22 
Fishery Bulletin 116(1) 
In this study, we consider several related topics con¬ 
nected to the potential effects of surface waves on sam¬ 
pling efficiency of the survey trawl. First, we describe a 
field experiment that we conducted, which was similar 
to those of O’Neil] et al. (2003) and Politis et al. (2012), 
to estimate the effects of vessel motion on the bottom 
contact of the bridles and footrope of the survey trawl. 
Second, we describe another field experiment that we 
conducted to estimate the effects of wave height on the 
herding of yellowfin sole (Limanda asperct ). The ex¬ 
perimental design focused on the trawl bridles, which 
has been shown to be the primary trawl component 
determining sampling efficiency of flatfishes (Somerton 
and Munro, 2001; Munro and Somerton, 2002). Third, 
we use the results of these 2 experiments to develop 
a mathematical model describing the effect of wave 
height on the herding of yellowfin sole. Fourth, we ex¬ 
amine how the annual biomass estimates of yellowfin 
sole determined from EBS trawl survey data, and the 
deviations between these estimates and those produced 
by the yellowfin sole stock assessment model (Wilder- 
buer et al. 1 ), are correlated with wave heights that the 
vessels encountered during the surveys. Finally, we 
consider the significance of these findings in terms of 
fisheries stock assessments. 
Materials and methods 
Description of the EBS survey trawl 
The EBS survey trawl, known as the 83-112 Eastern 
trawl, is a low-rise, 2-seam flatfish trawl designed for 
use on a smooth, soft sea bottom. A detailed descrip¬ 
tion is of its design and construction is presented in 
Stauffer (2004), however several aspects of its design 
are important for this study and are considered below 
(Fig. 1). The nylon net has a 25.5-m headrope and 34.1- 
m footrope which is 5.2 cm in diameter and constructed 
of wire rope with a single wrap of both polypropylene 
line and split rubber hose, but without bobbins or roll¬ 
ers to allow it to better conform to the smooth bottom 
in the EBS. The net is attached to a pair of 55-m wire 
bridles that are joined together and attached to a sin¬ 
gle 12.2-m wire door leg extension, which, in turn, is 
connected to two, 3-m chain door legs attached to 816- 
kg steel “V” doors (Fig. 1). Although this length of door 
leg extension is typical for many EBS survey vessels, 
door legs are custom built for each charter vessel and 
can vary from 8 tol5 m. Henceforth, when bridle mea¬ 
surements are specified aft of the doors, they include 
the length of the door extension for a specific vessel 
1 Wilderbuer, T. K., D. G. Nichol, and J. Ianelli. 2015. As¬ 
sessment of the yellowfin sole stock in the Bering Sea and 
Aleutian Islands. In Stock assessment and fishery evalua¬ 
tion report for the groundfish resources of the Bering Sea/ 
Aleutian Islands regions, p. 733-820. North Pacific Fishery 
Management Council, Anchorage, AK. [Available from web¬ 
site.] 
but exclude the lengths of the tail chains, which are 
standard for all vessels. 
Wave height and vessel motion 
Experimental design An experiment to estimate the 
performance of the survey trawl at various levels of 
wave height was conducted in the EBS from 14 through 
25 September 2003 aboard the chartered 38-m com¬ 
mercial stern trawler FV Vesteraalen. Because wave 
heights cannot be predicted sufficiently far in advance 
to plan a study of wave height, the experimental tows 
were completed opportunistically, in 2 parts, while the 
vessel was engaged in other studies. The first part, 
located near 55°58'N, 162°55 , W at a depth of 82 m, 
consisted of repetitive trawl tows that were 5 min in 
duration, measured from the time the trawl first con¬ 
tacted the bottom to the time the footrope left the bot¬ 
tom by using a bottom contact sensor (BCS; Weinberg 
and Somerton, 2006). The second part, located near 
55°10'N, 166°15'W at a depth of 115 m, consisted of 
repetitive tows that were 15 min in duration. During 
both parts, all operating procedures, except tow dura¬ 
tion (standard survey tow duration is 30 min) and the 
use of open codends, were consistent with EBS trawl 
survey sampling protocols (Stauffer, 2004). 
Various aspects of trawl geometry were measured 
on all tows. Along the footrope, the distance between 
the footrope and the seafloor (off-bottom distance) was 
measured to the nearest centimeter, at 1.5-s intervals 
and at 5 positions by placing BCS units at the center 
of the footrope, at 3 m to both sides of the center (the 
footrope corners), and on each wing 1 m behind the 
wing tip (Fig. 1). Along the bridles, off-bottom distance 
was measured at 6 positions by placing BCS units on 
the lower bridle at distances of 25, 40, and 50 m for¬ 
ward of the wing tip on both port and starboard sides 
of the trawl (Fig 1; measurements were made at all 
11 positions simultaneously). The BCS units measure 
the tilt angle between their points of bottom contact 
and their points of attachment to the trawl and were 
individually calibrated to allow prediction of off-bottom 
distance from tilt angle. Description of the BCS units 
and the methods for attaching them to the footrope and 
bridles are described in Weinberg and Somerton (2006). 
Heave (vertical movement in centimeters), pitch (an¬ 
gular motion on the along-vessel axis to the nearest 
0.1°), and roll (angular motion on the across vessel axis 
to the nearest 0.1°) were measured with a heave, pitch, 
and roll sensor (Teledyne TSS Ltd., 2 Watford, UK) at 
1-s intervals. The sensor, which was positioned near 
the center of motion of the vessel, allowed prediction 
of heave at one remote position on the vessel based on 
the x, y, and z coordinates of this position in relation 
to the sensor. We chose this position as the location 
where the starboard warp passed over the stern trawl 
2 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. 
