ACOUSTIC TARGET STRENGTH OF FOUR MAJOR CHINESE CARPS USING THE KIRCHHOFF-RAY MODE MODEL

Abstract

The four major Chinese carps (FMCCs)-silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), grass carp (Ctenopharyngodon idella), and black carp (Mylopharyngodon piceus)-are among the most commercially significant freshwater fish species in China. Accurate target strength (TS) measurements are essential for acoustic stock assessments, yet reliable TS data for FMCCs remain limited. This study utilized X-ray imaging to analyze 69 specimens across the four species, generating digital representations of their swim bladders and body morphologies. The Kirchhoff-ray mode (KRM) model was applied to estimate the TS of FMCCs and its variations based on sound wave frequency, pitch angle distributions, and morphological features. Results revealed that the TS was significantly affected by the pitch angle and varied greatly with frequency, TS was the highest within a pitch angle range of 65 degrees to 85 degrees, and the number of secondary TS peaks increased with higher frequencies. Regression equations for TS-body length (TS-BL) relationships, as well as standard b(20) formulations, were established for various pitch angle distributions (N[90 degrees, 5 degrees], N[90 degrees, 10 degrees], N[90 degrees, 20 degrees], and N[85 degrees, 15 degrees]) and frequencies (70, 120, 200 kHz). A comparison with previous ex situ experimental data highlighted some discrepancies between the results from the modeling approach and ex situ methods. This study provides fundamental data to support the stock assessment and species identification of FMCCs, contributing valuable insights for sustainable fisheries management.