Hydroacoustic technique for determination of the orientation of aggregated Baltic herring

The distribution of fish orientation is a very important factor influencing their target strength (𝑇𝑆), and thus the hydroacoustic assessment of fish abundance. A technique has been developed to estimate the orientation distribution of aggregated Baltic herring (Clupea harengus) by fitting the 𝑇𝑆 histograms obtained from the theoretical backscattering model to the measured 𝑇𝑆 histograms. By using available morphometry data of Baltic herring, a modified resonance scattering model to describe the backscattering by Baltic herring has been developed. Using this model, 𝑇𝑆 histograms were generated for different probability density functions (PDFs) of fish orientation, and then compared with the measured 𝑇𝑆 histograms. Based on the best fit to the measured histograms, the most likely distribution of herring orientation can be inferred.