Interpretation of acoustic data at two frequencies to discriminate between Antarctic krill (Euphausia superba Dana) and other scatterers

Abstract

A visual classification was carried out of echo trace characteristics derived from 113 integration intervals of 1 n mi (nautical mile) each taken from the Southern Ocean in the area of South Georgia. This separated the echo traces into five categories: three biotic and two abiotic in origin. The acoustic data were subsequently analysed in detail by comparing the acoustic parameter Mean Volume Backscattering Strength (MVBS) at 120 kHz with that at 38 kHz. The three biotic categories separated visually had significantly different mean AMVBS values (δMVBS = MVBS 120 kHz - MVBS 38 kHz). To investigate if the biotic categories could be identified solely on the basis of δMVBS values, maximum-likelihood techniques were used to analyse the multimodal distribution of a further 1286 δMVBS records. These analyses allowed ranges to be defined for AMVBS which separate three biotic categories. On the basis of the target strength properties of the most likely scatterers in the region, the three categories were identified as krill, organisms larger than krill (fish or squid) and organisms smaller than krill (small zooplankton). The acoustic techniques described in this paper indicate a useful methodology in the study of Southern Ocean pelagic organisms.