Modeling Study On The Target Strength Of Anchovy (Engraulis Japonicus)

Acoustic estimation is an important method for quantitative fish stock assessment. In acoustic fish stock estimation, the target strength of fish is a key parameter to convert the echo integral into absolute fish abundance, hence an important topic in fisheries acoustics. In this thesis, methods for fish target strength studies are reviewed; the acoustic scattering property and the target strength of anchovy (Engraulis japonicus), including their depth dependence, are then analyzed and estimated based on a modeling method. The main results are as follows.1. Based on literature studies, the definition and factors that may influence the target strength of fish are briefly described, and methods for the estimation of the target strength of fish are reviewed. Methods for fish target strength estimation falls into two categories, namely the experimental measurement methods and the theoretical modeling approximation methods. The experimental measurement methods include in situ measurement methods and controlled experiment methods; the modeling approximation include swimbladder models and the fish body models. This chapter provides a brief account of the major methods in these two categories, with emphasis on the introduction of those modeling methods.2. Based on sound scattering theory and a modeling approximation method, the acoustic scattering characteristics and the target strength of the anchovy in the Yellow Sea are estimated numerically. By neglecting the end effect, the scattering field of a finite-sized target is approximated by that of an infinite target. The scattering model is composed of a swimbladder model and a fish body model. The swimbladder is modeled by gas-filled prolate spheroid, and the fish body, exclusive of swimbladder, by liquid-filled prolate spheroid. Model parameters used for numerical calculations are based on an anchovy individual with a total length of 12.6 cm, the dimension of its swimbladder being extracted from soft x-ray images. The effective mean target strength is estimated from the sound scattering intensities at different angles obtained from model calculation and the tilt-angle distribution of anchovy using convolution technique, its tilt-angle (degree) distribution is assumed to be N (-3.9, 12.82). The result shows that anchovy is a highly directive scatterer. At 38 and 120 kHz, the maximum dorsal-aspect target strengths of the anchovy are -41.2 dB and -39.5 dB, respectively; and the effective mean target strengths are estimated to be -48.0 dB and -51.5dB,which agree reasonably well with those obtained from in situ measurements. In addition, the backscattering directivity, frequency dependent characteristics of the target strength and the contribution of swimbladder to the composite target strength of anchovy are analyzed and discussed. The results indicate that modeling study can play an important role in fish target strength study in China both as an important supplementary technique to direct measurement methods and as an effective approach to the better understanding of sound scattering property of fish.3. Based on the anchovy model described above and the same basic methods, the relationships of the acoustic scattering characteristics and target strength of anchovy with the resident water depth of the fish are studied numerically. The volume of the swimbladder in relation to the water depth is calculated according to Boyle's law, the range of the water depth is chosen from 0 m to 100 m. Numerical estimation of the mean target strength are calculated from the sound scattering directivity and the tilt-angle distribution of the anchovy using convolution technique, and its tilt-angle is assumed to conform to N(-3.9, 12.82). It shows that anchovy has clear frequency and depth dependent characteristics. At 38 kHz, the mean target strength of surface-layer adapted anchovy is -48.0 dB, but decreases to -51.3 dB when the depth increases to 100 m; at 120 kHz, the target strength of anchovy deceases from -51.5 dB to -56.1dB within the same depth range. In addition, the maximum value, the scattering directivity and the frequency characteristic of the target strength of anchovy are analyzed. This study enhanced our understanding on the depth dependence of the target strength of fish, and provided an important reference for the acoustic estimation anchovy at different depth layers. The method can also be used for the study of other fish similar to anchovy, thus provide technical support for the accurate estimation of fish stocks in China.