Practical target strength of free-swimming chub mackerel Scomber japonicus

Chub mackerel Scomber japonicus is a migratory fish widely distributed around Japan, and is an important fishery resource. However, target strength (TS) measurements of chub mackerel are limited, and the relationship between TS and fork length has not been fully clarified, despite its importance for the estimation of chub mackerel abundance. In this study, the TS–fork length (FL) relationship in chub mackerel was evaluated under realistic conditions. TS_mean and TS_max tended to increase with fork length at both 38 and 120 kHz, and TS histograms were bimodal for most individuals. In the TS_mean–FL relationship, when the coefficient a was fixed at 20 (the standard for fish with swim bladders), TS_cm (standardized by the square of the fork length) was −67.9 dB (r² = 0.70) at 38 kHz and −69.2 dB (r² = 0.45) at 120 kHz. Additionally, the swimming angle had a peak of around 0° with a mean of −1.23°, and the mean swimming speed was 0.16 FL/s with a standard deviation of 0.07 FL/s. TS reached a maximum between −20° and 0° and then decreased drastically as the swimming angle increased or decreased. Our results have practical implications for the management of chub mackerel.