Review and recommendations for the modelling of acoustic scattering by fluid-like elongated zooplankton: euphausiids and copepods

Abstract

Acoustic echo sounders are commonly used to survey zooplankton. An essential element in the methods is the acoustic scattering model, which relates acoustic echo data to meaningful biological parameters such as size and numerical density. Because of the importance of scattering models, there has been much development of models of increasing sophistication. With the increase in sophistication is an associated improvement in accuracy, but possibly at the cost of increased effort in implementing the model. Thus the practical question is which model provides sufficient accuracy for the scientific problem of interest. This paper presents a modelling study using a wide range of models, ranging from simple to complex representation of the animals, a synthesis of previously published laboratory scattering data from a variety of sources, and laboratory data presented for the first time. The focus is on fluid-like zooplankton (i.e., animals that do not support shear waves) with examples specific to euphausiids, shrimp, and copepods. The simplest model is the sphere with homogeneous material properties while the most complex is a high-resolution digitized form of each zooplankton taking into account surficial roughness and inhomogeneities of the material properties. The results show that there are conditions under which very simple, easy-to-use models can be used with reasonable accuracy while there are other conditions where the more complex models must be used. 2000 International Council for the Exploration of the Sea

Acoustic insights into the zooplankton dynamics of the eastern Weddell Sea

“…Copepods also perform diel and seasonal vertical migrations, but are considered rather weak scatterers of sound (Stanton and Chu, 2000) at the frequency of 75 kHz used here.…”

Distribution, patchiness, and behavior of Antarctic zooplankton, assessed using multi-frequency acoustic techniques

“…Expected echo intensities, or backscattering cross-sections (abs), were estimated based on the length of each individual determined by silhouette analysis and models of acoustic scattering appropriate to the individual's taxonomic group. These models were developed by Stanton et al (1994Stanton et al ( , 1998, are reviewed in Stanton and Chu (2000), and are sensitive to numerous parameters in addition to animal length, including animal orientation and material properties (Chu et al, 2000a; …”

“…In this study, however, we average volume backscattering over large depth ranges and horizontal intervals, and thereby average over a very large number of animals. When scattering is averaged over a distribution of animal lengths, the dips in target strength at particular size ranges are substantially reduced in magnitude, to 5 dB or less (Stanton et al, 1998;Stanton and Chu, 2000). Furthermore, the exact sizes of animals encountered in this study are such that the potential confounding effect of animal size on the relationship between volume backscattering and biomass is relatively small: the lengths of most of the net-sampled taxa (e.g., copepods, pteropods, siphonophores, and small euphausiids) were much smaller than the length at which the first dip in the target strength vs. size relationship occurs.…”

“…The equivalent cylinder used to represent the krill's shape was defined on the basis of the animal's average radius (ao) and 'acoustic' length (L), defined as the distance from the anterior of the eye to the end of the sixth abdominal segment (Table 3.1), following the approach of Stanton and Chu (2000). Other than length, the shape parameters were held constant for all simulations: a slight taper parameter T of 10 was used, and the crosssectional radius of the cylinder at its mid-section was related to length via a 0 = L/18.4.…”

“…Substantial progress has been made in the theoretical, physics-based modeling of the target strength of fluid-like crustacean zooplankton such as krill (reviewed in Stanton and Chu, 2000). State-of-the-art models employ the Distorted-Wave Born Approximation (DWBA) to estimate the scattering using a simplified description of the shape of the animal.…”