Measurement of target strength (TS) is essential for estimating the abundance of species using fisheries acoustics. With the advancement of broadband technology, continuous TS frequency responses have enhanced species differentiation. This study aimed to investigate whether physostomous and physoclistous fish exhibit differences in swim bladder size and shape, and whether such structural differences result in distinguishable TS characteristics. To this end, Pacific herring (Clupea pallasii), a physostomous species, and Pacific chub mackerel (Scomber japonicus), a physoclistous species, were selected for TS measurements. Experiments were conducted using the tether method in freshwater and seawater tanks, with calibrated split-beam echosounders (Simrad EK80) over 45–260 kHz. Measured TS values were compared with theoretical estimates from the Kirchhoff ray mode (KRM) model. The results show that maximum TScm increased with L/λ in both species. A normal tilt angle distribution (mean −5°, SD 15°) was assumed for average TScm. Herring showed a decreasing trend with increasing L/λ, while mackerel exhibited no clear trend. At L/λ 4.5, significant interspecific differences in TScm were observed: herring declined markedly, mackerel slightly increased. These findings support improved acoustic assessments, assist in species identification and provide a reference for differentiating between physostomous and physoclistous fish.
Wang et al. (2025) studied this question.