ICES Journal of Marine Science: Journal du Conseil Advance Access published online on July 24, 2007
ICES Journal of Marine Science: Journal du Conseil, doi:10.1093/icesjms/fsm110
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Diel variations in acoustic recordings of blue whiting (Micromesistius poutassou)
Institute of Marine Research, PO Box 1870 Nordnes, N-5817 Bergen, Norway
Correspondence to E. Johnsen: tel: +47 55 235355; fax: +47 55 238579; e-mail: espen.johnsen{at}imr.no
Johnsen, E., and Godø, O. R. 2007. Diel variations in acoustic recordings of blue whiting (Micromesistius poutassou). – ICES Journal of Marine Science, 64: 000–000.Annual landings of blue whiting (Micromesistius poutassou) in the northeast Atlantic have exceeded 2 million metric tonnes in recent years, and overexploitation is an increasing concern in terms of the sustainability of the fishery. The most important fisheries-independent dataset used for tuning the analytical stock assessment comes from the Norwegian surveys of blue whiting west off the British Isles. The survey is carried out in March/April during peak spawning, and improving its quality will have a direct positive effect on stock assessment. Here, we analyse diel effects on the abundance and vertical distribution as recorded by acoustics in 1995 and 1996 and from 1998 to 2002, and evaluate the potential effects on the survey estimates. On average, the acoustic density of blue whiting was
20% higher by day than by night. However, the diel bias varied considerably among years, and surprisingly, the acoustic density in shallow water (<350 m) was in general highest at night, when the blue whiting were distributed higher in the water column and more dispersed. The span in the vertical depth range increased considerably with bottom depth in water shallower than 550 m. In deeper water, where blue whiting had little or no bottom association, the day–night differences in vertical distribution were smaller and not affected by bottom depth. The inconsistency of the diel effect from year to year negatively affects the time-series used during annual stock assessments.
Keywords: acoustics, blue whiting, diel variation, spawning aggregation, vertical migration
Received 12 October 2006; accepted 12 June 2007.
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