Acoustic target strengths and swimbladder morphology of Chub mackerel Scomber japonicus in the Northwest Pacific Ocean

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 (b20) 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{degrees} (standard deviation (SD), 16.3[~]33.3{degrees}). 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{degrees}, respectively. These results can be used for the acoustic stock assessment of S. japonicus in the Northwest Pacific Ocean.