Target strength pattern measurements of Antarctic krill (Euphausia superba) by split-beam method in a small tank

Abstract

Target strength (TS) measurements were conducted for Antarctic krill (Euphausia superba) at 70 kHz on board RTV Umitaka-maru of Tokyo University of Marine Science and Technology in February 2003 during the Southern Ocean survey and TS patterns of 12 live Antarctic krill were successfully measured. We used a new precise method using the split-beam method in a small tank for TS measurement of small-size animals. We compared the measured results with the theoretical TS patterns by the Distorted Wave Born Approximation-based deformed cylinder model (DWBA model) and confirmed a good agreement at near the main lobe, however, the measured results were higher than the model predictions in the side lobe regions. Although the discrepancy was examined from several aspects such as body bending and scattering from pleopods, any factor was not decisive. In other words, it is likely that the discrepancy is caused by some factors combined. The stochastic version of the DWBA model showed a reasonable agreement with the measured TS in the side lobe regions. This phenomenon should be examined further from physical view points.