Rather-high-frequency sound scattering by swimbladdered fish

Abstract

A new model describes acoustic scattering by swimbladdered fish of lengths from at least 8 to 36 wavelengths. It represents a fish by an ideal pressure-release surface having the exact size and shape as the swimbladder. The backscattering cross section, or target strength, is computed by means of the Kirchhoffapproximation. To test the model, predictions of target strengths based on swimbladder morphometries of 15 gadoids of lengths from 31.5 to 44.5 cm are compared with conventional target strength measurements on the same, surface-adapted fish, anesthetized before acoustic measurement, and shock-frozen immediately afterwards. Details are given of the swimbladder morphometry. In essence, this consists of slicing the frozen fish with a microtome, photographing the exposed swimbladder cross sections, digitizing the contours, and triangulating the surface between pairs of contours on adjacent, parallel planes. Theory and experiment are compared through the dorsal and ventral aspect target strength functions, their averages, and simulated probability density functions.

Three-dimensional visualization of fish morphometry and acoustic backscatter

“…The current approximation of body and swimbladder morphology using equivalent cylinders make KRM model predictions at insonifying angles near head-and tail-on less reliable. Expanding the KRM model and/or further investigation into other models (e.g., finite element boundary element [Foote, 1985] or conformal mapping [Diperna and Stanton, 1994]) may be required to achieve accurate model predictions at all orientations.…”

“…Dorsal and lateral views provide two planes to derive three-dimensional representations. Ideally, three-dimensional imaging techniques such as CAT scans or flash freezing and reconstructing microtomed sections [Foote, 1985;Ona, 1990] could be used to obtain an accurate representation of morphology. At this time we do not know the degree of anatomical detail required to maximize accuracy of model predictions.…”

Physical Parameter Measurement and Theoretical Target Strength Estimation of Juvenile Cod (Gadus macrocephalus)

“…The HelmholtzKirchhoff integral used to develop an accurate and elaborate method to estimate back scattered sound from fish (Foote 1985;Foote and Traynor 1988). This approach was simplified by Clay (1991Clay ( , 1992 who incorporated Stanton's finite bent cylinder equation (Stanton 1989) and fluid-or gas-filled cylinders to model fish backscatter and has been validated for length and tilt (Jech et al 1995;Horne et al 2000).…”

“…TS pattern as function of tilt angle ranged from −75 o to 75 o , from head-down to head-up, respectively at 38 and 120 kHz. The tilt angle is defined as the angle between the fish axis and the horizontal plane (Foote 1985 o respectively. It means TS max was detected when the fish was in head-down or near-horizontal position in both frequencies.…”

Modeling of target strength of swimbladder fish at high frequencies.

“…In the Stanton's approach, function Q(z ,a, r) is found to be in the following form: (8) (9) Strictly speaking, in this equation the radius a must be constant (straight cylinder). However, in order to calculate scattering function from a slender body of revolution Junger9) relaxed this requirement.…”