Nero et al.: Low-frequency acoustic measurements of Merluccius productus 
331 
5 — Heceta North; 6 and 7 — Heceta South; 8 and 9 — 
Sebastian; and 10 and 11 — Mendocino. 
Just prior to the NRL survey, NMFS conducted a 
high-frequency acoustic-midwater trawl survey of 
Pacific hake. The survey began just south of San 
Francisco and ended off Cape Scott, Vancouver Is- 
land (Dorn et al., 1994). Generally, trawls were lo- 
cated inshore of the NRL stations. The NRL and 
NMFS surveys were separated by 43 days in the south 
(station 11, trawl 8), but only 5 days in the north (sta- 
tion 1, trawl 45)(Fig. 1). The NMFS survey provided 
information on fish size and abundance that could be 
compared with the results of the low-frequency acous- 
tic analysis. Comparisons were restricted to midwater 
trawls located close to the NRL stations within each of 
the above named latitudinal regions. A north-south 
trend in fish size was evident, although a gap occurred 
in the size distributions between 38 and 40 cm (Fig. 2). 
Trawls appeared to catch fish on either side of this gap; 
only a few trawls contained both size classes. This find- 
ing suggests that, at sea, different size classes remain 
spatially separate (Fig. 1). 
The NRL sound scattering measurements used an 
explosive sound source and a directional acoustic 
receiver. A 0.23-kg TNT charge detonated at 0.5 m 
depth provided a high source level over a wide fre- 
quency range. The shallow detonation depth allowed 
the gas bubble created by the explosion to vent to 
the surface and prevented the multiple sound pulses 
caused by bubble oscillations characteristic of charges 
detonated at greater depth. Measurement sequences 
consisted of 4 to 6 shots over a 30-min period. Because 
the ship typically drifted at 1-2 knots, the distance over 
which data were collected was on the order of a mile. 
The acoustic receiver used for these mea- 
surements consists of a thirty-two element 
line hydrophone, 0.9 m long, mounted 
along the axis of a conical reflector that 
had a height of 0.9 m and an open base 
with a diameter of 1.8 m. The purpose of 
the reflector was to map an annular area 
of the opening to each hydrophone element 
for sound entering the reflector parallel to 
the axis and to decrease sensitivity to 
sound entering the reflector from other 
directions. The receiver was originally de- 
signed for use at frequencies between 2.5 
and 20 kHz. By varying the number of ac- 
tive hydrophone elements, the receiver’s 
3-dB beam width can be maintained be- 
tween 10° and 20° in that frequency range. 
The eight elements nearest the vertex of 
the cone form a 45-cm aperture and are 
used for frequencies from 10 to 20 kHz. By 
doubling the active length with eight ad- 
ditional elements, a 90-cm aperture is 
formed that can be used for frequencies 
between 5 and 10 kHz, and all 32 elements 
form a 180-cm aperture that can be used 
below 5 kHz. Between 2.5 and 5 kHz, mea- 
sured beam patterns of the receiver show 
a main lobe with shape and 3-dB beam 
width similar to the main lobe of theoreti- 
cal beam patterns (Urick, 1983) for a plane 
circular array 1.8 m in diameter, whereas 
the side lobes are lower than those for a 
plane circular array. Below 2.5 kHz, the 
main beam widens beyond 20°, reaching 
38° at 1,600 Hz, 58° at 800 Hz, and 74° at 
500 Hz. 
Received signals from each TNT shot 
were amplified, high- and low-pass filtered 
NRL Stations 
+ 
NMFS Trawls 
by fish size 
• Hake<40 
o Hake>40 
❖ Mixed 
Longitude 
Figure 1 
Map of the U.S. west coast showing Naval Research Laboratory (NRL) 
stations and locations where pertinent National Marine Fisheries Ser- 
vice (NMFS) midwater trawls were undertaken. 
