Understanding the variability of fish acoustic target strength (TS) potentially improves the accuracy of fisheries acoustic survey. This study examines the effect of fish body bending by swimming activity on fish target strength. Bending shape of fish body and swimbladder were simulated from soft X-ray images of anesthetized walleye pollock. Target strength of bent fish was estimated using a Kirchhoff Ray-Mode model from simulated morphology as a function of fish pitch angle. TS patterns of whole fish and swimbladder did not change near the pitch angle at which maximum TS was obtained. But at the other pitch angle, TS patterns of bent fish differed from that of no bent fish. TS patterns of fish body changed at all the pitch angle. Decrease of maximum TS caused by fish bending was within 0.7 dB. Present study indicates that it is not necessary to take care of fish bending on TS estimation, when fish swimming behaviour is normal except for feeding time that fishes are more active.
