Side-aspect target strength measurement of swimbladdered fish using multi-frequencies system: Black sea bream (Acanthopagrus schlegeli)

Abstract

Besides dorsal-aspect target strength (TS) of the fish, side-aspect TS information is also acoustically important parameter in fisheries acoustics. In this study, the side-aspect TS of 11 black sea bream (Acanthopagurus schlegeli) were measured using a split-beam echosounder of 120, 200, and 420 kHz; total length of the fish ranged from 12.4 to 23.7 cm, and wet weight from 27.5 to 229.8 g. For the precise TS measurement with side-aspect angle, we used anesthetized and tethered specimens of known size while being rotated through 360° by means of a carousel structure. The side-aspect TS measurements of the fish were conducted by rotating the fish in the horizontal plane at 5° interval. The ping interval was 0.2 second and elapsed time at each angle was 30-60 second. As a result, the measured side-aspect TS data were fitted by sinusoidal function. The relationships between fish length and near full side-aspect TS were as follows: TS120 kHz = 21.46 log (TL)-67.5 (r = 0.70), TS200 kHz= 31.03 log (TL)-76.9 (r = 0.83), TS420 kHz= 30.79 log (TL)-72.2 (r = 0.77). For comparison, theoretically estimated side-aspect TS from the Kirchhoff ray mode (KRM) model, which based on swimbladder and body morphology, were compared with the measured TS.