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Split-beam sonar observations of targets as an aid in the interpretation of anomalies encountered while monitoring migrating adult salmon in rivers

Published online by Cambridge University Press:  15 March 2004

George M.W. Cronkite ,
Hermann J. Enzenhofer  and
Andrew P. Gray
George M.W. Cronkite*
Affiliation:
Pacific Biological Station, Department of Fisheries and Oceans, 3190 Hammond Bay Road, Nanaimo, BC V9T 6N7, Canada
Hermann J. Enzenhofer
Affiliation:
Cultus Lake Research Laboratory, Department of Fisheries and Oceans, 4222 Columbia Valley Highway, Cultus Lake, BC V2R 5B6, Canada
Andrew P. Gray
Affiliation:
Pacific Salmon Commission, 600-1155 Robson Street, Vancouver, BC V6E 1B5, Canada
*
[email protected]
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Abstract

The experiments described in this paper relate known target configurations under controlled conditions to acoustic characteristics of multiple moving fish. We wanted to further our understanding of the interactions between targets and the effects these interactions have on the measurement of the number of salmon migrating in rivers. Multiple targets in various configurations were passed through a horizontally oriented 4° × 10° beam from a split-beam echo sounder. The effects on measurements of target strength, detection probability and target location in the beam are presented. The observed target strength was not dependent on target velocity. There was a reduction in target detection due to the single-target selection criteria implemented by the hydroacoustic system. We mimicked the conditions in a river where a close range fish target may modify the beam geometry allowing detection of previously undetected targets. We demonstrated some of the effects resulting from moving targets into radial alignment and we demonstrated shadowing conditions that can cause extinction of target echoes.

Keywords

Split-beamMultiple targetsShadowingInterferenceTarget strength
Type
Research Article
Information
Aquatic Living Resources , Volume 17 , Issue 1 , January 2004 , pp. 1 - 12
Copyright
© EDP Sciences, IFREMER, IRD, 2004

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