Estimating the Target Strength of Sardine (Sardinops sagax) as a Function of Swimming Orientation

The swimming tilt angle of fish is one of the key factors influencing the estimation of target strength (TS). Therefore, understanding how TS varies with changes in swimming tilt angle is essential. This study employed the Kirchhoff-ray-mode (KRM) model to estimate TS and examine variations in the swimming tilt angle of sardines under flowing water conditions. Swimming tilt angles were measured at flow velocities of 30 and 50 cm/s. The KRM model was utilized to estimate TS for 17 sardine samples (total length: 13.0–24.6 cm) across four frequency bands (38, 70, 120, and 200 kHz). At a flow velocity of 30 cm/s, sardines swimming against the flow exhibited a mean swimming tilt angle of 4.0° ± 14.0°, with normalized mean TScm values of −64.7 dB at 38 kHz, −65.7 dB at 70 kHz, −66.4 dB at 120 kHz, and −66.9 dB at 200 kHz. At a flow velocity of 50 cm/s, sardines swimming against the flow showed a mean swimming tilt angle of −2.2° ± 10.1°, with normalized mean TScm values of −62.9 dB at 38 kHz, −63.7 dB at 70 kHz, −64.3 dB at 120 kHz, and −64.8 dB at 200 kHz. Considering the results of this study and the swimming behavior of sardines against the flow, the target strength of sardines swimming with the flow may be of less concern. Therefore, when conducting acoustic surveys, it is more efficient to account for flow velocity conditions rather than swimming direction.