180 
NOAA 
National Marine 
Fisheries Service 
Fishery Bulletin 
established 1881 ■<?. 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Reducing variability in bottom contact and net 
width of a survey trawl by restraining door 
movement and applying a constant ratio of 
warp length to depth 
Email address for contact author: ken.weinberg@noaa.gov 
Resource Assessment and Conservation Engineering Division 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115 
Abstract — A field study was con- 
ducted to examine methods to re- 
duce varying geometry of a demersal 
survey trawl net caused by changing 
depth and trawling speed and that 
could result in variable sampling ef- 
ficiency. A reduction in varying trawl 
net geometry is important because 
variance in indices of abundance is 
the result of variability in sampling 
efficiency, as well as animal density. 
Trawl performance measures con- 
sidered were door and wing spread 
and the contact of the footrope and 
lower bridles with the seabed. Three 
treatments were tested for their ef- 
fects on these measures: 1) standard 
towing procedures, 2) door spread re- 
strained by a restrictor line attached 
between the trawl warps ahead of 
the doors, and 3) doors similarly re- 
stricted in conjunction with a modi- 
fied scope ratio. Generalized linear 
modeling showed that both depth 
and trawl speed significantly af- 
fected trawl measures in nearly all 
cases. The restrictor line reduced the 
effect of depth on spread and, to a 
lesser extent, on bottom contact of 
the footrope; however, it was ineffec- 
tive at reducing the effect of trawl 
speed over the speed range observed. 
The combination of a restrictor line 
and modification of the scope ratio 
to achieve a consistent upward pull 
on the doors was most effective in 
maintaining trawl shape to our tar- 
get dimensions. 
Manuscript submitted 11 June 2014. 
Manuscript accepted 10 February 2015. 
Fish. Bull.: 113:180-190 (2015). 
doi: 10.7755/FB. 113.2.6 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Kenneth L. Weinberg (contact author) 
Stan Kotwicki 
Demersal otter trawls used in both 
commercial fisheries and resource 
assessments lack a rigid frame; in- 
stead, sheer on the trawl doors and 
various operational factors (e.g., 
warp length and towing speed) are 
used to hold them in their fishing 
configuration. From the perspective 
of commercial fisheries, the lack of a 
rigid frame is convenient because it 
allows for easy stowage of the trawl 
when it is on deck. However, from 
the perspective of resource assess- 
ment, this flexibility contributes to 
variability in trawl shape and likely 
has a subsequent effect on sampling 
efficiency (Godp and Engas, 1989; 
Koeller, 1991; Walsh, 1992; Weinberg 
et al., 2002). This idea challenges a 
fundamental assumption with bot- 
tom trawl surveys for estimating 
indices of population abundance for 
stock assessments. That assumption 
is that the sampling efficiency of the 
trawl remains the same from tow to 
tow, across the survey area, and over 
time so that the survey-estimated 
catch per unit of effort accurately re- 
flects changes in the distribution and 
abundance of fish stocks. 
The relationship between trawl 
shape and its sampling efficiency is 
directly affected by 3 common pro- 
cesses: 1) horizontal herding, defined 
as the horizontal movement of fish 
into or out of the net path, 2) ver- 
tical herding, defined as the vertical 
movement of fish into or out of the 
net path, and 3) escapement, defined 
as and limited here to fish passage 
beneath the footrope. The effective- 
ness of horizontal herding can be af- 
fected by shifts in the bridle angle 
of attack due to changes in door and 
wing spread. This region of the trawl 
is critical for capture efficiency be- 
cause it generates both visual and 
tactile stimuli (Main and Sangster, 
1981) that, along with other fac- 
tors such as temperature (Ryer and 
Barnett, 2006) and trawl speed, will 
determine whether fish (particularly 
flatfishes) are herded into the path of 
the net or, conversely, elude capture 
by passing over, under, or through the 
bridles (Somerton and Munro, 2001). 
The effectiveness of vertical herding 
can be affected by changes in the 
height of the net opening that make 
it possible for fish above the hea- 
drope to pass over the top or swim 
down into the trawl opening. For 
semipelagic species, additional visu- 
al, auditory, and pressure cues from 
the vessel, warps, doors, bridles, and 
wing mesh contribute to this process 
