Acoustic estimation of size distribution and abundance of zooplankton

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Abstract
A series of investigations were undertaken to observe and describe the sound backscattering process from larger zooplankton (euphausiids). The target strength versus frequency, size, and aspect angle of the organism was measured. The target strength is highly dependent on the density and sound speed contrasts between the target and the medium, and both these parameters were measured. From the target strength observations it was concluded that the fluid sphere model was insufficient as a scattering model for krill. Observations of the tilt angle distribution of krill at natural field conditions showed that they were distributed over a large region of tilt angles. The deficiencies of the fluid sphere models required the development of a new scattering model based on experimental data. This model predicts a decreasing target strength versus frequency in the geometric scattering region. The backscattering spectra of euphausiids were better described by our empirical model than by the fluid sphere models. Applying the empirical model to estimate size distribution and biomass of krill, we found strong correlation between the acoustically estimated distributions and those from net catches.