Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 836
Title: New broadband methods for resonance classification and high-resolution imagery of swimbladder-bearing fish using a modified commercial broadband echosounder
Author: T. K. Stanton, D. Chu, J. M. Jech, J. D. Irish
Publication Year: 2010
Journal: ICES Journal of Marine Science
Volume: 67
Pages: 365-378
Keywords: acoustic scattering, broadband, echosounder, fish, resonance
Abstract: A commercial acoustic system, riginally designed for seafloor applications, has been adapted for studying fish with swimbladders. The towed system contains broadband acoustic channels collectively spanning the frequency range 1.7100 kHz, with some gaps. Using a pulse-compression technique, the range resolution of the echoes is 20 and 3 cm in the lower and upper ranges of the frequencies, respectively, allowing high-resolution imaging of patches and resolving fish near the seafloor. Measuring the swimbladder resonance at the lower frequencies eliminates major ambiguities normally associated with the interpretation of fish echo data: (i) the resonance frequency can be used to estimate the volume of the swimbladder (inferring the size of fish), and (ii) signals at the lower frequencies do not depend strongly on the orientation of the fish. At-sea studies of Atlantic herring demonstrate the potential for routine measurements of fish size and density, with significant improvements in accuracy over traditional high-frequency narrowband echosounders. The system also detected patches of scatterers, presumably zooplankton, at the higher frequencies. New techniques for quantitative use of broadband systems are presented, including broadband calibration and relating target strength and volume-scattering strength to quantities associated with broadband signal processing.
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