Interpretation of multi-frequency acoustic data: effects of fish orientation

One goal of fisheries acoustics is to develop objective classification or identification methods to automate allocation of acoustic backscatter to species. Classification schemes rely on consistent relationships for successful apportionment of acoustic backscatter to species. A method is developed that compares frequency-dependent volume backscatter from an acoustical survey of Atlantic herring (Clupea harengus) to investigate the potential for classifying herring. Predicted backscattering patterns by a Kirchhoff-ray approximation are used to explain the observed relationships and evaluate the potential for classification of multi-frequency data. Combining predicted backscatter with observations of the frequency-dependent volume backscatter gave approximately 40% classification success, which is not sufficient for survey purposes. However, this method highlighted potential consequences that fish orientation may have on classification schemes and density and abundance estimates. This method of comparing multi-frequency volume backscatter appears to be beneficial for detecting behavioral changes by groups of fish, which may be used to select target strength values for density or abundance estimates. Utilizing predicted target strengths from numerical or analytical solutions or approximations, appropriate target strengths could be selected and would provide more accurate estimates of fish density and abundance.