded east of 36°W (2 m−2). Recruitment indices for 1- and 2-year-old krill were low in the western part of the survey area, the outflow from the Bellingshausen Sea/Antarctic Peninsula region ( R 1 = 0.0 , R 2 = 0.11 ) indicating spawning failure and/or poor recruitment for several years. In the eastern part of the survey area, mostly the outflow of the Weddell Sea, recruitment indices were high and above the long-term average ( R 1 = 0.60 , R 2 = 0.72 ), suggesting a population with constant and successful reproduction, recruitment and mortality. The distribution and structure of the krill population did not show any clear relationship to the position of the major oceanic fronts in the Scotia Sea.