Acoustic characterisation of the broad-scale distribution and abundance of Antarctic krill (Euphausia superba) off East Antarctica (30-80°E) in January-March 2006

Abstract A large-scale oceanographic survey (BROKE-West) was undertaken off East Antarctica in the austral summer of 2005/06. Throughout the survey, multi-frequency echosounder data and ancillary environmental data were collected to determine the distribution and abundance of Antarctic krill ( Euphausia superba ) and to explore its broad relationship with the bio-physical environment. The acoustic data were analysed using three different methods to provide measurements of krill abundance that can be set in context with previous studies. Based on the most recently developed acoustic method, the mean biomass-density of krill across the survey area (1.3 million km 2 ) was estimated to be 24 g m −2 . Total biomass was estimated to be 28.75 million tonnes (Mt) with a coefficient of variation (CV) of 16.18%. This biomass estimate has been used by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to update the precautionary catch limit for krill in this area (CCAMLR Division 58.4.2) from 0.450 to 2.645 Mt. Overall, krill were widely distributed at relatively low densities: 25% of the 2-km along-track echo-integration intervals were devoid of krill, 50% registered densities of 1 g m −2 of krill or less and 75% registered densities of 12 g m −2 or less. Mean densities were highest in the waters to the south of the Southern Boundary (SB) of the Antarctic Circumpolar Current (ACC), while the waters to the north of the Southern ACC Front (SACCF) were almost devoid of krill. Half of the cumulative krill density across the survey was found within 80 km of the 1000 m isobath (the shelf break), and 40% within 40 km. This was mostly due to particularly high densities (up to 4400 g m −2 ) around the shelf break on 3 of the 11 transects surveyed. The majority of acoustic krill detections were in the top 100 m of the water column, centred around 50 m depth.