Weinberg et al.: Survey selectivity for Gadus macroceph 
367 
0 . 25 - 2.0 2 . 0 - 2. 5 2 . 5 - 3.0 3 . 0 - 7. 0 
Depth layer (m) 
7 . 0 - 16.0 
Figure 5 
Boxplots of demersal fish backscatter per unit of area (sa) 
at 5 depth layers within 16 m of the sounder-detected bot- 
tom (n-20 tows), determined from acoustic data collected at 
a frequency of 38 kHz in August 2013 in the eastern Bering 
Sea. The horizontal line across the shaded box indicates the 
median value, the shaded box indicates the first and third 
quartiles, the lines outside the shaded box indicate a distance 
of 1.5 times the interquartile range above the third quartile 
and below the first quartile, and the plus marks indicate out- 
liers outside these lines. 
lantic cod and deduced that changes in towing 
speed would affect the catching efficiency of this 
species. In their study, Atlantic cod were sub- 
jected to water velocities that were slower than 
our towing speeds, but water temperatures were 
close to those in our study (2.6°C). At the towing 
speeds used by fishermen in the northeast Atlan- 
tic (1.0 m/s), Atlantic cod were able to maintain 
sustained swimming speeds for 10 min, but at a 
speed of 1.5 m/s (the slow towing speed in our 
study), they could maintain swimming speed for 
only 1 min. If Pacific cod swimming abilities are 
indeed similar to those of Atlantic cod, then, giv- 
en the towing speeds of 1.5 m/s or greater used in 
our experiment, we expect that Pacific cod maxi- 
mum sustained swimming speeds would not be 
enough to elude capture even during a haul last- 
ing 22.5 min, the shorter tow duration used in 
our experiment. 
Large Pacific cod do not escape capture by out- 
swimming the survey trawl, as indicated by our 
study results: catches when towing at the fast 
speed were no different than catches when tow- 
ing at the slow speed. This result indicates that, 
once Pacific cod reach the trawl mouth, they lack 
the means to swim fast enough or long enough 
to escape forward around the wing ends. In situ 
video evidence shows that this species tends to 
hold station in front of the footrope for only brief 
periods before slipping back into the net (Rose®). 
Large Pacific cod are unlikely to swim over the 
net because acoustic backscatter indicates that 
most Pacific cod, when in the presence of a trawler, oc- 
cur very close to the bottom within the vertical fish- 
ing dimensions of the trawl. In addition, findings from 
previous studies on gadid behavior indicate that trawl 
gear elicits a diving response in fish, not a rising re- 
sponse. The remaining avenues for escapement that 
could explain lowered trawl efficiency are 1) large Pa- 
cific cod could swim through the small mesh of the sur- 
vey net, an option that is physically impossible and 2) 
they could escape beneath the footrope, the frequency 
of which has been previously shown to be negligible 
(Weinberg et ah, 2002). 
If large Pacific cod are not outswimming the trawl, 
perhaps they are swimming over the headrope — a no- 
tion that would also explain a drop in selectivity for 
large fish related to both trawl sampling efficiency and 
availability. Here, we used fish backscatter to within 
0.25 m of the seabed to assess the vertical distribu- 
tion of Pacific cod near the seafloor during our experi- 
ment. This process discards potential backscatter from 
fish in the acoustic dead zone (Ona and Mitson, 1996), 
which is located very close to the seabed and could be 
an area of concern for an absolute estimate of all fish 
Sa- However, the distribution of fishes within the dead 
zone is less important for our main interest of detect- 
® Rose, C. R. 2010. Unpubl. data. Resour. Assess. Conserv. 
Eng. Div., Alaska Fish. Sci. Cent., NOAA, Seattle, WA 98115. 
ing Pacific cod occurrence in relation to the headrope 
height of the trawl; indeed, if most Pacific cod are in 
the acoustic dead zone, they clearly are not above the 
headrope height during vessel passage. 
Analysis of acoustic backscatter collected during 
towing indicated that only 4% of the total backscat- 
ter attributed to Pacific cod occurred above the height 
of the survey headrope, although the backscatter was 
measured at the vessel rather than at the net itself, 
meaning that any upward movement of fish after ves- 
sel passage would be undetected. Again, there are no 
previous studies on the vertical swimming behavior 
of Pacific cod in relation to trawls from which we can 
draw inferences. Studies of walleye pollock (Kotwicki 
et ah, 2013) and Atlantic cod show that these 2 com- 
mercially important gadids were stimulated to dive, 
rather than rise; their response to trawl warps may be 
both acoustically, as well as visually, driven according 
to Handegard and Tjpstheim (2005). This behavior is 
also acknowledged by commercial fishermen who tend 
to drag their nets below semipelagic schools. There is, 
therefore, little reason to believe that Pacific cod swam 
over the headrope during this experiment. 
Nichol et al. (2007) did, however, on the basis of 11 
archival tags, provide evidence of an off-bottom portion 
of the Pacific cod vertical distribution during daylight 
hours (the time during which the EBS survey is con- 
ducted) when the fish were in an undisturbed state 
