Target Strength Estimation of Juvenile Cod (Gadus macrocephalus) using an Acoustic Scattering Model

Abstract

Recent years, hydro-acoustical survey has been applied to measure a small marine organism like juvenile fish or zooplankton to manage marine environment as well as fisheries abundance. In the survey, target strength (TS) of the species is used as a scale factor to convert the acoustical scattering to fish biomass or abundance. The relationship between TS and body length for the species must be established accurately, because the accuracy of TS directly and largely affect to the accuracy of fish abundance estimated by hydro-acoustical survey. Small marine organism is difficult to measure the acoustic scattering directly. Thus physical model as an alternative method can be applied to estimate the values of TS. In order to estimate acoustic scattering from small juvenile cod (Gadus macrocephalus) with the model, the contrast (fish body to medium ratio) of the density and the sound-speed, which are required during the calculation were measured. The results shown that the measured density contrasts of juvenile cod varied between 1.003 and 1.029 (with mean 1.014 and standard deviation (S.D) 0.01). In the other hand, sound-speed contrasts varied between 1.039 and 1.041 (mean 1.041; S.D 0.001). The relationship between averaged TS and TL in cm established by Kirchhoff-ray mode (KRM) model at 38 kHz and 120 kHz were TS = 20 log TL - 68.7 and TS = 20 log TL - 68.9, respectively.