154 



Fishery Bullelin 105(1 



3 

 2,5 



O) 

 ^ 1.5 



04 06 08 01 012 014 016 018 02 



L3 (m3) 



Figure 1 



Weight (W) of each boccacio (Sehaates paticispinis) versus length-to-the-third 

 power (L'^) (dark dots), and linear least-squares fit (dashed line), and the 

 mean deviation A is shown below the equation. 



intensities in the tank were estimated from the M 

 recorde(i time series. The coherent component repre- 

 sents 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 Sit) of the coher- 

 ent to the incoherent intensities decreased exponentially 

 with the scattering mean free path l^ of the fish (de 

 Rosny and Roux, 2001): 



S(t)-- 



S,.(f) 

 SM) 



= exp 



l ( cNor^ 



where the bracketing [ ] designates the average for mul- 

 tiple receivers. 



The scattering mean free path is related to the total 

 scattering cross section of a single fish in the tank ia-,,), 

 the sound speed (c), the number of fish (N). and the 

 volume (V). Multiple receivers may be used simultane- 

 ously to reduce the heterogeneities of the acoustical 

 field on the coherent and incoherent intensities in the 

 tank. Knowing N, c, and V, Oj (normalized to a single 

 fish) was estimated from the exponential decay of Sit). 

 Thus, Oj, 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/St) into twenty narrow frequency bands. Each 

 band corresponded to the bandwidth of the transmitted 

 chirp, divided by twenty. 



The signal acquisition system (Fig. 2) consisted of 

 a function generator CompuGen 1100 (GageApplied, 

 Montreal, Canada) internally clocked with two 16-bit 

 CompuScope 1610 (GageApplied, Montreal, Canada) 

 dual-channel acquisition boards in a portable computer. 

 The internal clocking of the function generator and the 

 acquisition boards allowed perfect timing between the 

 emitted and the recorded signals. Ensembles of M=100 

 chirps were transmitted over 50 ms every other second 

 for three frequency bandwidths from 10 to 40 kHz with 

 an ITCIOOIB emitter if,=25 kHz); 30 to 70 kHz with an 

 ITC1032 emitter (/;=56kHz); and 60 to 150 kHz with 



