Broadband characteristics of Pacific herring (Clupea pallasii) and Pacific chub mackerel (Scomber japonicus) target strength identified using the tether method

Target strength (TS) is essential for acoustic fish biomass estimation. Traditional narrowband surveys, which are constrained by single-frequency TS measurements, limit species identification. In contrast, broadband technology, which utilises continuous frequency ranges, enhances differentiation through TS frequency response. The aim of this study was to investigate whether the structural differences in swim bladders between physostomous and physoclistous fish produce distinguishable TS characteristics. Pacific herring (Clupea pallasii) and Pacific chub mackerel (Scomber japonicus) were selected to represent physostomous and physoclistous species, respectively. TS measurements were conducted using the tether method in freshwater and seawater tanks, employing calibrated split-beam quantitative echo sounders (Simrad EK80 scientific echosounder system) within the range of 45–260 kHz. After the acoustic measurements, the theoretical TS estimated by the Kirchhoff ray mode model was compared with the measured results. The maximum TS_cm increased with increasing L/λ (ratio of length (L) to wavelength (λ)) for both species. The tilt angle distribution was assumed to be represented by a normal distribution (mean value: −5°, standard deviation: 15°) regarding average TS_cm. Pacific herring showed an overall decreasing trend, whereas Pacific chub mackerel showed no significant trend. At L/λ < 4.5, significant differences were observed in maximum and average TS_cm and L/λ characteristics, with Pacific herring exhibiting a marked decrease and Pacific chub mackerel showing a slight increase. These results may improve the accuracy of fishery resource assessments of these two species, assist in their identification, and provide a reference for differentiating between physostomous and physoclistous fish.