Estimation and examination on target strength of Bambooleaf wrasse (Pseudolabrus japonicus) by Kirchhoff-ray mode model.

Abstract

Fish target strength (TS) is a key parameter in the acoustic estimation of fish abundance. The fish TS could be estimated through by in-situ, ex-situ measurement with dual and split beam system, and numeric or theoretical models based on fish morphology. In general, measurements become a main approach to apply for fish abundance estimation. Theoretical model estimation for fish TS is a powerful method to guide and verify the measurements, and substitute the measurement for the fish which is hard to apply the direct measurement method. In the experiment, TS patterns for the 20 samples (LT ranged from 13.7 to 21.3 cm) of live Bambooleaf wrasse (Pseudolabrus japonicus) was estimated theoretically with Kirchhoff ray mode model and TS values for 18 live fish samples were additionally measured around zero tilt angle of swimming aspect by a tethered method using 120 kHz to verify theoretical values. The results of this study explained as follow: swim-bladder was easy to be covered by unexpected objects and destroyed during the frozen sample treatment and fail in extraction of swim-bladder morphology with X-ray photo; digitizing intervals to extract fish body and swim-bladder morphology in X-ray photos significantly affected on the calculation of TS pattern; TS pattern changes in the value range of a sound speed and density ratio for general fish flesh are relatively small within 1 dB; and there was a good agreement between measurement and theoretical calculation, and the correlation between averaged TS and BL could be finally calculated as = 20logTL-71.6 dB (r2=0.69) with the theoretical method.