Acoustic estimate of the krill (Euphausia superba) density between South Shetland islands and South Orkney Islands, Antarctica, during 2002/2003 austral summer

Abstract

Acoustic survey for density and biomass estimate of Antarctic krill, Euphausia superba, was conducted in the large area between South Shetland Islands and South Orkney Islands, during November 30-December 30, 2002. Considering oceanographic and geographic properties, the study area was divided into six sub-regions. Acoustic system and frequency used in the survey were quantitative echo sounder (Simrad EK500) and 38, 120-kHz split beam transducers. In order to discriminate krill aggregations in all acoustic signal, difference of mean volume backscattering strength (ΔMVBS) method of the two frequencies was introduced. Averaged krill density for the overall surveyed area was 23.5 g/m2, and spatially averaged estimates of krill density were 44.9 g/m2 (north of the South Shetland Islands), 30.3 g/m2 (Bransfield Strait), 11.3 g/m2 (near the Elephant Island), 13.6 g/m2 (north of the Elephant Island), 18.1 g/m2 (between Elephant Island and South Orkney Islands) and 21.7 g/m2 (northwest of the South Orkney Islands) at each sub-area. In the two sub-regions with surveyed area, estimated krill biomass in the north of the Elephant Island was 0.315 million tones with a CV of 18.35% (6,766 mile2), and between Elephant Island and South Orkney Islands was 1.26 million tones with a CV of 9.45% (20,299 mile2). As a whole, the krill density in the early summer season was low level, comparing with that of January-February. This suggested that major krill swarms in the around South Shetland Islands were reached in the mid-summer seasons from western part of the Antarctic Peninsula, and the low krill density also affects the density variation of the krill between Elephant Island and South Orkney Islands.