Nero et al.: Low-frequency acoustic measurements of Merluccius productus 
335 
E. 
x: 
Q_ 
CD 
Q 
S, (db) 
Frequency (Hz) 
Figure 3 
Volume reverberation at NRL station 1, during the day. 
Hz) than at station 11 in the south (1,400 Hz). At 
night the resonance peaks of the individually inte- 
grated layers differed, with the peaks of the shallow 
sublayers occurring at about 1,000 Hz and the reso- 
nance peaks of the deep sublayers occurring at about 
1,500 Hz. 
Bladder size and number 
The inverse analysis resulted in the determination 
of a distribution of ESR for each station grouped by 
geographic area (Fig. 8). The inversion gave an al- 
most exact fit to the scattering data (curves and 
points in Figs. 6 and 7) indicating that the estimates 
of the distributions of ESR were quite good. Overall, 
the largest radii are near 2.2 cm, peaks occur be- 
tween 1.2 and 1.8 cm, there is a dip at 0.8 cm, and 
smaller peaks occur between 0.5 and 0.7 cm. With 
the weight-length regression for hake given earlier, 
a swimbladder volume to fish volume ratio of 0.03, 
and if variabilities in either are ignored, L = 28 r. 
Thus, the inversion tells us that, if the larger scat- 
terers are hake, they are between 22 and 62 cm long 
with the greatest number between 34 and 50 cm (ig- 
noring that variabilities increase the size range 
slightly). Figure 2 indicates that NMFS caught hake 
between about 25 and 55 cm length, with the great- 
est number between 28 and 46 cm length. Hence, we 
believe the estimated size range agrees well with 
trawl data, and we feel confident in our assumption 
that the scatterers with ESR of 0.8 to 2.2 cm are, 
indeed, Pacific hake. 
Scatterers with ESR below 0.8 cm were present at 
all stations with the greatest numbers occurring be- 
tween 0.5 and 0.7 cm (Fig. 8). According to their ESR, 
these fish are probably a mixture of mesopelagic fish 
and northern anchovy. This is consistent with find- 
ings of Kalish et al. (1986) who caught several spe- 
