Modelling the effect of swimbladder compression on the acoustic backscattering from herring at normal or near-normal dorsal incidences

Abstract

Abstract 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.