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 fi sh. The model provides a basis for choices of sonar carrier frequencie s. 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 w ere used to represent body flesh. For carrier frequencies above the br eathing mode resonance, swimbladders were modeled as a series of short gas-filled volume elements of cylinders. A Kirchhoff-ray approximatio n was used to compute the high-frequency acoustic scattering. A low mo de solution for a gas-filled cylinder was used to compute the low-freq uency ''breathing mode resonance.'' All contributions were added coher ently. The scattering lengths L, or target strength=20 log\L/L(0)\ (wh ere L(0) is reference length) were sensitive to fish orientation relat ive to the sonar beam. Theoretical target strengths were compared to t he 38-kHz cod data. Agreement was good.