Sound scattering by several zooplankton groups. I. Experimental determination of dominant scattering mechanisms

Abstract

The acoustic scattering properties of live individual zooplankton from several gross anatomical groups have been investigated. The groups involve ͑1͒ euphausiids ͑Meganyctiphanes norvegica͒ whose bodies behave acoustically as a fluid material, ͑2͒ gastropods ͑Limacina retroversa͒ whose bodies include a hard elastic shell, and ͑3͒ siphonophores ͑Agalma okeni or elegans and Nanomia cara͒ whose bodies contain a gas inclusion ͑pneumatophore͒. The animals were collected from ocean waters off New England ͑Slope Water, Georges Bank, and the Gulf of Maine͒. 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 large tank at sea using live fresh specimens. Individual echoes as well as averages and ping-to-ping fluctuations of repeated echoes were studied. The material type of each group is shown to strongly affect both the overall echo level and pattern of the target strength versus frequency plots. In this first article of a two-part series, the dominant scattering mechanisms of the three animal types are determined principally by examining the structure of both the frequency spectra of individual broadband echoes and the compressed pulse ͑time series͒ output. Other information is also used involving the effect on overall levels due to ͑1͒ animal orientation and ͑2͒ tissue in animals having a gas inclusion ͑siphonophores͒. The results of this first paper show that ͑1͒ 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 shape͒, ͑2͒ the gastropods produce echoes from the front interface and possibly from a slow-traveling circumferential ͑Lamb͒ wave, and ͑3͒ the gas inclusion of the siphonophore dominates the echoes, but the tissue plays a role in the scattering and is especially important when analyzing echoes from individual animals on a ping-by-ping basis. The results of this paper serve as the basis for the development of acoustic scattering models in the companion paper ͓Stanton et al., J. Acoust. Soc. Am. 103, 236-253 ͑1998͔͒.

Acoustic scattering of broadband echolocation signals from prey of Blainville's beaked whales : modeling and analysis

“…With respect to the inverse problem, research on acoustic scattering from marine organisms has shown that different groups of animals can be classified by the frequency spectra of their backscattered signal (Martin et al, 1996;Stanton et al, 1998b). It is unclear, however, to what degree dolphins, or other toothed whales, use features of broadband, acoustic backscattering to aid in the classification of prey.…”

“…Acoustic scattering spectra of marine organisms are characterized by unique interference patterns specific to target size, shape, material properties, and orientation (Stanton et al, 1998b;Lavery et al, 2002;Reeder et al, 2004). In order to perform a quantitative, statistical analysis of the prey echoes compiled in this study, an algorithm was created to quantify the structure of the spectra.…”

Estimating Gulf of Maine zooplankton distributions using multiple frequency acoustic, video and environmental data

“…Driven by a need for more accurate scattering predictions, complex mathematical models of geometric shapes that resemble zooplankton have been created (Stanton et al, 1994a(Stanton et al, , 1998aYe, 1997;Stanton and Chu , 2000b). Scattering predictions from these models have been compared to scattering measurements from actual zooplankton and ha,ve shown good agreement (Stanton et oJ, 1998a).…”

“…Zooplankton thnt are fluid-like in their body composition occur throughout the oceans. (Chu and Stanton (1998), Stanton et al (1998a)) have found that near broadside incidence there are generally two main from the animal: one at the front interface of the body, and a second echo from a wave that propn.gates through the animal's body, reflects off the interfa.ce at the far side of the animal, propagates back through the animal's body and finally is detected by the receiver (Fig. 2-1a).…”

“…The hardware a.nd methodologies used to record echoes matched those used for tank-based measurements of zooplankton backscatter conducted at sea and on land (Stanton et al) 1994a(Stanton et al) , 1998a except for the addition of two hardware fiters (Krohn-Hite Model :3200) that were needed to noise from the ROV vVe collected acoustic measurements from several hundred free-ranging siphonophores over a three day period. vVithin the limits of resolution of the video the animals were identifìed…”