364 
Fishery Bulletin 1 14(3) 
Length (cm) 
Figure 2 
Length-frequency distribution of Pacific cod (Gadus macrocephalus), by towing 
speed, determined from 48 tows conducted for this study in the eastern Bering 
Sea during August 2013. Slow tows had a speed of 1.5 m/s, and fast tows had a 
speed of 2.1 m/s. Length intervals are given in fork lengths. 
the moment they were sampled by the echosounder on 
the vessel. 
The difference between frequencies (120 and 38 
kHz) in mean volume backscattering strength (Sy, dB 
re 1/m; cf MacLennan et ah, 2002) was used to identify 
backscatter consistent with that of fishes with swim 
bladders. For analysis, backscatter data were grouped 
in bins, each of which had a resolution of 20 pings (hor- 
izontal) by 5 m (vertical). Bins for which the difference 
between at 120 kHz and at 38 kHz was between 
-10 and 8 dB were classified as backscatter that in- 
dicated fish (De Robertis et ah, 2010). Only bins for 
which the backscatter had a signal-to-noise ratio of at 
least 10 dB (De Robertis and Higginbottom, 2007) were 
included in the analysis. 
Fish backscatter per unit of area (s^, m^/nmi^) was 
then integrated by using an Sy integration threshold 
of -70 dB in several depth layers referenced to a 0.25- 
m backstep above the seabed echo (0.25-2.0, 2. 0-2. 5, 
2. 5-3.0, 3. 0-7.0, and 7.0-16.0 m). The upper bound of 
the first depth interval matched the mean headrope 
height of this experiment. Similarly, a height of 2.5 m 
corresponds to a survey-wide average headrope height 
for the 83-112 eastern trawl used to assess Pacific cod 
in the EBS (Nichol et al., 2007), a height of 7.0 m cor- 
responds to a survey-wide average headrope height 
for the poly-Noreastern trawl used by the NOAA 
Alaska Fisheries Science Center to assess Pacific cod 
in the Gulf of Alaska region (Nichol et ah, 2007), and 
a height of 16.0 m corresponds to an estimated effec- 
tive fishing height for the 83-112 eastern trawl that is 
used to assess walleye pollock in the EBS (Kotwicki et 
ah, 2013) and that perhaps may apply to Pacific cod 
as well. 
The results of this analysis of backscatter data 
were examined for evidence of fish above the headrope 
height (mean: 2.0 m) during the time the demersal 
trawl was in contact with the seabed, after accounting 
for horizontal setback of the trawl behind the vessel 
(approximately 3-4 min depending on vessel speed). 
The acoustic assessment was restricted to the deep 
study site because catches there consisted almost ex- 
clusively of Pacific cod and fiatfishes and, therefore, it 
was reasonable to assume that any backscatter that 
indicated fish with swim bladders was a result of the 
presence of Pacific cod. It was not possible to make 
such an assumption for data collected at the shallow 
study site because the catches there were dominated 
by walleye pollock, which cannot be acoustically distin- 
guished from Pacific cod. 
Results 
Summary of catches 
During the 48 experimental tows, 1462 Pacific cod, 
ranging in size from 34 to 105 cm FL, were caught, 
but only 2 fish were larger than 90 cm FL (Fig. 2). 
Of the captured fish, 701 individuals were large (>55 
cm FL) and included in further analyses. The bottom 
temperatures during the experiment ranged between 
2.6°C and 2.7°C. 
Swept area 
Of the 24 paired tows, 16 pairs had reliable net men- 
suration data with which we could test differences in 
swept area by pair. The mean difference in swept area 
between paired tows (fast and slow) was -0.072 ha, 
a variance that was not significant (^=-0.492, df=15, 
P=0.63). The fast tows swept a greater area than that 
swept by the slow tows during half of the pairs (8 of 16 
tows). Conversely, the slow tows swept a greater area 
than that swept by the fast tows during the other half 
of pairs. Bottom contact sensors provided reliable data 
