Acoustic scattering properties of juvenile jack mackerel Trachurus japonicus based on a scattering model and ex situ target strength measurements

The purpose of this study was to clarify the acoustic scattering properties of juvenile jack mackerel Trachurus japonicus, based on theoretical calculations and target strength (TS) measurements for determining the optimum frequency with which to estimate the biomass using a scientific echosounder. The theoretical TS of the juvenile jack mackerel was estimated using a scattering model (PSM) based on the morphometry of the swim-bladder. The fork length (L) ranged from 7.5 to 12.9 cm (26 specimens). These theoretical TS values corresponded with ex situ measurement results of the TS. The normalized max. TScm and mean TScm, i.e. the TS normalized by the square of body length (L, cm), at each frequency corresponding to those commonly used for acoustic surveys (18, 38, 50, 70, 120, and 200 kHz), ranged from -66.2 to -68.4 dB, and -68.7 to -69.9 dB, respectively. In addition, these TS differences between pairs of frequencies (ranging from 18 kHz to 200 kHz) within the tilt angle distributions having a mean of 0 degrees and standard deviation of 15 degrees were relatively small.