Correlation between zooplankton population and the volume-scattering coefficient

Abstract

Reverberation backscatter measurements have been conducted in Puget Sound and the Strait of Georgia. The acoustic measurements were made in the frequency spectrum common to commercial fishing sonar systems. Using a downward-looking transducer, the acquired reverberation backscatter data are used to calculate the volume-scattering strength coefficient of the water column, a coefficient analogous to the target strength of a unit volume of water along the acoustic axis of the transducer array. Concurrent with the acoustic measurements, vertical biological trawls are conducted using a 1-m2 plankton net to collect the zooplankton samples from the water column directly below the measurement transducer. The biological samples are preserved for analysis of settled volume per depth interval and separation by taxa. Correlation is apparent between zooplankton layers and acoustic backscatter. A typical dense layer of zooplankton, nominally 100 ft thick, can result in a 10-dB increase in the volume-scattering-strength coefficient over that depth interval. Adjacent regions in the water column having a lesser volume-scattering coefficient are seen to have a lesser zooplankton population.