ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on March 11, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(4):623-635; doi:10.1093/icesjms/fsn025
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Silent ships do not always encounter more fish: comparison of acoustic backscatter recorded by a noise-reduced and a conventional research vessel
1 National Marine Fisheries Service, Alaska Fisheries Science Center, Resource Assessment and Conservation Engineering Division, 7600 Sand Point Way NE, Seattle, WA 98115, USA
2 Institute of Marine Research, PO Box 1870, Nordnes N-5817, Bergen, Norway
Correspondence to A. De Robertis: tel: +1 206 526 4789; fax: +1 206 526 6723; e-mail: alex.derobertis{at}noaa.gov
De Robertis, A., Hjellvik, V., Williamson, N. J., and Wilson, C. D. 2008. Silent ships do not always encounter more fish: comparison of acoustic backscatter recorded by a noise-reduced and a conventional research vessel. – ICES Journal of Marine Science, 65: 623–635.The extent to which fish avoid approaching research vessels is an important source of uncertainty in fisheries surveys. Vessels radiate noise at the frequencies where fish hearing is most sensitive, and noise is thus thought to be the primary stimulus for vessel avoidance. In an effort to minimize vessel avoidance, international standards for noise emission by research vessels have been established. Although vessels meeting these criteria are now in service, the effectiveness of noise quietening on vessel avoidance remains poorly understood. The new, noise-reduced, RV "Oscar Dyson" (OD) will augment the conventionally constructed research vessel, "Miller Freeman" (MF) and serve as the primary platform in conducting acoustic surveys of walleye pollock (Theragra chalcogramma) in Alaska. To investigate whether noise-reduction measures result in differential avoidance, which would bias the pollock abundance time-series, we conducted an inter-vessel comparison of acoustic backscatter recorded by OD and MF during a survey of walleye pollock in 2006 in the eastern Bering Sea. Overall, we found no evidence for differences in vessel avoidance that would impact the echo integration results of adult pollock. Analysis of pollock depth distributions from both vessels suggests that there is a comparatively greater diving response to OD, with the reaction taking place primarily after the vessel has passed and for fish shallower than 90 m. Given that the change in vertical distribution is after the fish have been detected by the echosounder, this reaction should not influence echo-integration measurements. The results indicate that use of the OD rather than the MF is unlikely to bias the Bering Sea survey time-series through changes in vessel avoidance by adult walleye pollock.
Keywords: acoustics, hearing, noise-reduced vessel, survey, vessel avoidance, walleye pollock
Received 14 May 2007; accepted 27 December 2007; advance access publication 11 March 2008.
Present address: Norwegian Institute of Public Health, PO Box 4404, Nydalen, N-0403 Oslo, Norway
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