In this study, Target strength for multi-frequency (38 kHz, 70 kHz, 120 kHz and 200 kHz) of chub mackerel (Scomber japonicus) was estimated using by the KRM model. The body shape of the Chub mackerel was described by an X-ray system and the body length of 20 individuals ranged from 16 cm to 28 cm. The swimbladder tilt angle ranged between – 8 and – 14°, the maximum TS value according to the swimming angle of chub mackerel was – 33.0 dB at – 11°. The averaged TScm according to fork length was – 66.02 dB at 38 kHz, – 66.50 dB at 70 kHz, – 66.00 dB at 120 kHz and – 67.35 dB at 200kHz, respectively.

This study was focused on acoustic scattering characteristics of jack mackerel (Trachurus japonicus) at frequency 38, 70, and 120 kHz by Kirchhoff-ray mode (KRM) model. The body length (BL) of 16 individuals ranged in 12.2~22.0 cm (mean±S.D.: 17.8±3.2 cm) and the swimbladder length ranged in 4.2~8.6 cm (mean±S.D.: 6.6±1.6 cm) and the swimbladder cross section ranged in 1.7~6.6 cm2 (mean±S.D.: 3.8±1.6 cm2). This result shows that results correlate well between the BL and the length and cross section of swimbladder. The swimbladder angle ranged in 7~12°and the maximum TS values ranged in –16~–5°at tilt angle. The averaged TS-to-BL relationship were TS38kHz=20log10BL-65.33 (R2=0.66), TS70kHz=20log10BL-65.90 (R2=0.67), and TS120kHz=20log10BL-66.65 (R2=0.65). These results can be used fundamental data in order to estimate distribution and biomass of jack mackerel by using hydro-acoustic method.

The indirect target strength (TS) estimation method which uses the Kirchhoff ray mode model (KRM model) was discussed to apply for a biomass estimation in the water of mixed species. TS of 25 live scorpion fishes for 120kHz were measured by a tethered method and of others dominant 5 species in the marine ranching ground of Jeju coastal water including a scorpion fish were also estimated by KRM model. The measurement TS of scorpion fish well agreed with the theoretical values and the standard formula of scorpion fish was estimated as TS120kHz=20Log (L)-72.9 (r2=0.67). TScm values estimated on trial to each sample of dominant 5 species were from -69.3dB to -75.1dB at 120kHz and they were in the general range of swimbladdered fish. It was clarified that TS by KRM model can be used to estimate fish biomass estimation by increasing a sample number and is more effective under the condition that there is rare TS information for inhabiting species in mixed-species area.