Nero et al.: Low-frequency acoustic measurements of Merluccius productus 
339 
after the NMFS survey. At all other NRL 
inshore stations, densities were similar to 
the NMFS survey estimates. At all offshore 
NRL stations, densities remained high. 
Comparisons between the offshore NRL sta- 
tion data and the NMFS data were not pos- 
sible because the NMFS survey did not ex- 
tend beyond bottom depths of approxi- 
mately 1,200 m. The offshore NRL measure- 
ments, approximately 50 km beyond the 
NMFS survey, gave densities as high as 300 
kg/ha in layers at 300 to 400 m depth. 
An examination of swimbladder behav- 
ior in hake was conducted on the NRL data 
by determining the relation between the 
acoustically measured swimbladder radii 
and depth for all stations (Fig. 10). In this 
analysis, it was assumed that hake at one 
station do not exhibit a size-dependent 
stratification in the water column. At night, 
hake were spread throughout the water col- 
umn, allowing estimates of average ESR for 
several distinct layers at some stations. 
During the day, the hake were in a more 
compact single layer, resulting in only one 
estimate. 
In Figure 10 curve, “A” depicts the shape 
of the relation between swimbladder radii 
and depth that is expected if the gas vol- 
ume changed according to Boyle’s Law and 
the bladder radius decreased with increas- 
ing depth. If hake maintain a near constant 
volume of gas in their bladders by adding 
or removing gas, then they would have a 
constant swimbladder radius, as depicted 
by line “B” in Fig. 10. There are no clear 
trends in the data. However, several weak 
relationships are worth noting. First, data 
taken from less than 100-m depth at sta- 
tions 5 and 7 show unusually small radii 
suggesting some size-dependent stratification. Ex- 
cluding these two values gives a weak but signifi- 
cant correlation for the remaining data (correlation 
coefficient=-0.373). In addition, two sets of data, from 
stations 9 and 11, and possibly from station 5, ex- 
hibit a decrease in bladder radius with depth. Sta- 
tions 6 and 7 show an increase in radius with depth 
whereas station 4 shows maximum radius at middepth. 
Discussion 
Pacific hake are believed to be the main cause of 
midwater scattering at frequencies of 1-2 kHz along 
the continental shelf and slope of the west coast of 
the United States (Figs. 6-7). No other large bodied 
midwater fishes are as abundant as hake (Ware and 
McFarlane, 1989). In addition, hake made up 97% of 
the catch in midwater trawls taken during the NMFS 
triennial acoustic-midwater trawl survey in the sum- 
mer of 1992. 3 Other small fishes, mesopelagic fish, 
and northern anchovy may be abundant but will reso- 
nate well above 2 kHz. They are believed to be the 
source of scattering above 3 kHz in the upper 200 m 
at night (Fig. 7). 
3 1992. Alaska Fisheries Science Center, Natl. Mar. Fish. Serv., 
NOAA, 7600 Sand Point Way NE, Seattle, WA 98115- 
0070. Unpubl. data. 
