Page 30 - Acoustics Today

Fig. 2. A photo album of just a few types of zooplankton from the Bering Sea including: euphausiid (a), copepod (b), gastropod (c), larval pollock (d), two types of amphipods (e, f), crab zoea (g), jellyfish (h), and a siphonophore (i). The ruler shown in a-g is in cm whereas the jellyfish and siphonophore are approximately 20 and 3 cm in diam- eter respectively.
from the animal. For other fish and zooplankton, the scatter- ing from the animal can be modeled as a fluid-like object where the animal is represented by a geometric shape contain- ing a fluid with different acoustic impedance than the sur- rounding seawater (Stanton, 1998; Stanton, 1990). Many of these models are not perfect as they have a uniform composi- tion within the body volume which we know is not true since muscle, bone, and other organs and tissues are different. Simple geometric shapes (e.g., spheres and spheroids) of these structures produce the simplest mathematical scattering mod- els. However, these shapes may not accurately represent the actual scatterer shape. Thus a variety of more accurate but complex shapes have been used ranging from cylinders (Chu et al., 1993; Stanton et al., 1998c) to high-resolution (mm) computerized tomographic measurements (Lavery et al., 2002). Much work has been done recently in detailed adjust- ments to many of these scattering models (Demer and Conti, 2003a,b). Despite these approximations, these models have been tested with measurements from animals (Gorska et al., 2005; Ianelli et al., 2009; ) and have been found to be accurate enough for use in stock assessment purposes.
Although fish can be considered a diverse group of ani- mals with different body shapes, sizes, and composition; they appear to be relatively uniform when compared to the diver- sity of the zooplankton. The groups represented with the word zooplankton represent many different phyla, ranging from crustaceans to mollusks to cnidarians to fish (Fig. 2). If you exclude the microzooplankton and only include meso- zooplankton (those you can see without a microscope), their size ranges over four orders of magnitude from small mm- long copepods to siphonophores which can be 10 m in length. In addition to the size differences within this group,
there are a large variety of body morphologies from fish lar- vae to crustaceans (the most abundant zooplankton group) to snails with and without shells to the gelatinous organisms which may or may not contain a gas-inclusion. Siphonophores are a particularly fascinating zooplankton (Biggs, 1977; Rogers et al., 1978; Gould, 1984; Mackie et al., 1987;). These are gelatinous zooplankton related to coral polyps which are often described as jellyfish, but they are morphologically and phylogenetically distinct from the more common moon jellies, ctenophores, or blobs of goo you may see washed up on the beach. These animals have a structure called a pneumatophore which contains a small volume (often spherical with a diameter of a few mm) of gas (Pickwell et al., 1964 ). Like fish, siphonophores use this body part to control their position in the water column (Warren et al., 2001). Several of these animals, specifically pteropods containing a calcium carbonate snail-like shell and the afore- mentioned siphonophores, can be very strong acoustic scat- terers (Stanton et al., 1998a). So strong that the scattering from a single pteropod or siphonophore can be equivalent to that from tens of thousands of crustacean (fluid-like) zoo- plankton (Stanton et al., 1994a; Stanton et al., 1998c).
So to make sense of acoustic survey data, one must have a good handle on the types of animals that lie beneath the surface. Using ground truthing data such as net or video tows, one can calculate the relative abundance of the various scatterer types and by applying theoretical scattering models, one can estimate how much of each taxa would contribute to the total backscatter that the survey measures (Warren and Wiebe, 2008). Ideally one size or type of animal would dom- inate the scattering in a region. And in that case, it is fairly straightforward to estimate biological information from
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