Sound scattering by live zooplankton and micronekton: Empirical studies with a dual-beam acoustical system

Measurements and analyses are presented of the backscattering of 420‐kHz sound by 43 individual animals of representative zooplanktonic and micronektonic taxa. Direct measurements of an individual’s target strength were made with a commercial dual‐beam sonar system in an enclosure filled with filtered seawater deployed off a dock at Friday Harbor, Washington. The dependence of target stengths upon individual length, wet weight, and dry weight was investigated. In addition, the ‘‘target strength’’ and statistical variations of echo amplitude due to variations in shape and orientation of the organism were compared with acoustic scattering models involving different shapes (the general shapes of the sphere, and straight and uniformly bent finite cylinders were used along with attempts to take into account roughness). It was found that: (1) backscattering cross sections are proportional to volume of the organisms rather than area as would be predicted by a sphere scattering model, (2) mean target strength bas...

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