Angle-dependent target strength variability of Kawakawa (

Accurate target strength (TS) measurements are essential for understanding fish acoustic backscatter, which is influenced by factors such as frequency and orientation. This study analyzed TS values of Kawakawa (Euthynnus affinis) samples (fork length 27.4 cm) using a broadband echosounder and examined the effect of different tilt angles on TS. Measurements were conducted in an acoustic tank with a Simrad EK80 Wide Band Transceiver equipped with a split-beam transducer over the 160-260 kHz range. Dead fish samples were positioned under the transducer at tilt angles of -45°, 0°, and +45°, and data were processed using ESP3 software. Results showed angle-dependent TS variations: at 0°, TS ranged from -31.72 dB to -19.00 dB; at +45°, from -40.86 dB to -22.88 dB; and at - 45°, from -41.33 dB to -24.37 dB. These findings demonstrate that TS is strongly dependent on fish orientation, emphasizing the utility of broadband acoustics for detailed fish characterization.