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Fishery Bulletin 96(2), 1998 
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Depth (m) 
Figure 1 0 
Mean equivalent spherical radius (ESR) in sublayers at 
night (dark symbols) and daytime (open symbols with same 
shape) for NRL stations. Stations 1, 8 , and 10 are day- 
time only. Dashed lines are (A) the calculated ESR for a 
swimbladder exhibiting constant mass of gas (decreases over 
depth), and (B) constant volume of gas (level with depth). 
tions. There is some evidence that different cohorts 
stay together and remain separate from other size 
classes along the same section of coast. In the NMFS 
trawl data, most trawls show two predominant size 
groups, large hake (modal size >40 cm) and small 
hake (modal size <40 cm). These two groups rarely 
occurred in the same trawls. Where the two size 
groups overlapped (between 38°N and 43.5°N), two 
trawls caught large numbers of both size groups, two 
trawls caught large fish, and six trawls caught small 
fish. Some of the trawls with small fish were made 
near NRL stations 7 and 8. We hypothesize that at 
the time of the NRL survey (35 to 39 days after the 
NMFS survey), these small hake may have moved 
slightly inshore or south of the NRL stations and were 
replaced by larger fish. 
Our interpretation of resonance and calculation of 
swimbladder size and fish size required assumptions 
of the relation of swimbladder radius, r, to fish length, L. 
We also assumed that this relation was constant with 
depth. As gadoids migrate they do adjust the gas vol- 
ume of the swimbladder (Sand and Hawkins, 1973). 
Unfortunately our examination of swimbladder behav- 
ior in hake is inconclusive. The results of examining 
radius as a function of depth suggest that hake do 
compensate somewhat for pressure changes by add- 
ing and removing gas; however, they do not appear 
to compensate completely. We caution that this con- 
clusion is conjectural and could be an artifact of size- 
dependent stratification of hake in the water column. 
Overall, bias introduced in our estimates by not ac- 
counting for the possible addition and removal of gas 
to the swimbladders of fish descending and ascend- 
ing could have resulted in, assuming the worst case, 
an underestimate of length and biomass by 10% and 
35%, respectively. This bias would not have affected 
our estimates of fish numbers because they were 
dependent on the number of radii independent of 
associated fish size. 
Comparison of the NMFS survey estimates of sur- 
face density with the NRL estimates of surface den- 
sity produced several important findings. First, al- 
though there were spatial and temporal differences 
in the surveys and although different acoustic tech- 
niques were used, the similarities between the sur- 
veys were encouraging and suggest that good accu- 
racy is obtained with both techniques. Second, an 
NRL measurement at one station (one-mile drift) may 
be influenced by local minima or maxima in hake 
density. This hypothesis is suggested by the high 
values at station 1 where only a few days previously, 
in NMFS samples, much lower values were obtained. 
By chance at station 1, NRL may have made a mea- 
surement over an isolated concentration of fish. 
Third, relatively high densities of fish were seen at 
offshore stations in NRL sampling. Some NMFS 
acoustic transects suggest a high abundance of fish 
near the offshore limit of several transects taken near 
43°N (Dorn et al., 1994). This high abundance would 
indicate that, at times, high densities of hake occur 
offshore, probably as a result of a particular set of 
as-yet-unknown oceanographic conditions. 
This study has two important implications for fish- 
eries surveys. First, hake may occur in high num- 
bers, up to 300 kg/ha, approximately 50 km offshore 
of the outer edge of traditional fisheries acoustic sur- 
veys. Second, at these offshore sites the peak in the 
main scattering layer of hake may at times occur at 
depths of 400 to 450 m. At these depths hake could 
be difficult to assess by conventional 38-kHz fisher- 
ies sonars; therefore underestimates of the abun- 
dance of hake over deep water are possible. 
Acknowledgments 
We are indebted to J. J. Traynor, Alaska Fisheries 
Science Center, NMFS, for helping coordinate this 
incidental experiment and for providing a large 
amount of unpublished data. We thank N. V. 
Lombard, the Captain and crew of the USNS Wilkes, 
and E. Beeson, senior NAVOCEANO representative, 
for providing logistic support and assistance at sea. 
We also thank several anonymous reviewers who pro- 
vided critical reviews of the manuscript. This research 
was supported by the Office of Naval Research. 
