What can we learn about small bladder fish by measuring backscattered sound?

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Abstract
Our paper is limited to the scattering of high-frequency acoustic signals from small bladder fish because that is our data base. The frequencies are above the bubble resonant frequency of the swim bladder. We measure the probability density function (PDF) of fish echoes from fish in situ. Since we use a directional transducer, we also include the beam pattern effect and random locations of fish in the sonar beam. We have measurements of caged fish for which the attitude and behavior are controlled. The PDF of the fish echoes depend on fish behavior and the length of the fish. We fit the two-parameter Rice PDS, i.e., the envelope of a sine wave plus ramdon noise, to the data. For small fish and very quiet fish, the sine wave component is much larger than the random component. Large active fish have a large random component and the PDF tends to the Rayleigh PDF. We are constructing acoustic models of the fish by combining, measured scattering cross sections versus aspect, measured PDF, and behavior. Even though our studies were made at high frequencies the spatial resolution of the single transducer was not large enough to image the fish. It would be a lot easier to measure fish lengths and behavior directly if we had high resolution imaging sonars. [This work was supported by a grant from the Office of Naval Research.]

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