Relationship between acoustic backscattering strength and density of zooplankton in hte sound-scattering layer

Hydroacoustic sampling in the ocean is a useful technique for studying the biomass and structure of the distribution of marine organisms, including zooplankton. Acoustical and biological sampling of zooplankton in the sound-scattering layer (SSL) off the east coast of Oshima Peninsula, northern Japan has been conducted during the last 10 years to obtain the scale factor for converting the backscattering strength to biological density. The volume backscattering strength (SV) was measured at 25, 50, 100, and 200 kHz, while an IKMT (Isaacs-Kidd Midwater Trawl) and a Norpac (North Pacific standard net) were used to sample the biological organisms in the SSL. During drifting observation at dusk, the maximum speed of the upward migration of the SSL toward the surface was 3 m min, and the SV changed over a range from "80 dB to "50 dB. Vertical tows of the Norpac indicated that euphausiids were the major zooplankton species of the migrating SSL. For quantitative analysis, the IKMT was towed horizontally about one hundred times, and the calculated density of zooplankton showed a maximum of 10 g m. We assumed that the acoustic reflectivity of the individual plankton was related to the size of the plankton squared, and that the acoustic backscattering strength of biomass was proportional to the distribution density. Results of regression analysis showed a linear relationship between the log of zooplankton density ñ (mg m) and the acoustic volume backscattering strength SV (dB), with correlation coefficients greater than 0.5 at all frequencies.