Weinberg and Kotwicki: Reducing variability in bottom contact and net width of a survey trawl 
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m, experimenting with 3 differing restrictor posi- 
tions while also testing 3 different warp lengths 
or scope ratios, our proxy for varying vertical 
wire angles, for their effect on wing spread. A 
review of the survey database showed that for 
the most recent 5 years before this experiment, 
95% of tows had an average wing spread of at 
least 15 m (Fig. 2A). Of the more uncommon 
tows with wing spreads <15 m, most occurred 
in shallower water (depths <39 m) and had a 
warp length of 137 m, the shortest length per- 
mitted by the survey (Fig. 2B). For the purpose 
of this study, we considered 15 m to be the ef- 
fective minimum survey wing spread. Therefore, 
because the primary objective of this study was 
to keep trawl geometry constant across survey 
depths, we targeted a 15-m wing spread for all 
tows in which the restrictor was deployed. With 
the trawl doors at the surface, 3 restrictor posi- 
tions of 70, 116, and 162 m forward of the doors 
were explored. Testing resulted in the position at 
116 m yielding wing spreads closest to our target 
spread of 15 m. 
Our third experimental treatment, in which 
the restrictor line and a modified scope ra- 
tio were used, called for setting an appropriate 
amount of trawl warp to minimize the variability 
in the upward pulling force on the trawl door. A 
constant upward pulling force at all depths was 
considered desirable because it would ensure 
that the door weight in contact with the bottom 
was equal, regardless of depth, enabling more 
consistent ground sheer of the doors necessary to 
spread the trawl. The pulling forces exerted on 
a trawl door are divided into vertical (upward 
pulling force; U) and horizontal (forward pulling force; 
F) forces that can be related to each other with this 
equation: 
U = F x tan(u), (1) 
where v = the angle between the trawl warp and the 
seabed. 
This equation indicates that, to achieve constant U, 
it is necessary to keep F and v constant. F depends 
on trawl speed and the friction force produced by door 
contact with the substrate. Other than maintaining 
a constant vessel speed over ground, we were unable 
to control for either one of these variables in the ex- 
periment. However, angle v was controlled in the third 
treatment by setting an appropriate length of wire to 
keep v constant and minimize the variability in U. The 
length of warp needed at each depth location was de- 
termined with this equation: 
Warp length = depth / sin(y). (2) 
We tested 3 different warp lengths, 229, 274, and 
366 m, at the depth of 71-m, that represented 3 dif- 
ferent wire angles, 11.2°, 15.1°, and 18.2°. The 274-m 
warp length or 15.1° wire angle gave the most constant 
wing spreads near our 15-m target and coincidentally 
was closest to the 14.7° angle used by Engas and Ona 1 
to keep approximately two-thirds of the door weight in 
contact with the bottom. On the basis of this equation: 
Warp length = depth / sin(15.1°), (3) 
our 3 depth sites (21, 104, and 150 m) required the use 
of 81, 400, and 576 m of trawl warp, respectively, ap- 
proximating a 3.8:1 scope ratio. Because the treatment 
that used a restrictor with a modified scope ratio called 
for 81 m of warp at our shallow experimental depth 
site and our restrictor position was 116 m forward of 
the doors, no restrictor was used and the trawl blocks 
served as the restrictor instead. 
Data analyses 
For any given trawl measure, all 3 towing treatments 
in a haul were excluded from analyses if one or more 
of the treatments produced unusable data. Examples 
of events that produced unusable data include sensor 
malfunction or suspected trawl failure caused by the 
collapse of a door or the encounter of a derelict crab 
pot. The remaining treatment data were screened for 
