Three-dimensional visualization of acoustic backscattering and models by alewife

Abstract

The advent of nontraditional underwater acoustic instrumentation (e.g., multibeam and sector-scanning sonar) and incorporating fish behavior in quantitative fisheries assessment require that backscatter be measured and modeled at a variety of aspect angles. Acoustic backscattering measurements were obtained in a large laboratory tank using live alewife (Alosa pseudoharengus). An individual alewife for each series of measurements was tethered and rotated in two planes of orientation (dorsal/ventral and lateral). The alewife were insonified with a broadband (40–100 kHz) chirp signal and bistatic scattering geometry was used. Backscattering amplitudes for all angles of orientation (3-D scattering ambit) were modeled using a Kirchhoff ray-mode model [C. S. Clay and J. K. Horne, J. Acoust. Soc. Am. 96, 1661–1668 (1994)] and digital images of the fish body and swimbladder morphometry. Visualization of the scattering ambit provides a quantitative examination of the effects of fish orientation on echo amplitude. Comparisons of backscattering amplitudes from the model and measurements along the dorsal/ventral and lateral planes are given. The utility of broadband measurements for fish backscattering amplitude measurements and the integration of acoustic models in fisheries assessments are discussed. [Work supported by ONR, NOAA/NMFS, and CICOR.]