Spatial distribution and swarm characteristics of Antarctic krill around the South Shetland Islands

The spatial distribution and swarm characteristics of Antarctic krill were studied using the swarm-based method established in SG-ASAM-17. Acoustic data were collected from the FV Fu Rong Hai using Simrad EK60 echosounder (38/70/120 kHz) during four transect surveys conducted in December 2013, March 2015, January 2016 and February 2018 around South Shetland Islands. The swarm characteristics, including Volume Backscattering Strength (Sv), Nautical Area Scattering Coefficient (NASC), depth, thickness and length of swarms were estimated from the 120kHz data. The full spatial distributions of acoustic density were obtained with Kriging space interpolation, which showed that krill usually aggregated as swarms in a patchy fashion and the mean krill densities in December 2013 and February 2018 were markedly higher than the other two years. Meanwhile, much more swarms (1055) were detected in February 2018 than the other three years. The results also indicated that more krill swarms located at deeper water in March 2015 compared with those in December 2013, January 2016 and February 2018, although the majority of swarms were also found in the upper 100 m layer. The swarm-based method provides a promising way not only for biomass estimation but also for the study of swarm characteristics.