ACOUSTIC SCATTERING CHARACTERISTICS OF SEVERAL ZOOPLANKTON GROUPS

The acoustic echo levels from zooplankton are strongly dependent upon the acoustic frequency and size, shape, orientation, and material prop erties of the animals. Because of the great number of species of zoopl ankton, it is practical to study the acoustic properties of species gr ouped by their gross anatomical similarity. Zooplankton from several m ajor groups are discussed: fluid-like (decapod shrimp, euphausiid, sal p), hard elastic shelled (gastropod), and gas-bearing (siphonophore). The results from laboratory tests show that the plots of (single ping) target strength versus acoustic frequency have a distinct pattern for each animal type. For example, the plot for euphausiids when ensonifi ed at broadside incidence contained a series of broadly spaced deep nu lls; the plot for gastropods either had more tightly spaced nulls or a flat spectrum; the plot for siphonophores either had a less consisten t pattern of nulls or a flat spectrum. The nulls from the euphausiid d ata were sometimes as deep as 30 dB below surrounding levels. The patt erns are linked to the physics of the scattering process and modeled m athematically. In addition, key results on these animals from Stanton et al. (1994a, ICES J. Mar. Sci., 51: 505-512) are summarized to furth er illustrate the variability in scattering characteristics of the ani mal groups (for example, data from 2-mm-long gastropods show that they produce a level of echo energy per unit biomass approximately 19 000 (i.e., 43 dB) times greater than that of 30-mm-long salps). The impact of these observations on design and interpretation of acoustic survey s is discussed. Very importantly, drawing a simple relationship betwee n echo energy and biomass for regions containing a complex assemblage of zooplankton would be greatly flawed. (C) 1996 International Council for the Exploration of the Sea