329 
Abstract .—Low-frequency volume 
scattering measurements were con- 
ducted by the Naval Research Labora- 
tory (NRL) on the upper slope, slope 
base, and abyssal plain along the U.S. 
west coast between the Strait of Juan 
de Fuca, Washington, and Cape Mendo- 
cino, California. Comparisons of swim- 
bladder radii estimated from resonances 
with those estimated from fish lengths 
obtained from quasisynoptic National 
Marine Fisheries Service (NMFS) trawl 
catches of Pacific hake, Merluccius pro- 
ductus, strongly suggest that the major 
source of the low-frequency scattering 
were Pacific hake. Estimates of hake den- 
sity from the NRL low-frequency mea- 
surements and the NMFS acoustic, 
midwater trawl survey on the slope gave 
comparable values of 195-439 kg/ha and 
91-369 kg/ha respectively. However, NRL 
measurements, up to 50 km offshore of 
the outer limit of the NMFS survey, found 
densities of 270-300 kg/ha in layers 
peaking at 225 to 450 m depth. This 
finding suggests that high densities of 
hake may occur farther offshore of the 
traditional limit of NMFS surveys and 
at depths which may be difficult to sur- 
vey with conventional fisheries sounders. 
Manuscript accepted 2 October 1997. 
Fishery Bulletin 96:329-343 ( 1998). 
Low-frequency acoustic measurements 
of Pacific hake, Merluccius productus, 
off the west coast of the United States 
Redwood W. Nero 
Charles H. Thompson 
Richard H. Love 
Naval Research Laboratory, Code 7174 
Stenms Space Center, Mississippi 39529-5004 
E-mail address for R.W. Nero:woody.nero@nrlssc.navy.mil 
Pacific hake, Merluccius productus, 
are the most abundant large mid- 
water fish on the continental shelf 
and slope of the west coast of the 
United States during summer 
(Ware and McFarlane, 1989). Hake 
winter between Point Conception, 
CA, and Baja California. In spring 
they migrate northward to feed in 
the productive waters along the con- 
tinental shelf from northern Cali- 
fornia to Vancouver Island. They 
remain in this area from May 
through October. The migration be- 
tween the winter spawning grounds 
and the summer feeding grounds is 
size structured, i.e. related to fish 
length and optimum swimming ve- 
locity (Francis, 1983; Smith et al., 
1992), with the larger fish swim- 
ming faster and farther. Large fish 
migrate farthest north to near 
Vancouver Island, smaller fish 
reach only as far as southern Oregon 
and northern California (Stauffer, 
1985). 
A midwater trawl fishery targets 
hake occurring in dense feeding ag- 
gregations along the shelf break in 
midsummer ( Stauffer, 1985). Since 
1977, the National Marine Fisher- 
ies Service (NMFS) has conducted 
a triannual acoustic, midwater 
trawl survey to determine hake dis- 
tribution and abundance (Dorn et 
al., 1994). During the 1992 survey, 
NMFS integrated volume scattering 
with a high-frequency (38-kHz) echo 
sounder and corroborated fish iden- 
tity and size with midwater trawls. 
Volume scattering was converted to 
fish biomass with a volume scatter- 
ing to biomass regression (Dorn et 
al., 1994). 
Coincident with the 1992 fishery 
survey, the Naval Research Labo- 
ratory (NRL) conducted an acous- 
tic survey of volume scattering us- 
ing a broad-band low-frequency 
measurement system (Thompson 
and Love, 1996). Because the NRL 
survey was conducted quasisynop- 
tically with the fisheries survey, 
comparisons were possible between 
the two surveys. 
Although echo sounders at rela- 
tively high frequencies (>20 kHz) 
are a standard tool in fisheries sur- 
veys (MacLennan and Simmonds, 
1992), a broad-band low-frequency 
sound at the natural resonance of 
the population of fish swimbladders 
at depth have been used as an alter- 
native tool (Holliday, 1972; 1977a). 
A variety of acoustic models has 
been developed to help predict and 
understand various aspects of this 
resonance (Anderson, 1950; An- 
dreeva, 1964; Love, 1978; Stanton, 
1989; Clay, 1991, Feuillade and 
Werby, 1994). With these models, 
the acoustic resonance of a popula- 
tion of fish swimbladders at depth 
can provide quantitative informa- 
tion about the fish population. The 
spectrum of the resonance is depen- 
