Abstract
The target strength (TS) of fish varies with acoustic frequency, fish size, fish activity, orientation, swimbladder and so on. Among these, the swimbladder is recognized as having the most important effect on TS. The dorsal aspect TS as a function of tilt angle of fish caught by bottom trawling in the East China Sea was measured at two frequencies (25 and 100 kHz). The swimbladder sizes (length and height) and inclination angle of the swimbladder to the lateral line of the fish were measured by X-ray photographs. TS and swimbladder data were then compared and analyzed, and the following results were obtained:
(1) The eight species of fish, except the silver pomfret, have swimbladders, but the angle and size of the swimbladder varies.
(2) For bladder fish, the angle of the maximum dorsal aspect TS agrees with the broadside direction of the swimbladder.
(3) The dorsal aspect TS of the silver pomfret is clearly lower than that of bladder fish at the two frequencies.
(4) The maximum and averaged dorsal aspect TS values of the silver pomfret, were lower by about 7-8 dB, 8-10 dB at 25 kHz, and by 8-10 dB, 9-11 dB at 100 kHz than two other bladder fish (yellow sea bream and whitefin crevalle).
