Biomass of Antarctic krill (Euphausia superba) in the eastern sector of the CCAMLR Division 58.4.2

In February and March 2021 an acoustic-trawl survey was carried out to estimate the biomass of Antarctic krill (Euphausia superba) in the eastern sector of the CCAMLR Division 58.4.2 (area = 775,732 km².) The survey was run from the Australian Research Vessel RV Investigator operating a calibrated EK80 scientific echosounder and an RMT-8 + 1 net. Krill were identified using the swarms-based technique and estimated mean areal biomass density was 6.4 gm^-2. Differences were found between day and night mean areal biomass densities (t-test, p = 1.2e-07) and the statistical distribution of biomass densities (KS-test, p <2e-16). Day-night biomass density differences were suggestive of krill migrating to shallow water at night and so being missed by hull mounted echosounders. We therefore used the day time only observations which resulted in a 35% reduction in total transect length from 1,327 nautical miles reduced to 861 nautical miles. Day time mean areal biomass density was 8.3 gm^-2 with a total biomass 6.477 million tonnes, with CV = 28.9%.