Multi-frequency acoustic measurement of zooplankton

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1.A method for size estimation of small organisms composing sound scattering layrs in the sea was developed using 4 frequencies and the Kristensen model. In this study, it was assumed that scattering layrs consist of a dominant size class. Then, equivalent spherical radius of the dominant size was estimated by the procedure based on least squares and rank conformity.2.The size estimate was compared to size composition of organisms taken by net sampling. While zooplankton taken by net tended to be smaller than acoustic size estimate, both were consistent with one another in a relative sense, i.e.large, small, and medium sizes. The current system has a limitation in resolution of size composition because of having only 4 frequencies, but it is useful for rapid and continuous measurement with wide coverage where zooplankton composition is simple. Since, at present, research vessels are generally equipped with acoustic instruments similar to our system, the method developed in this study can be applied widely.3.The volume scattering strength was corrected by size estimated above. The relationship between the corrected volume scattering strength and logarithmic total volume of zooplankton taken by net was expressed by first-order equation which was common for 4 frequencies. The constant term, which is theoretically to be zero, remaining in this equation is due to an error in measurement and/or difference from real biomass by net-avoidance. Main causes for the error are probably biovolume estimation of organisms sampled by net and parameter values given in the scattering model, which are needed to be examined further.