Acoustic determination of Antarctic krill biomass at South Georgia (Subarea 48.3) during winter

During the austral winter of 2022, three 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, corresponding with the start, 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. An RMT1 net with 610 um mesh net was deployed to target krill swarms to estimate the length-frequency of krill for target strength models. Krill swarms were identified using the CCAMLR swarms-based method. Krill areal biomass in the area normally used by the krill fishery (Eastern Core Box (ECB)), using combined day and night transect data, was 268,521 tonnes in May, 249,822 tonnes in July and 76,828 tonnes in September. Krill density was 45.3 g m^-2, 42.1 g m^-2 and 13.0 g m^-2 over the same time period. Distinct diurnal variability in biomass was observed, with significantly higher biomasses detected at night in July and September. There was less diurnal variability in May. We observed what appeared to be a westerly movement of krill throughout the winter period, with the lowest biomasses observed in the ECB during September.