Nero et al.: Low-frequency acoustic measurements of Merluccius productus 
34! 
Table 2 
Volume reverberation estimates of the density and depths of occurrence 
Washington-California coast. Numbers in parentheses following densities 
radii included in the estimates, ind. = individuals. 
of probable fish swimbladders at stations along the 
in the 0.25 to 0.7 cm radius category are the smallest 
Day 
Night 
Station 
no. 
Depth (m) 
Density 
(ind./l,000 m 2 ) 
Depth ( 
m) 
Density 
(ind./l,000 m 2 ) 
Peak 
Range 
Peak 
Range 
0.25-0.7 cm radius 
1 
200 
125-450 
162 (0.35) 
— 
— 
— 
4 
— 
— 
— 
70 
50-550 
4,855 (0.25) 
5 
380 
250-525 
21 (0.45) 
70 
50-550 
7,974(0.25) 
6 
225 
125-475 
30 (0.4) 
<50 
100-525 
393 (0.3) 
7 
n/a 
175-450 
6 (0.4) 
75 
50-500 
1,440(0.25) 
8 
325 
150-550 
17 (0.4) 
— 
— 
— 
9 
400 
200-600 
10 (0.45) 
75 
50-550 
1,695 (0.25) 
10 
325 
150-550 
6 (0.45) 
— 
— 
— 
11 
300 
100-600 
14 (0.4) 
175 
125-500 
648 (0.3) 
0.8-2. 5 cm radius 
1 
200 
125-450 
79 
— 
— 
— 
4 
— 
— 
— 
245 
100-550 
99 
5 
380 
250-525 
28 
375 
50-550 
25 
6 
225 
125-475 
96 
200 
100-525 
45 
7 
220 
175-450 
87 
300 
50-500 
62 
8 
350 
150-550 
39 
— 
— 
— 
9 
400 
200-600 
46 
425 
100-550 
34 
10 
450 
375-550 
7 
— 
— 
— 
11 
325 
100-600 
48 
275 
125-500 
46 
More likely possibilities are biogeographic effects 
of species composition and fish size. One possibility 
is that the NRL measurements were not on hake but 
another fish such as jack mackerel or chub mack- 
erel, which may not have exhibited a north-south 
trend in fish size. However, the predominant fish in 
the area sampled were hake. Trawls close to the NRL 
stations contained only 1% jack and chub mackerel. 4 
Another possibility is that large hake migrated south- 
ward with the NRL survey. This is also unlikely. The 
NRL survey took place in the last two weeks of Au- 
gust. At about the same time (17 Aug) NMFS was 
still collecting large hake in trawls near 51°30'N 
(Queen Charlotte Sound). Furthermore, the commer- 
cial fishery caught large hake off Washington as late 
as September and October (Dorn et al., 1994). 
4 1992. Alaska Fisheries Science Center, Natl. Mar. Fish. Serv., 
NOAA, 7600 Sand Point Way NE, Seattle, WA 98115- 
0070. Unpubl. data. 
Table 3 
Surface density estimates (kg/ha) of Pacific hake as deter- 
mined by NMFS acoustic-midwater trawl survey and NRL 
measurements (day only). Data are grouped by NMFS geo- 
graphic regions. Numbers in parentheses indicate the num- 
ber of stations included in the averages for the NRL stations. 
Bottom depth (m) 
NMFS NRL inshore NRL offshore 
Region 1,000-1,200 900-1,500 m 2,400-3,100 
Canada South 
91 
439 (1) 
— 
Southern 
Columbia 
369 
348 (1) 
270 (2) 
Eureka 
238 
195 (2) 
300 (2) 
The most reasonable hypothesis to explain the dis- 
crepancy between trawl and acoustic estimates in 
hake size in the south of the study area is that the 
same cohort of hake was encountered at all NRL sta- 
