Weinberg and Kotwicki: Reducing variability in bottom contact and net width of a survey trawl 
181 
(Engas and God0, 1989a; Somerton, 2004; De Robertis 
and Wilson, 2006; De Robertis and Handegard, 2013). 
The effectiveness of a bottom trawl footrope, in relation 
to fish escapement beneath it (particularly in relation 
to escapement of flatfishes), can be affected by changes 
in the distance from the bottom during periods when 
it loses contact (Engas and Godp, 1989b; Walsh, 1992). 
For these reasons, we suspect that minimizing changes 
in trawl geometry should lead to better consistency in 
trawl efficiency and, hence, to more precise estimates 
of abundance. 
Of the environmental and operational factors that 
affect net geometry, towing depth, towing speed, warp 
length, and substrate type are most important (Godo 
and Engas, 1989; Weinberg and Kotwicki, 2008). 
Changes in these factors generally produce changes in 
the horizontal spread of the trawl doors and net width. 
For many trawl designs, increasing the horizontal 
opening results in a decrease in the vertical opening 
and can increase the distance of the footrope from the 
seabed (von Szalay and Somerton, 2005). One method 
that has been shown to successfully reduce the spread 
of a trawl is to deploy a restrictor line (Fig. 1) — a line 
attached between trawl warps ahead of the trawl doors 
to restrain door movement (Engas and Ona 1 ’ 2 ). 
In May 2005, the Alaska Fisheries Science Center 
(AFSC) of the U.S. National Marine Fisheries Service 
conducted an experiment with a restrictor line on the 
standardized trawl gear that is used in the annual bot- 
tom trawl survey in the eastern Bering Sea (hereafter 
referred to as the survey). In this study, we extended 
the work of Engas and Ona 1 ’ 2 by using generalized lin- 
ear modeling (GLM) to analyze the effects of the re- 
strictor on footrope and bridle contact with the seabed. 
More specifically, using the trawl performance criteria 
of 1) consistent door and wing spread, and 2) reduced 
distances of the footrope and lower bridles from the 
seabed, we examined which of 3 towing treatments 
best minimized changes in trawl geometry. 
Materials and methods 
Vessel, trawl gear, and instrumentation 
Trawling operations were conducted aboard the 40-m 
stern trawler FV Aldebaran that was chartered for 
the 2005 survey of crab and groundfish resources in 
the eastern Bering Sea (Lauth and Acuna 3 ). This ves- 
1 Engas, A. and E. Ona. 1991. A method to reduce survey 
bottom trawl variability. ICES Council Meeting (C.M.) Doc- 
uments 1991/B:39, 6 p. 
2 Engas, A. and E. Ona. 1993. Experiences using the con- 
straint technique on bottom trawl doors. ICES Council 
Meeting (C.M.) Documents 1993/B:18, 10 p. 
3 Lauth, R. and E. Acuna (compilers). 2007. 2005 bottom 
trawl survey of the eastern Bering Sea continental shelf. 
AFSC Processed Rep. 2007-1, 164 p. [Available from Alaska 
Fish. Sci. Cent., NOAA, Natl. Mar. Fish. Serv., 7600 Sand 
Point Way NE, Seattle, WA 98115.] 
sel is equipped with raised, split trawl winches filled 
with compacted, solid-core trawl warps 28.6 mm (1.125 
in) in diameter. Warps were measured and marked 
in accordance with standardized survey procedures 
(Stauffer, 2004). 
The 83-112 bottom trawl used in this study is the 
same as the one used in the annual AFSC resource as- 
sessment surveys. It is a low-rise, 2-seam flatfish trawl 
characterized by a headrope that is 25.3 m (83 ft) long 
and a simple footrope that is 34.1 m (112 ft) long. The 
footrope is 5.2 cm in diameter and is composed of steel 
cable wrapped in a split rubber hose for protection. It 
is weighted with 75 kg of chain to which the netting 
is attached. The nylon net is made of 102-mm stretch- 
mesh panels forward of the intermediate section, 89- 
mm stretch-mesh throughout the remainder of the net, 
and has a 32-mm mesh liner inside the codend. It con- 
nects on each side to a steel V-door, which is 1.8x2. 7 m 
and weighs 816 kg (in air), by a pair of 3-m-long door 
legs, a 12.2-m-long door leg extension, and a pair of 
54.9-m-long bare cable bridles. 
Our restrictor line, with a length of 10.5 m, was 
nearly 2 m longer than the 8.6-m distance between 
the trawl blocks to facilitate deployment over the stern 
with the trawl doors in the water and beginning to 
spread. It consisted of 3 lengths of braided Spectra 4 
line (each 12.7 mm in diameter): a 6-m-long middle 
section capable of bridging the stern ramp and a pair 
of detachable 2-m-long end sections for tethering each 
side to the trawl warps. Quick assembly of the 3 sec- 
tions was accomplished by using a combination of ham- 
merlocks, G-hooks, and flat links. The starboard side of 
the restrictor connected to a loop of 25.4-mm-diameter 
Duralon braid sheathing tied directly to the starboard 
warp with a variant of the hitch knot that is slip-free 
yet easy to untie. The port side of the restrictor ter- 
minated with a large slip hook that snapped loosely 
around the warp, allowing it to slide freely and, there- 
fore, preventing the restrictor from parting if the net 
was askew during deployment or while towing. 
Time-synchronized data were collected from numer- 
ous shipboard and trawl-mounted instruments. Vessel 
position and speed over ground (hereafter referred to 
as towing speed ) were measured at 2-s intervals with 
GPS satellite navigation. The speed of the trawl as 
it moved through the water (hereafter, referred to as 
trawl speed) was measured along the trawl axis to the 
nearest 0.1 kn, at approximately 20-s intervals, with 
a Scanmar acoustic trawl speed sensor (TrawlSpeed 
HC4-TSS, Scanmar AS, Asgardstrand, Norway) mount- 
ed to the center of the headrope. Wing spread and door 
spread were measured acoustically to the nearest 0.1 
m, at 4-s intervals, with NetMind sensors (Northstar 
Electronics Inc., Vancouver, Canada). The distance of 
the footrope from the seabed was measured to the 
nearest centimeter at 0.5-s intervals with bottom con- 
4 Mention of trade names or commercial companies is for iden- 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
