ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on June 12, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(5):1066-1074; doi:10.1093/icesjms/fsm074
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The complexity of narrowband echo envelopes as a function of fish side-aspect angle
1 Alaska Department of Fish and Game, 333 Raspberry Road, Anchorage, AK 99518, USA
2 School of Aquatic and Fishery Sciences, University of Washington, PO Box 355020, Seattle, WA 98195, USA
3 Pacific Biological Station, Nanaimo, British Columbia, Canada V9T 6N7
Correspondence to D. L. Burwen: tel: +1 907 267 2225; fax: +1 907 267 2401; E-mail: debby_burwen{at}fishgame.state.ak.us
Burwen, D. L., Nealson, P. A., Fleischman, S. J., Mulligan, T. J., and Horne, J. K. 2007. The complexity of narrowband echo envelopes as a function of fish side-aspect angle. – ICES Journal of Marine Science, 64: 1066–1074.High-frequency, narrowband acoustic signals may contain more information on fish size and orientation than previously thought. Our observations of dual frequency identification sonar (DIDSON) images of fish orientation paired with split-beam echo envelopes helped clarify why metrics such as echo duration have performed better than target strength measurements when predicting salmon lengths at side aspect. Fish orientation has a pronounced effect on the duration and shape of split-beam echo envelopes from large (80–130 cm) salmon insonified at side aspect. At near-normal aspect angles, echo envelopes are unimodal, symmetrical, and resemble echo envelopes from calibration spheres. With increasing oblique-aspect angle, echo shapes become less symmetrical as the number of peaks increases, and echo duration and amplitude become more variable. Using angle and range coordinates, peaks in an echo envelope can be traced to their origin on a DIDSON image. At oblique-aspect angles, discrete peaks develop that are reflected from regions close to the head and tail. In addition, the distance between peaks increases with increasing aspect angle and is larger than can be explained by swimbladder length.
Keywords: aspect angle, DIDSON sonar, echo duration, side aspect, species classification, split-beam echosounder, target strength
Received 19 September 2006; accepted 25 April 2007; advance access publication 12 June 2007.
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