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Measurements of total scattering spectra from bocaccio

Fishery Bulletin, Jan, 2007 by Stephane G. Conti, Benjamin D. Maurer, Mark A. Drawbridge, David A. Demer

Marine sportfishing in southern California is a huge industry with annual revenues totaling many billions of dollars. However, the stocks of lingcod and six rockfish species have been declared overfished by the Pacific Fisheries Management Council. As part of a multifaceted fisheries management plan, marine conservation areas, covering many million square nautical miles, have been mandated. To monitor the recovery of the rockfish stocks in these areas, scientists are faced with the following challenges: 1) multiple species of rockfish exist in these areas; 2) the species reside near or on the bottom at depths of 80 to 300 m; and 3) they are low in numerical density. To meet these challenges, multifrequency echosounders, multibeam sonar, and cameras mounted on remotely operated vehicles are frequently used (Reynolds et al., 2001). The accuracy and precision of these echosounder results are largely dependent upon the accuracy of the species classification and target strength estimation (MacLennan and Simmonds, 1992).

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Broad bandwidth characterization of sound scatter from marine organisms has some potential for remotely classifying fish species (Conti and Demer, 2003), shapes and sizes (Conti et al., 2005), behaviors (Conti et al., 2006b), and to validate models for target strength estimation (Demer and Conti, 2003). All of these studies have employed variants of a new method for measuring the broad bandwidth total scattering cross section ([[sigma].sub.T]) of animals moving in a reverberant tank.

With the new method, the total scattering cross section ([[sigma].sub.T]) of live animals in tanks is obtained from a comparison of the coherent and incoherent acoustical intensities reverberated in a tank (de Rosny and Roux, 2001, 2003). The accuracy of this measurement technique was shown by using standard metal spheres (Demer et al., 2003). This technique was successfully used on krill (Demer and Conti, 2003; Conti et al., 2006a), fish (Conti and Demer, 2003), and humans (Conti et al., 2004). In our study, we explored the potential and limitations of the method to characterize the broad bandwidth sound scattering from bocaccio (Sebastes paucispinis).

Materials and methods

The total scattering cross section, ([[sigma].sub.T], of bocaccio was measured over acoustic frequencies ranging from 10 to 150 kHz with a group of fish (n=20) swimming freely in a large, insulated fiberglass tank at Hubbs-SeaWorld Research Institute, San Diego, CA, on 1 and 2 July 2004. The tank had 5.1 cm of foam insulation on the exterior, measured 2.44 m in diameter, and was filled with seawater to a depth of 1.37 m (V [volume]=6.4 [m.sup.3]). The pool was thermostated at approximately 12[degrees]C. The acoustic measurement technique and a variety of its applications have been well documented (de Rosny and Roux, 2001; Conti and Demer, 2003; Demer and Conti, 2003; Demer et al., 2003; Conti et al., 2004). However, the general procedure and details of these experiments are presented here for convenience and clarity.

Each of the 20 fish was handled one time, a week prior to the experiment, to measure their weight (W) and total length (L). These data were summarized and plotted in graphs (Table 1, Fig. 1).

[FIGURE 1 OMITTED]

An emitter transmitted M acoustical pulses into the tank every other second ([delta]T=2s). The corresponding reverberation time-series [h.sub.k](t) were simultaneously recorded on multiple receivers while the fish were swimming between consecutive shots. The boundaries, volume, as well as the positions of the emitter and the receivers in the tank remained identical during the measurements.

The time series [h.sub.k](t) were composed of echoes from the boundaries of the tank and the fish. For two consecutive time series [h.sub.k](t) and [h.sub.k+1](t), the contributions from the boundaries of the tank were identical, whereas the contributions from the fish were not. The coherent

[S.sub.c](t) = 1/M [M.summation over (k=1)][h.sub.k](t)[h.sub.k+1)(t)

and incoherent

[S.sub.i](t) = 1/M [M.summation over (k=1)][h.sup.2.sub.k](t)

intensities in the tank were estimated from the M recorded time series. The coherent component represents the acoustical intensity reverberated by the fixed boundaries of the tank. The incoherent component also accounted for the acoustical intensity scattered by the fish. When the positions of the fish were uncorrelated between consecutive pulses, the ratio S(t) of the coherent to the incoherent intensities decreased exponentially with the scattering mean free path [l.sub.s] of the fish (de Rosny and Roux, 2001):

S(t)=[[S.sub.c](t)/[S.sub.i](t)] [approximately equal to] exp (-t c/[l.sub.s]) [approximately equal to] exp (-t cN[[sigma].sub.t]/V),

where the bracketing [] designates the average for multiple receivers.

The scattering mean free path is related to the total scattering cross section of a single fish in the tank ([[sigma].sub.t]), the sound speed (c), the number of fish (N). and the volume (V). Multiple receivers may be used simultaneously to reduce the heterogeneities of the acoustical field on the coherent and incoherent intensities in the tank. Knowing N, c, and V, [[sigma].sub.T] (normalized to a single fish) was estimated from the exponential decay of S(t). Thus, [[sigma].sub.T] averaged over 10 to 150 kHz was estimated for a single bocaccio, and its total scattering spectrum was similarly estimated after filtering the recorded time series [h.sub.k](t) into twenty narrow frequency bands. Each band corresponded to the bandwidth of the transmitted chirp, divided by twenty.

Measurements of total scattering spectra from bocaccio

Fishery Bulletin, Jan, 2007 by Stephane G. Conti, Benjamin D. Maurer, Mark A. Drawbridge, David A. Demer

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