Effective and accurate use of difference in mean volume backscattering strength to identify fish and plankton

Abstract

Acoustic species identification is very important for fisheries' operations and surveys. One of the most promising methods for identification is to utilize the difference of mean volume backscattering strengths (ΔMVBS) among frequencies. Improvement of this technique is the aim of this study. The ΔMVBS must be obtained for a common observation range among frequencies so that the difference can be attributed solely to frequency characteristics of the sound scattering of targets organisms. We derived the common observation range of at least up to 150 m for our quantitative echosounder operating at 38 and 120 kHz with the same beam widths of 8.5°. We related ΔMVBS data obtained off northeastern Japan to specific marine organisms combined with the swimming depth and water temperature information. The echoes with small ΔMVBS (−1 dB<ΔMVBS10 dB) to krill ( Euphausia pacifica ). The changing pattern of ΔMVBS suggested complicated behaviour between species such as predator and prey interaction. In order to obtain reliable and detailed information the integration cell should be small and the ΔMVBS should be displayed as an echogram in an absolute colour scale.