Acoustic scattering characteristics of several zooplankton groups Academic Article uri icon

abstract

  • The acoustic echo levels from zooplankton are strongly dependent upon the acoustic frequency and size, shape, orientation, and material properties of the animals. Because of the great number of species of zooplankton, it is practical to study the acoustic properties of species grouped by their gross anatomical similarity. Zooplankton from several major groups are discussed: fluid-like (decapod shrimp, euphausiid, salp), 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 ensonified at broadside incidence contained a series of broadly spaced deep nulls; the plot for gastropods either had more tightly spaced nulls or a flat spectrum; the plot for siphonophores either had a less consistent pattern of nulls or a flat spectrum. The nulls from the euphausiid data were sometimes as deep as 30 dB below surrounding levels. The patterns are linked to the physics of the scattering process and modeled mathematically. In addition, key results on these animals from Stanton et al. (1994a, ICES J. Mar. Sci., 51: 505-512) are summarized to further illustrate the variability in scattering characteristics of the animal 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 surveys is discussed. Very importantly, drawing a simple relationship between 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

publication date

  • April 1996