Page 31 - Acoustics Today

Fig 3. Time to go fishing as a National Marine Fisheries Service vessel deploys a net trawl to capture fish as part of an acoustic and trawl stock assessment survey in the Bering Sea.
Fig. 4. What comes up in the net are Alaskan pollock which may not look familiar to you, but you’ve likely eaten it as fish sticks, fish sandwiches, or imitation crab- meat. Despite its small size relative to other commercially fished species such as hal- ibut, salmon, or cod, more pounds of pollock are landed in the United States every year than any other fish species.
acoustic data. Antarctic krill surveys have been conducted acoustically for several decades by several nations (Reiss et al., 2008; Kasatkina et al., 2004). Krill dominate the ecosys- tem in terms of scatterer biomass in many locations in Antarctica and elsewhere, and they can be discriminated from larger fish scatterers by measuring the difference in scattered energy at two or more acoustic frequencies (David et al., 1999; Miyashita and Aoki, 1999; Lawson et al., 2006, 2008; De Robertis et al., 2010). However, many other regions in the world have multiple types or sizes of animals that are contributing to the overall scattering signal which makes this process much more difficult, or in some cases impossible.
Differences in animal behavior which might seem insignif- icant to a biologist can be very important to an acoustician. The
angle (or tilt) of a scatterer can cause very large differences in scattered energy at frequencies in the geometric scattering regime (McGehee et al., 1998; Warren et al., 2002). Surprisingly we know very little about how fish and zooplankton orient themselves in the water column, and how their orientations vary during regular behaviors such as vertical migration, schooling, dispersion, predator avoid- ance, feeding, or reproduction. In prac- tice, assumptions must be made about the orientation distributions of these organ- isms which are very rarely species and site specific due to a paucity of information about these animal characteristics and the difficulty in making these measurements. Optical sampling methods can provide this information (Davis et al., 1992; Endo, 1993; Benfield et al., 2000) so combining them with acoustical sampling systems is a productive method (Sameoto, 1980;
Miyashita et al., 1996; Wiebe et al., 2002).
Other important parameters needed to properly model
the scattering from fish and zooplankton are the material properties of the organism. These values describe the acoustic impedance of the organism and how it differs from the sur- rounding seawater, which has a very large impact on the amount of energy these animals scatter. The importance of these properties has been known from the very early stages of acoustic modeling (Smith, 1954; Greenlaw and Johnson, 1982), however most studies do not make these measure- ments, as they can be time-consuming and occasionally diffi- cult to make at sea. In practice, scientists (including myself) use material property values from the literature which may be based on different species or groups of animals in a different part of the world. One of the most frequently-cited studies is Foote (1990) which described the material properties of Antarctic krill. The values from this study have been applied by other scientists to model the scattering from animals rang- ing from gelatinous zooplankton to almost every type of crus- tacean in habitats ranging all over the world. Again the pauci- ty of information about these parameters is a severe limitation in accurately describing the scattering from zooplankton as very small differences in these values (a few percent or less) can cause order of magnitude differences in the scattered ener- gy (Chu and Wiebe, 2005). Recently, several research groups (including my own) have been making more measurements of this type on many different types of animals including: coastal crustacean (Forman and Warren, 2010) and gelatinous zoo- plankton (Warren and Smith, 2007), larval fish (Chu et al., 2000), Antarctic krill (Chu and Wiebe, 2005 done in situ!) and salps (Wiebe et al., 2010), and several types of zooplankton from the Bering Sea (Køgeler et al., 1987; Smith et al., 2010).
An interesting discovery of our work in the Bering Sea was that we did not observe differences in material properties (or target strength) for the three different species of krill that we captured and studied. What did cause a difference in the
30 Acoustics Today, July 2012