330 
Fishery Bulletin 96(2), 1 998 
dent on swimbladder size, depth, and fish behavior 
(Sand and Hawkins, 1973; Love, 1993). The scattered 
sound at resonance is almost completely omnidirec- 
tional, independent of fish aspect (Feuillade and 
Werby, 1994). As a result, resonance measurements 
are useful for inferring the size and abundance of 
the swimbladders. An inverse solution of measured 
scattering levels can be used to generate a size-fre- 
quency distribution of swimbladders (Holliday, 
1977b), from which fish size can then be inferred. 
On the U.S. west coast, Pacific hake, measuring 
45 cm in fork length and occupying depths of 50 to 
500 m, are expected to be resonant near 500 to 1,500 Hz 
(Love, 1978). Midwater trawls show that no other 
large pelagic fish are as abundant as hake (Dorn et 
al., 1994). Other fishes expected to contribute to scat- 
tering are anchovy and mesopelagic fishes (Kalish 
et al., 1986). Both are much smaller than hake and 
at depths of 50 to 500 m should only contribute to 
scattering above 2 kHz. 
This study reports on low-frequency volume scat- 
tering measurements from what are assumed to be 
Pacific hake at depths of 50 to 500 m over the conti- 
nental slope off the Strait of Juan de Fuca and the 
Oregon-California coasts. Bioacoustic models are 
used to estimate the number and size of swim- 
bladders at depth. These results are compared with 
estimates of swimbladder size and abundance ob- 
1 1992. Alaska Fisheries Science Center, Natl. Mar. Fish. Serv., 
NOAA, 7600 Sand Point Way N.E., Seattle, WA 98115- 
0070. Unpubl. data. 
tained from quasisynoptic acoustic data and mid- 
water trawl catches collected by the National Ma- 
rine Fisheries Service. 1 A comparison of swimbladder 
size is problematic. Fish can either let their swim- 
bladders compress according to Boyle’s law during 
descent, or they can actively add gas to the swim- 
bladder to maintain neutral buoyancy. In addition, 
the relation between fish size and swimbladder vol- 
ume is also highly variable within a group of fish 
and is affected by feeding and other aspects of fish 
behavior (Ona, 1990). We consider these possibili- 
ties and suggest several hypotheses about the 
swimbladder behavior of Pacific hake. 
Materials and methods 
Measurements 
The NRL low-frequency acoustic measurements were 
made from 14 to 29 August 1992 aboard the USNS 
Wilkes (T-AGS-33) in conjunction with a Naval 
Oceanographic Office (NAVOCEANO) survey. The 
NRL conducted measurements at 9 stations located 
on the continental slope and abyssal plain between 
the Strait of Juan de Fuca and Cape Mendocino 
(Table 1; Fig. 1). Stations 2 and 3 off the Washington 
coast were missed owing to heavy seas. Because we 
expected a north-south trend in the size composi- 
tion of the hake, stations at similar latitude were 
grouped and named according to their proximity to 
geographic features: station 1 — La Perouse; 4 and 
Table 1 
Volume reverberation stations conducted by the Naval Research Laboratory. 
Station 
no. 
Date 
Time 
(local [h] ) 
Day or 
night 
Lat. 
N 
Long. 
W 
Bottom depth 
(m) 
i 
14 Aug 
1836-1920 
day 
48°26' 
126°20' 
900 
4 
18 Aug 
0120-0223 
night 
44°44' 
125°04' 
1,100 
5 
19 Aug 
1325-1420 
day 
44°45' 
125°42' 
2,800 
5 
20 Aug 
0145-0240 
night 
44°38' 
125°43' 
2,900 
6 
21 Aug 
0000-0105 
night 
43°28' 
125°40' 
3,100 
6 
21 Aug 
0925-1032 
day 
43°27' 
125°38' 
3,100 
7 
23 Aug 
0025-0109 
night 
43°26' 
125°04' 
1,070 
7 
23 Aug 
0858-0935 
day 
43°30' 
125°06’ 
1,245 
8 
24 Aug 
0905-0936 
day 
42°18' 
124°54' 
1,025 
9 
25 Aug 
2212-2248 
night 
42°19' 
125°40' 
2,800 
9 
26 Aug 
0856-0927 
day 
42°23' 
125°39' 
2,800 
10 
27 Aug 
0925-1001 
day 
40°15' 
125°35' 
2,400 
11 
28 Aug 
0117-0200 
night 
40°04' 
125°09' 
1,465 
11 
29 Aug 
0931-1006 
day 
40°03' 
125°08' 
1,500 
