Acoustic models and target strengths of the Atlantic cod (Gadus morhua)

Abstract

Acoustic models should realistically represent the fish body form. To display the form, anesthetized Atlantic cod were soft x rayed to image inflated swimbladders and skeletal elements. The fish lengths ranged from 177 to 804 mm and the corresponding mass range was 48 to 4700 g. The x rays give good images of the swimbladder and skeleton. The fish images were projected at the same length to compare the relative shapes of their bodies and swim bladders. The relative shapes of the 177- to 412-mm cod were about the same. The maximum fish body heights and widths were about 0.18 and 0.13 of total length. The relative swimbladder lengths were about 0.25 of the total fish lengths and the maximum diameters were about 0.05 of the total lengths. The swimbladder of the 804-mm cod had a relative length of 0.16 and maximum diameter of 0.07 total length. From the shapes of the fish, it was evident that simple bent cylinders were not adequate. The integration of the scattering amplitudes over length [in C. S. Clay, J. Acoust. Soc. Am. 89, 2168–2179 (1991)] was replaced by a summation over a finite number of cylinders. One set of cylinders represents the fish flesh and another set represents the swimbladder. All of the computations were done numerically. Interactions between the swimbladder and fish body were ignored. The scattering amplitudes S(f) were converted to target strengths by using the 10 log[S(f)S*(f)]. [Work supported by the Office of Naval Research and the Memorial University of Newfoundland.]