SOUND-SCATTERING BY SEVERAL ZOOPLANKTON GROUPS - I - EXPERIMENTAL-DETERMINATION OF DOMINANT SCATTERING MECHANISMS

The acoustic scattering properties of live individual zooplankton from several gross anatomical groups have been investigated. The groups in volve (1) euphausiids (Meganyctiphanes norvegica) whose bodies behave acoustically as a fluid material, (2) gastropods (Limacina retroversa) whose bodies include a hard elastic shell, and (3) siphonophores (Aga lma okeni or elegans and Nanomia cara) whose bodies contain a gas incl usion (pneumatophore). The animals were collected from ocean waters of f New England (Slope Water, Georges Bank, and the Gulf of Maine), The scattering properties were measured over parts or all of the frequency range 50 kHz to 1 MHz in a laboratory-style pulse-echo setup in a lar ge tank at sea using live fresh specimens, Individual echoes as well a s averages and ping-to-ping fluctuations of repeated echoes were studi ed, The material type of each group is shown to strongly affect both t he overall echo level and pattern of the target strength versus freque ncy plots, In this first article of a two-part series, the dominant sc attering mechanisms of the three animal types are determined principal ly by examining the structure of both the frequency spectra of individ ual broadband echoes and the compressed pulse (time series) output. Ot her information is also used involving the effect on overall levels du e to (1) animal orientation and (2) tissue in animals having a gas inc lusion (siphonophores). The results of this first paper show that (1) the euphausiids behave as weakly scattering fluid bodies and there are major contributions from at least two parts of the body to the echo ( the number of contributions depends upon angle of orientation and shap e), (2) the gastropods produce echoes from the front interface and pos sibly from a slow-traveling circumferential (Lamb) wave, and (3) the g as inclusion of the siphonophore dominates the echoes, but the tissue plays a role in the scattering and is especially important when analyz ing echoes from individual animals on a ping-by-ping basis, The result s of this paper serve as the basis for the development of acoustic sca ttering models in the companion paper [Stanton et al., J. Acoust. Sec. Am, 103, 236-253 (1998)]. (C) 1998 Acoustical Society of America.