Weinberg et al.; Survey selectivity for Gadus macroceph 
365 
Table 2 
Catch of large (>55 cm in fork length) Pacific cod (Gadus macrocephalus) at fast (Cf) 
and slow (Cg) towing speeds for each pair of tows conducted for this study in the 
eastern Bering Sea in August 2013. Pairs 1-10 were deep tows. Pairs 11-24 were 
shallow tows. The expected catch (Ce) was calculated on the basis of the length-based 
selectivity curve from the 2013 National Marine Fisheries Service stock assessment 
as described in the Materials and methods section. 
Tow pair 
Cf 
Cg 
Cf/Cg 
^e/^s 
1 
17 
22 
0.8 
33 
1.5 
2 
16 
6 
2.7 
8 
1.4 
3 
14 
10 
1.4 
19 
1.9 
4 
11 
15 
0.7 
26 
1.8 
5 
5 
12 
0.4 
18 
1.5 
6 
7 
26 
0.3 
42 
1.6 
7 
5 
5 
1.0 
8 
1.5 
8 
7 
9 
0.8 
15 
1.7 
9 
9 
9 
1.0 
15 
1.6 
10 
15 
13 
1.2 
19 
1.5 
11 
14 
14 
1.0 
20 
1.4 
12 
19 
42 
0.5 
57 
1.4 
13 
22 
10 
2.2 
14 
1.4 
14 
17 
15 
1.1 
19 
1.3 
15 
17 
16 
1.1 
22 
1.4 
16 
22 
36 
0.6 
50 
1.4 
17 
11 
27 
0.4 
38 
1.4 
18 
22 
19 
1.2 
31 
1.6 
19 
16 
23 
0.7 
32 
1.4 
20 
16 
15 
1.1 
20 
1.4 
21 
15 
8 
1.9 
13 
1.6 
22 
7 
7 
1.0 
10 
1.5 
23 
6 
7 
0.9 
9 
1.3 
24 
16 
9 
1.8 
13 
1.4 
on all tows, indicating that trawl footropes were firmly 
in contact with the substrate and providing evidence 
for our decision to use all 24 pairs of data in subse- 
quent analyses. 
Effect of towing speed on catches 
Fast tows had larger catches of large Pacific cod in only 
10 of 24 paired tows. In those 10 pairs, the catches 
from fast tows were 1.1 to 2.7 times (mean: 1.6 times) 
greater than the catches from slow tows (Table 2). A 
sign test indicated that larger catches were not sig- 
nificantly more frequent in fast tows (successes=10, tri- 
als=24, P=0.924); larger catches in at least 18 of the 24 
pairs would be required for significance (P<0.05). 
The mean difference d between In(cf) and In(cs) of 
-0.08 (SD 0.55) was approximately normally distrib- 
uted according to a goodness-of-fit test (xV2-i=l-226, 
P=0.54; Fig. 3). The mean of Cf/Cg was 1.1 (SD 0.58) 
(range: 0.3-2. 7; Table 2). A paired ^-test indicated that 
the difference in Inicatch) between fast and slow tows 
was not statistically significant (^23=-0.69, P=0.50). The 
expected mean ratio of the catch of large Pacific cod in 
fast tows over slow tows (Cf/Cg) was 1.5 (range: 1.3-1. 9). 
If the expected ratio of 1.5 were true, then the power 
of a 1-sided ^-test (Hg;. d>0) would be 97% in rejecting 
Ho (Table 3). 
Vertical distribution 
Demersal fish backscatter was fairly low, as would be 
expected given the low numbers of Pacific cod captured. 
The strongest demersal fish backscatter (S^ — 45 dB) 
appeared very close to the acoustically detected sea- 
bed; fish backscatter farther off the seabed was gener- 
ally weaker in comparison (Sy — 65 dB). The demersal 
fish backscatter observed below the average headrope 
height of 2.0 m during this study was a very large frac- 
tion of fish backscatter integrated over all depth lay- 
ers examined (median proportion: 0.96; Fig. 4). In an 
absolute sense, the highest demersal fish backscatter 
values were found within the depth layer of 0.25-2.0 m 
(Fig. 5); the median fish in this layer was more than 
14 times that in any other depth layer. 
Discussion 
We failed to detect a difference between slow (1.5 m/s) 
and fast (2.1 m/s) towing speeds in the rates at which 
