1National Fisheries University, Shimonoseki, Yamaguchi 759-6595, 2National Research Institute of Fisheries Engineering, Hasaki, Ibaraki 314-0408, 3Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, 4Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
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 swimbladder. 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° and standard deviation of 15° were relatively small.