186 
Fishery Bulletin 113(2) 
Table 2 
Generalized linear model coefficients from the eastern Bering Sea study conducted by the Alaska Fisheries Science Center in 
2005 on variation in trawl geometry by towing treatment, no restrictor, survey standard scope (NRSS); presence of a restric- 
tor line, survey standard scope (RSS); and presence of a restrictor line, modified scope (RMS), describing the relationship 
between the mean net performance measures as affected by depth and trawl speed. Only coefficients from models found to 
be significant are shown. Coefficients of multiple determination (R 2 ) expressed as a percentage also are presented. Common 
intercepts and slopes indicate that we did not detect significant differences between treatments, respectively. 
Door Wing Footrope Footrope Footrope Lower Lower 
spread spread center corner wing bridle 30 m bridle 40 m 
Depth 
Speed 
Depth Speed 
Depth Speed 
Depth Speed 
Depth Speed 
Depth Speed 
Depth Speed 
Common 
intercept 
Common 
10.9416 
-0.2354 
2.0952 
3.1684 
0.0550 
-1.3598 
slope 
NRSS 
intercept 
NRSS 
37.7859 
37.7859 
1.3135 
0.4942 
0.0188 
0.0054 
0.0067 1.0321 
0.0074 1.3755 
slope 
0.1878 
1.8372 
0.0253 
0.0265 
RSS 
intercept 
16.5546 
16.5546 
RSS slope 
RMS 
0.0258 
10.8316 
0.0084 
0.0244 
intercept 
22.2891 
22.2891 
RMS slope 
R2 
-0.0248 
8.9876 
0.0014 
0.0226 
percentage 
95.27 
84.79 
93.59 
80.68 
18.06 
20.89 
38.40 
positively affected distances off bottom at the 2 bridle 
positions, increasing with increasing depth and speed, 
but no differences between treatments were detected 
(Table 2). 
Discussion 
Horizontal herding 
Our primary objective was to reduce the effects of 
depth and coincidental changes in trawl speed on the 
geometry of our survey trawl. Specifically, we wanted 
to determine which of 3 towing methods would best 
control the variability in the spread of the doors and 
wings to achieve the average minimum 15-m wing 
spread seen in recent surveys. We found that attaching 
a restrictor when we used the standard survey scope 
ratios successfully reduced the variability in mean 
wing spread to within 1.7 m across the typical range 
in survey depths — considerably less than the 8.3-m 
range seen during surveys (Fig 2A). Best results were 
obtained when a restrictor line was used with a modi- 
fied scope ratio (3.8:1) to provide a constant upward 
pulling force on the trawl doors. 
With mean door and wing spread data, we were able 
to estimate bridle angles of attack at each depth site 
and for each treatment. Maintaining consistent bri- 
dle angles minimizes inconsistencies in fish-size and 
species-specific selectivity inherent in the horizontal 
70- 
° 
o e 
65- 
a/ 
60- 
-o 
I 55- 
CO 
* * 
I 50- 
^ s' 4- 1- 
AR — 
i" A i 
HQ 
1 + r 
40- 
§ + 
+ 
i i i i i i 
40 60 80 100 120 140 
Depth (m) 
Figure 3 
The effect of depth on mean door spread during the 
eastern Bering Sea field experiment conducted by the 
Alaska Fisheries Science Center in 2005 to reduce the 
variability in a survey trawl’s geometry. Data were 
fitted with a generalized linear model. Three towing 
treatments were applied: no restrictor line with stan- 
dard scope (circle, solid line), restrictor line with stan- 
dard scope (triangle, dashed line), and restrictor line 
with modified scope (plus sign, dotted line). 
