In order to improve the accuracy of abundance estimation of Japanese anchovy (Engraulis japonicus) by acoustic surveys using a quantitative echosounder, the shape of the swimbladder and the relationship between the target strength (TS) and the body length Lb were investigated. Specimens were soaked in formalin solution just before the experiments to prevent the gas in the swimbladder from leaking out through the anus. The body length ranged from 5.6 to 11.2 cm (44 specimens). The body length was measured from the anterior margin of the snout to the posterior margin of the scales. The swimbladder lengths of the Japanese anchovy obtained from the soft X-ray images were 1.4 to 1.9 times longer than those of walleye pollock given the same body length. The TS patterns according to the orientation angle of the body at 38 kHz were measured in a freshwater tank after the soft X-ray imaging and were in good agreement with the model predictions. The TS-Lb relationship, TS = 20 log10 Lb - 64.0, was obtained by averaging the TS patterns over the orientation distribution (mean = 0° and s.d. = 10°). Also the depth-dependent TS according to Boyle's law was TS = 20 log10 Lb - (20/3) log10 (1 + z/10) - 63.4, where z represents the depth.
