Acoustic determination of Antarctic krill biomass at South Georgia (Subarea 48.3) during winter: results from two winter seasons

During the austral winters of 2022 and 2023, six acoustic surveys were carried out to estimate the biomass and distribution of Euphausia superba (Antarctic krill) on the northern shelf of South Georgia. Surveys were undertaken in May, July and September 2022 and April, July and September 2023, corresponding with the beginning, middle and end of the krill fishing season, carried out on board the MV Pharos SG using an EK80 split-beam echosounder with 38 and 120 kHz transducers. Data on krill length-frequency were obtained from both standard and target deployments of an RMT1 net with 610 um mesh net, and krill fishery data, and used for target strength models. Krill swarms were identified using the CCAMLR swarms-based method. In 2022, krill areal biomass in the area normally used by the krill fishery (Eastern Core Box (ECB)), using combined day and night transect data, was 268,571 tonnes in May, 245,816 tonnes in July and 75,676 tonnes in September. In 2023 it was 574,249 in April, 63,389 tonnes in July and 8,641 tonnes in September. Krill biomass density in 2022 was 45.3 g m^-2, 41.5 g m^-2 and 12.8 g m^-2 in May, July and September respectively, and in 2023 was 96.9 g m^-2, 10.7 g m^-2 and 1.5 g m^-2 in April, July and September. Diurnal variability in biomass was observed, although this varied between years and across seasons. We also highlight the similar NASC to biomass conversion factors obtained between net and fishery data despite different length-frequency distributions in 2023, in contrast with 2022 where distributions varied less yet the conversion factor varied more.