Surface water circulation in krill fishing areas near the South Shetland Islands
This paper describes tracks followed by six surface drifting buoys that were deployed in shelf and oceanic waters to the north of the South Shetland Islands. Each buoy was deployed with a drogue attached at a depth of 30 m to assess flow patterns at the depth range where krill tended to be most abundant. The buoys were tracked using the ARGOS satellite positioning system. Based on buoy tracks, the surface circulation to the north of the South Shetlands shelf was described as a slow sluggish southwest current with a series of small-scale circular and semi-circular flows in the shelf region, whereas the circulation in the oceanic region was described as the strong northeast current. Between these counter-flowing currents a shear current was found in the slope region. The direct current measurements demonstrated that the oceanographic regime in these waters is not well described by geostrophic calculations. The picture obtained of the surface circulation was considered in conjunction with the distribution of krill catches taken in the area by Japanese vessels during the period from 1981 to 1994. It is suggested that the sluggish and shear currents observed to the north of the islands may have properties which contribute to the formation of stable krill concentrations in the area. The observed long-distance movements of three buoys also suggested that oceanic currents may transport krill from the South Shetland Islands to the South Georgia and South Orkney Islands on a time scale of 110 to 120 and 85 days respectively.