Indirect Methods of Fish Target Strength Estimation in Absolute and Logarithmic Domain

The problem of indirect fish target strength estimation from data collected using single-beam system is based on transforming probability density function (PDF) of measured fish echo level into fish target strength PDF estimate. However, when estimating fish biomass the backscattering cross-section estimates are rather used for transforming echo voltages or echo integration outputs into fish density estimates. This implies, that rather non-logarithmic magnitudes should be used during echo data processing. The paper presents the results of estimation conducted on sample survey data and processed as well as in a logarithmic domain of fish target strength as in an absolute domain of backscattering cross-section. As the inverse problem, which has to be solved in the estimation procedure, in absolute domain is more ill-conditioned than its counterparts in logarithmic domain, the Adaptive Expectation Maximization and Smoothing (AEMS) inverse method is used. Additionally the results of transforming target strength PDF into backscattering cross-section PDF is compared with results obtained using direct processing in an absolute domain.