ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on February 5, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(2):226-237; doi:10.1093/icesjms/fsm190
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The influence of beam position and swimming direction on fish target strength
1 School of Aquatic and Fishery Sciences, University of Washington, Box 355020, Seattle, WA 98195, USA
2 NOAA Fisheries, Alaska Fisheries Science Center, 7600 Sand Point Way NE, Building 4, Seattle, WA 98115, USA
Correspondence to M. J. Henderson: Department of Fisheries Science, Virginia Institute of Marine Science, College of William and Mary, PO Box 1346, Gloucester Point, VA 2306, USA tel: +1 804 684 7588; fax: +1 804 684 7327; e-mail: mhender{at}vims.edu
Henderson, M. J., Horne, J. K., and Towler, R. H. 2007. The influence of beam position and swimming direction on fish target strength. – ICES Journal of Marine Science, 65: 226–237.Fish orientation is consistently identified as a major influence on fish target strength (TS). Generally, orientation is defined as the tilt angle of the fish with respect to the acoustic transducer, whereas a more accurate definition includes tilt, roll, and yaw. Thus far, the influences of roll and yaw on fish TS have only been examined cursorily. We used in situ single-target data to create fish tracks, to estimate fish tilt and yaw, and correlated these estimates with TS. The results show that tilt, yaw, and beam position have a significant influence on fish TS. To investigate further how yaw and beam position affect TS, we calculated the expected backscatter from each fish within simulated fish aggregations using a backscatter model. The TS of individual fish at 38 and 120 kHz varied by as much as 11 and 19 dB with changes in yaw and beam position. Altering the fish’s tilt, yaw, and beam position resulted in TS differences of 14 and 26 dB at 38 and 120 kHz, respectively. Orientation had a minimal influence on an aggregation’s average TS if the aggregation had a variable tilt-angle distribution and was dispersed throughout the acoustic beam.
Keywords: beam position, orientation, target strength, target tracking, yaw
Received 28 March 2007; accepted 7 August 2007; advance access publication 5 February 2008.
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