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Observations of controlled moving targets with split-beam sonar and implications for detection of migrating adult salmon in rivers

Published online by Cambridge University Press:  15 March 2002

George M. W. Cronkite
Pacific Biological Station, Department of Fisheries and Ocean, 3190 Hammond Bay Road, Nanaimo, BC V9T 6N7, Canada
Hermann J. Enzenhofer
Cultus Lake Salmon Research Laboratory, Department of Fisheries and Oceans, 4222 Columbia Valley Highway, Cultus Lake, BC V2R 5B6, Canada
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When measuring the flux of migrating salmon in rivers there are often multiple fish in the beam simultaneously. To obtain accurate measurements of flux, an understanding of the effects of multiple targets is required. Multiple targets in various configurations were passed through a horizontally-oriented 4° × 10° beam from a split-beam echo sounder. The effects on target strength, detection probability, spatial and temporal patterns of echoes, and measurement of target position are presented. When multiple targets were in the beam at the same range, the target strength was positively biased, whereas, when they were in-line at different ranges, the target strength and detection probability were negatively biased for the shadowed targets. Targets at the same range, but separated in the vertical or horizontal direction, produced characteristic patterns in the X vs. ping and Y vs. X plots. Similar patterns were found in routine observations of migrating salmon, allowing the identification of some multiple-target events. Identification of these events can aid in the correct interpretation of migrating fish data for flux measurement.

Research Article
© Elsevier, Inra, Ifremer, Cemagref, CNRS, 2002

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