CCAMLR

In this study, we calibrated a commercial echosounder (ES70) installed on a krill fishing vessel to use it in estimating biomass of Antarctic krill (Euphausia superba). The method of calibration was to analyze the difference between the bottom backscattering strength of the commercial echosounder (i.e. ES70) and the scientific echosounder (i.e. EK60) at some transects designated by CCAMLR. 38kHz and 120kHz were used for the calibration, and krill swarm signals, obtained from multi frequencies, was examined to verify the calibration result. The analysis result confirmed the possibility of calibration by bottom backscattering strength, since the proportion of krill swarm signals within 2 dB < S_{V120kHz‒38 kHz} < 12 dB (i.e. a common ΔMVBS range of 38kHz and 120kHz to be an indicative of Antarctic krill) over the total acoustic signals were 26.95% and 92.04%, respectively before and after the calibration.

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.