156 



Fishery Bulletin 105(1) 



to the frequency to the fourth power in the Rayleigh 

 domain. For these fish, the transition from the Rayleigh 

 to geometric regimes occurred at frequencies around 15 

 kHz (Fig. 4). 



The spectra of Oj- are shown for over 60 narrow band- 

 widths (Fig. 4) after we averaged the results from all 

 of the experiments. Each of the three frequency bands 

 were resolved into 20 narrow bandwidths. Further nar- 

 rowing of the filters on the reverberation time series 

 served only to increase the standard deviation of the 

 measurements without providing more information 

 on the spectra. The mean a^. was in agreement with 

 the previous broad bandwidth average. The a-j, and its 

 standard deviation increased at the lowest and highest 

 frequencies, likely because of a decrease in signal-to- 

 noise ratios. 



The back scattering target strength TS=101ogiQ(a^,) 

 for these fish can be compared to the target strength 

 from a rigid sphere in water of radius a for which the 

 resonances are damped by the surrounding flesh. For 

 this type of scatterer, the target strength at high 



ka : 



2nf 



is equal to \Q\og^Q(mi^) 

 diameter 90 mm, lOlogjQ 



For a rigid sphere in water of 

 (TO2)=-22dB. The swimbladder 



0.01 

 N 



-I 



-in 005 



-CM 



^^^^^ 



10 



15 



20 

 67(5) 



25 



30 



0,015 



N 

 STI 



C II 



0.01 



0.005 



10 



15 



20 

 bT{s) 



25 



30 



0.015 



N 

 ,1 



0.01 



0,005 



—I 1— 



40 



20 

 87(5) 



Figure 3 



Total scattering cross section a-j., normalized to one bocaccio iSebastes pau- 

 cispinis), versus time between shots, 6T, for three frequency bands. The mea- 

 surements (dotted lines) for the lowest and highest frequencies band were 

 repeated 11 times, and 20 times for the center frequencies band, and averaged 

 (dashed dark lines). For short time between shots, Oj. was biased because of 

 slow fish motion. The measurements stabilized with increasing time between 

 shots,6r. 



is gas filled and can be considered a hard scatterer and 

 its resonances are damped by the surrounding flesh. As 

 a first-order approximation, the theoretical predictions 

 for the air bubble can be used to estimate the target 

 strength of the fish by adjusting the radius of the rigid 

 sphere to the size of the fish swimbladder. 



Discussion 



The mean Oj from 10 to 150 kHz was measured from 

 boccacio with a mean length of 468 mm. It is roughly 

 equivalent to that from a 90-mm-diameter rigid sphere 

 in water. This result is in agreement with the approxi- 

 mate size of their swimbladder. Thus, the spectrum of 

 the total scattering cross section for boccacio can be 

 measured with this technique. 



Despite these positive results, the following should be 

 considered for future experiments. These measurements 

 were made against the frequency from a heterogeneous- 

 size group of a single species of fish. As such, the re- 

 sults do not permit comparisons of Oj. with frequency, 

 and animal species and morphological features (e.g., 

 size, shape, length, sex, etc.). Measurements should be 

 made of individual fish. 



It should also be noted that the bocaccio in these 

 experiments moved very slowly. Simply increasing the 

 time to achieve incoherence be- 

 tween pulses may itself introduce 

 systematic and random measure- 

 ment error because of instabili- 

 ties in the medium. That is, some 

 additional incoherence can result 

 from bubbles and fluctuations in 

 the water temperature, sound 

 speed, volume, and water surface. 

 The magnitude of this incoherence 

 could eclipse the differences in Oj. 

 between individual fish. In these 

 measurements, bubbles and motion 

 of the air-water interface caused by 

 breaking bubbles and fish motion 

 were noticeable visually but had 

 only minor effects on the data. 



The most significant shortcom- 

 ing of these experiments was a 

 generally low signal-to-noise ra- 

 tio because of the tank size and 

 material properties. This low ratio 

 caused appreciable measurement 

 uncertainty at the lowest and 

 highest frequencies. The water 

 volume was large for the projected 

 signal intensities, and the fiber- 

 glass boundaries were not very 

 reflective in comparison to other 

 materials such as stainless steel 

 and glass used in previous experi- 

 ments. Therefore, to obtain and 

 compare measurements of Oj. from 



35 



40 



"T r 



35 



40 



