© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Modelling the effect of swimbladder compression on the acoustic backscattering from herring at normal or near-normal dorsal incidences
a Institute of Oceanology of Polish Academy of Sciences ul. Powstancow Warszawy 55, PL-81-712 Sopot, Poland
b Institute of Marine Research PO Box 1870, Nordnes, N-5817 Bergen, Norway
*Correspondence to N. Gorska; tel: +48 58 551 72 81; fax: +48 58 551 21 30. e-mail: gorska{at}iopan.gda.pl; egil.ona{at}imr.no.
Inaccuracy in herring target strength can be an important source of bias in the acoustic assessment of several important herring stocks. New acoustic data on herring target strength (Ona et al., 2001, submitted for publication; Ona, 2003) confirm previous suggestions and evidence on a possible reduction of the size of the herring swimbladder as a result of its compression with increasing water depth. Theoretical work for a better understanding of the acoustic scattering from herring over its entire depth distribution may therefore be essential for improving abundance estimation. This study supplements the analysis, conducted by Gorska and Ona (2003) for herring averaged-backscattering cross-section. The modal-based, deformed-cylinder model (MB-DCM) solutions, presented in that paper, are used. The sensitivity of the herring backscattering cross-section in case of normal or near-normal dorsal incidences is studied with respect to frequency, contraction factors of the swimbladder dimensions and some fish morphological parameters. The study is important for a better understanding of not only the backscattering by individual fish for the dorsal incidence, but also the depth- and frequency-dependencies of the mean-backscattering cross-section. The theoretical results have been applied in the interpretation of the actual measured target-strength data on adult herring.
Keywords: depth-dependent target strength, herring, modelling, normal or near-normal dorsal incidences, swimbladder compression
Received 26 August 2002; accepted 10 July 2003.
1 tel: +47 55 23 85 00; fax: +47 55 23 85 84.
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