Density and biomass of Antarctic krill around South Shetland Islands using by 2-dB difference method

The dB-difference and characteristics of krill aggregation inhabiting Subarea 48.1, which includes the Elephant Island peripheries and the west and south of the South Shetland Islands, were estimated to distinguish Antarctic krill, using acoustics. From April 13 to 24, 2016, acoustic data were collected along 24 survey lines using the frequencies 38 and 120 kHz, and midwater trawling was performed at seven stations. Using the difference between the dB values of two volume backscattering strength (Sv) frequencies (38 and 120 kHz), a clear acoustic distinction could be made between Antarctic krill (4.9 to 12.0 dB) and fish (-4.0 to -0.2 dB). The distributions and mean Sv of krill aggregations in the Elephant Island peripheries and south of South Shetland Islands were higher than those in the west of the South Shetland Islands. The mean length/height ratio of krill aggregations in the west of the South Shetland Islands (64.5) was higher than that in the south (35.9) and the Elephant Island peripheries (33.8), with the length of the aggregations exceeding their height. It was evident that most krill aggregations were distributed between the surface layer (less than 10m) and 200m depth.