Weinberg and Kotwicki: Reducing variability in bottom contact and net width of a survey trawl 
185 
Table t 
Means and standard deviations (SD) by trawl performance measure, depth of site (shallow, middle, and deep), and 
towing treatment observed during the eastern Bering Sea study conducted by the Alaska Fisheries Science Center in 
2005 to reduce variability in the geometry of a demersal survey trawl. Treatments included standard survey procedure, 
adding a restrictor line to constrain door movement, and adjusting scope ratio with the restrictor line in place. Trawl 
spreads were measured in meters, and the distances of the footrope and lower bridle from the seabed were measured 
in centimeters at various points along their lengths (fwd=forward of wingends). 
Treatment 1: no restrictor, standard survey scope 
Trawl measure 
Shallow ( n= 
■28) 
Middle (n= 20) 
Deep (re=28) 
Mean 
SD 
n 
Mean 
SD 
n 
Mean 
SD 
n 
Door spread (m) 
46.44 
2.23 
7 
66.88 
2.47 
4 
70.44 
1.59 
10 
Wing spread (m) 
15.15 
0.76 
9 
17.88 
0.24 
4 
18.05 
0.13 
5 
Headline height (m) 
3.28 
0.16 
12 
2.96 
0.12 
8 
2.77 
0.11 
14 
Footrope center (cm) 
1.84 
0.14 
9 
3.36 
0.05 
4 
5.33 
0.36 
10 
Footrope corner (cm) 
2.73 
0.28 
17 
3.27 
0.15 
8 
5.09 
0.31 
19 
Footrope wing (cm) 
3.44 
0.74 
17 
3.41 
0.49 
8 
4.14 
0.52 
17 
Bridle, 30 m fwd (cm) 
3.16 
0.66 
15 
2.52 
0.21 
6 
4.45 
1.61 
17 
Bridle, 40 m fwd (cm) 
2.59 
0.71 
12 
2.46 
0.45 
6 
3.75 
0.71 
19 
Treatment 2: restrictor, standard survey scope 
Shallow ( n= 
28) 
Middle (re=20) 
Deep (n= 28) 
Trawl measure 
Mean 
SD 
n 
Mean 
SD 
n 
Mean 
SD 
n 
Door spread (m) 
45.19 
4.59 
12 
51.11 
3.82 
4 
50.59 
0.61 
10 
Wing spread (m) 
14.49 
0.78 
12 
15.37 
0.54 
4 
15.98 
0.71 
8 
Headline height (m) 
3.40 
0.23 
14 
3.25 
0.20 
8 
3.16 
0.12 
14 
Footrope center (cm) 
1.79 
0.16 
12 
2.84 
0.34 
4 
5.10 
0.26 
9 
Footrope corner (cm) 
2.75 
0.46 
23 
3.39 
0.32 
8 
5.17 
0.53 
20 
Footrope wing (cm) 
3.22 
0.71 
21 
3.30 
0.63 
7 
4.12 
0.58 
20 
Bridle, 30 m fwd (cm) 
3.06 
0.65 
19 
2.50 
0.28 
7 
4.37 
1.33 
18 
Bridle, 40 m fwd (cm) 
2.42 
0.60 
16 
2.27 
0.24 
6 
3.67 
1.06 
15 
Treatment 3: restrictor, modified scope 
Shallow ( n= 
28) 
Middle (n= 20) 
Deep (n= 28) 
Trawl measure 
Mean 
SD 
n 
Mean 
SD 
n 
Mean 
SD 
n 
Door spread (m) 
43.57 
0.80 
9 
45.00 
5.93 
4 
44.08 
2.45 
9 
Wing spread (m) 
14.46 
0.43 
12 
14.32 
0.93 
4 
15.07 
0.63 
6 
Headline height (m) 
3.38 
0.16 
14 
3.27 
0.25 
7 
3.13 
0.17 
13 
Footrope center (cm) 
1.78 
0.11 
11 
2.71 
0.29 
4 
4.82 
0.41 
9 
Footrope corner (cm) 
2.61 
0.37 
23 
3.19 
0.20 
8 
5.20 
0.56 
18 
Footrope wing (cm) 
3.44 
0.77 
22 
3.28 
0.75 
7 
4.03 
0.63 
15 
Bridle, 30 m fwd (cm) 
3.03 
0.68 
17 
2.48 
0.25 
8 
4.03 
1.44 
16 
Bridle, 40 m iwd (cm) 
2.49 
0.44 
18 
2.17 
0.25 
6 
4.25 
1.63 
13 
ratio was slightly more efficient than the use of the re- 
strictor line with the standard scope ratio at mitigating 
the depth effect at the center of the footrope (Table 2, 
Fig. 5). The effect of trawl speed was small and positive 
for all treatments. The interaction term, between trawl 
speed and treatment, was not significant, indicating no 
difference in the effect of trawl speed between treat- 
ments (Table 2). 
Distance of the bridle off bottom 
Changes in mean distances of the bridle off bottom 
were nominal, less than 2 cm across all 3 depth sites. 
For our control treatment, mean observed distances off 
bottom ranged from 3.2 to 4.4 cm and from 2.6 to 3.8 
cm for the 30 and 40 m positions, respectively (Table 
1). The GLM indicated that both depth and trawl speed 
