Hydrography and acoustic biomass estimates of Antarctic krill (Euphausia superba) near the South Shetland Islands, Antarctica, during January 2000: preliminary results

To estimate the abundance and distribution of Antarctic krill (Euphausia superba) in the vicinity of South Shetland Islands (SSI), a hydroacoustic/oceanographic survey was conducted by the R/V Onnuri from 9 to 19 January 2000. The acoustic data were obtained using a Simrad EK500 echosounder operating at 38, 120, and 200 kHz, from 8 transects comprising the South Shetland Islands (SSI) mesoscale box (total transect length = 459 nautical miles, area = 11,313 n mile2). Krill were collected with a Bongo net (mesh size: 0.333 mm, 0.505 mm) to determine their size composition and stage of development. In addition, Conductivity, Temperature and Depth (CTD) and on-station Acoustic Doppler Current Profiler (ADCP) were deployed to understand the physical structure of the water column at 11 stations.
The hydrography was differentiable between the offshore region and coastal area. Offshore stations showed the presence of ‘Winter Water’ in mid layer and Circumpolar Deep Water at greater depths. Offshore water and coastal water were also seen to the north and south of Elephant Island, respectively. ADCP results indicate the water flowing from the Bransfield Strait out to the offshore region around the east of King George Island.
The length-weight relationship of krill sampled during the survey was w = 0.0035*(L)3.2108 where w was the mass (mg) and L was the total length (mm) ; the median length was 50 mm. The conversion factor for integrated backscattering area to areal krill biomass density at 120 kHz was 0.1556. The mean density of krill near the SSI area was 12 g/m2 with a coefficient of variance (CV) of 14.5%. The mean density and CV in the Bransfield strait was 9 g/m2 and 20.3 %. Krill swarms of relatively high densities occurred to the north of Smith Island, north and east of King George Island and north and south of Elephant Island.