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ICES Journal of Marine Science: Journal du Conseil Advance Access published online on September 4, 2007
ICES Journal of Marine Science: Journal du Conseil, doi:10.1093/icesjms/fsm135
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© 2007 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Acoustic and stiffness properties of gillnets as they relate to small cetacean bycatch

T. Aran Mooney1,, Whitlow W. L. Au1, Paul E. Nachtigall1 and Edward A. Trippel2

1 Department of Zoology, Hawaii Institute of Marine Biology, University of Hawaii, 46-007 Lilipuna Rd, Kaneohe, HI 96734, USA
2 Fisheries and Oceans Canada, St Andrews Biological Station, St Andrews, New Brunswick, Canada E5B 2L9

Correspondence to T. A. Mooney: tel: +1 808 2475063; fax: +1 808 2475831; e-mail: mooneyt{at}hawaii.edu

Mooney, T. A., Au, W. W. L., Nachtigall, P. E., and Trippel, E. A. 2007. Acoustic and stiffness properties of gillnets as they relate to small cetacean bycatch – ICES Journal of Marine Science, 64: 1–9.

Small cetaceans are incidental bycatch in gillnet fisheries. Dolphin and porpoise bycatch has been reduced by the use of barium sulphate-enhanced gillnets. This decreased entanglement is likely the result of either higher acoustic reflectivity or greater stiffness for barium nets. To address these variables, our study quantified the acoustic reflectivity and stiffness of six net types including barium sulphate, iron oxide-enhanced and control demersal gillnets of sizes which typically target cod (Gadus morhua) and monkfish (Lophius americanus). Acoustic reflectivity, or target strength (TS), was assessed using dolphin and porpoise-like sonar signals from 0° to 40°. TS values were used to calculate likely detection ranges. Barium sulphate- and iron oxide-enhanced nets showed increased reflectivity compared with control nets, with the barium sulphate nets generating the highest TS values. Dolphins should detect these nets in time to avoid contact, but porpoises, with typically lower source levels, may not detect nets at a range great enough to avoid entanglement. Barium sulphate line was significantly stiffer than comparable nylon line. All lines lost stiffness when soaked in seawater for 24 h. Barium sulphate nets proved stiffer and more acoustically reflective, and both factors are likely important in reducing harbour porpoise bycatch.

Keywords: bottlenose dolphin, bycatch, gillnet, harbour porpoise, net stiffness, target strength

Received 20 December 2006; accepted 26 July 2007.


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