Acoustic target strengths and swimbladder morphology of Chub mackerelScomber japonicusin the Northwest Pacific Ocean

Abstract The Northwest Pacific chub mackerel ( Scomber japonicus ) is one of the most productive, economically important fishery resources worldwide. Due to fluctuations in their abundance and distribution, there is a pressing need to accurately assess this species and to ensure total allowable catch limits are followed. Acoustic target strength (TS; dB) measurements of Scomber japonicus were conducted at 38, 70, and 120 kHz using a split-beam echosounder of individuals from nine size groups (mean fork length, 10.8∼28.3 cm) swimming freely in a net cage within a seawater tank. An underwater camera was utilized to simultaneously measure the swimming angle. A least-squares regression analysis revealed that when the slope was constrained to 20, as per the generally applicable morphometric equation, the resulting values for the constant term ( b 20 ) were ‒67.7, ‒66.6, and – 67.3 dB at 38, 70, and 120 kHz, respectively. The mean swimming angle of S. japonicus across the groups was –10.5∼9.6° (standard deviation (SD), 16.3∼33.3°). In addition, the ratio of swimbladder height to swimbladder length, swimbladder length to fork length, and tilt angle of the swimbladder (mean ± SD) were 0.191 ± 0.060, 0.245 ± 0.055, and 9.6 ± 3.0°, respectively. These results can be used for the acoustic stock assessment of S. japonicus in the Northwest Pacific Ocean.