Acoustic measurements of zooplankton distributions in the sea

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Abstract
Acoustic volume-scattering strengths were recorded at frequencies of 0·54, 1·16, 1·80, and 3·08 MHz from the upper 100 m of the water column off southern California. Concurrent pump samples of zooplankton were collected from discrete depths at the time of the acoustic recordings. Three predictive scattering models were used to calculate the volume-scattering strengths of these samples. Positive correlations between the calculated values from the pump samples and in situ field measurements were produced from all three scattering models. Correlations produced with the truncated fluid-sphere model values were slightly higher than correlations generated by data from the other two models, especially at a frequency of 1·80 MHz. Acoustic scattering at 1·80 MHz appeared to result primarily from organisms in the 0·4 to 0·6 mm equivalent spherical radius size class which included the copepods Calanus pacificus and Rhincalanus nasutus (2·5 to 3·5 mm in length). Correlations between calculated and measured volume-scattering strengths were highest at 3·08 MHz and decreased with decreasing frequency. A similar trend occurred when comparing measured volume-scattering strengths with the volume of zooplankton collected. Positive correlations between the measured acoustic values and both the results of the predictive models and the zooplankton volumes indicated that the acoustic values reflected the distribution of zooplankton in the water column.

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