ACOUSTIC MODELS OF FISH - THE ATLANTIC COD (GADUS-MORHUA)

Acoustic fish models should represent the fish body form. The Atlantic cod were used to model the acoustic scattering function of teleost fish. The model provides a basis for choices of sonar carrier frequencies. Anesthetized live Atlantic cod ranging from 156 to 380 mm (SL) were ''soft'' x-rayed to image inflated swimbladders and skeletal elements. Maximum body heights and widths were 0.18 and 0.13 of fish lengths. Lengths and diameters of swimbladder were approximately 0.25 and 0.05 of the fish lengths. A series of short-length fluid-filled cylinders were used to represent body flesh. For carrier frequencies above the breathing mode resonance, swimbladders were modeled as a series of short gas-filled volume elements of cylinders. A Kirchhoff-ray approximation was used to compute the high-frequency acoustic scattering. A low mode solution for a gas-filled cylinder was used to compute the low-frequency ''breathing mode resonance.'' All contributions were added coherently. The scattering lengths L, or target strength=20 log\L/L(0)\ (where L(0) is reference length) were sensitive to fish orientation relative to the sonar beam. Theoretical target strengths were compared to the 38-kHz cod data. Agreement was good.