Frequency dependence of sound backscattering from live individual zooplankton

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Abstract
A broadband laboratory chirp sonar was used to investigate the frequency characteristics of the backscattering by two individual live decapod shrimps (Palaemonetes vulgaris). The measurements illustrate that for fixed shape and orientation, the backscattering amplitude varies dramatically with frequency with as low as 30-dB-deep nulls at certain frequencies (30 dB corresponds to a factor of 1000 in an echo integration value). The overall echo level and frequency characteristics vary from ping to ping as the animal changes shape and orientation. The fluid bent cylinder model [T. K. Stanton (1989), J. Acousl. Soc. Am. 86:691–705] describes the single-realization normal incidence data reasonably well, especially the frequencies at which the peaks and nulls occur. Our theoretical analysis only involved physical parameters that were either directly measured or taken from the literature where physical parameters involving similar animals are published. As expected, once the data are averaged across a number of pings (as one would do in a field measurement), the nulls are mostly “filled in” and the backscatter versus frequency curve becomes smoother with variability of the order 5–8 dB (factor of 3–6 in echo integration value). These data are useful for predicting volume scattering strengths observed in the field by taking averages with respect to shape and orientation.

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