Multifrequency echosounder data were collected during the 2006 BROKE-West summer survey of Division 58.4.2 for the purposes of estimating the unexploited biomass (B0) of Antarctic krill (Euphausia superba) and its associated coefficient of variance (CV). This paper updates the version submitted to WG-EMM in 2006 (WG-EMM-06/16) because a reanalysis of the data has resulted in amendments to the acoustic estimates of krill mean biomass density, biomass and variance. The mean acoustic biomass density of krill, integrated to 250 m depth across the entire survey stratum (1.31 million km2), was 9.48 g m-2. B0 was estimated to be 12.46 million tonnes with a CV of 15.15%. Krill were widely distributed at relatively low densities throughout the survey area; only 13% of the 2-km-alongtrack echo-integration intervals were devoid of krill, 50% of intervals registered densities of 1 g m-2 of krill or less, and 80% of intervals registered densities of 10 g m-2 or less. Mean densities were highest in the waters to the south of the Southern Boundary of the Antarctic Circumpolar Current, particularly in waters to the west which were within the influence of the Weddell Gyre. Half of the cumulative density across the survey was found within 120 km of the 1000 m isobath (the shelf-break/ slope), and 40% within 50 km. This was mostly due to very high densities (up to 1400 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. The krill distributions inferred from both the acoustic data and from net catches were considered in the context of the physical oceanography, from which a case is presented for the subdivision of Division 58.4.2 into smaller, more biologically homogeneous areas. A qualitative critical appraisal of the methods is included by way of contribution to ongoing discussions about acoustic survey and analysis methods for krill.
Numéro du document:
WG-EMM-07/33
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