21 
National Marine 
Spencer F. Baird r ' V 
Fisheries Service 
Fishery Bulletin 
First U S. Commissioner \ : r _ ff 
of Fisheries and founder 
NOAA 
established in 1881 
of Fishery Bulletin WiI7 
The effects of wave-induced vessel motion on the 
geometry of a bottom survey trawl and the herding 
of yellowfin sole (Limanda aspera) 
Email address for contact author: dsomerton4@gmail.com 
Resource Assessment and Conservation Engineering Division 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE, Building 4 
Seattle, Washington 98115-6349 
Abstract-Bottom trawl surveys 
need constant catchability over time 
to produce estimates of relative 
abundance that are most informa¬ 
tive for stock assessment modeling. 
However, environmental conditions 
during surveys can potentially pro¬ 
duce both interannual and long-term 
changes in catchability. Here we 
consider the effects of one environ¬ 
mental variable, sea state, on ves¬ 
sel motion, trawl performance, and, 
presumably, catchability. We present 
the results of 2 field experiments 
performed on chartered commercial 
trawlers typical of those used by the 
National Marine Fisheries Service 
for conducting its eastern Bering 
Sea bottom trawl survey. The first 
experiment shows that wave-induced 
vessel motion is transmitted down 
the towing warps, creating vertical 
oscillation in both trawl bridles and 
the footrope, which potentially influ¬ 
ence both the herding and footrope 
escapement of fish. The second ex¬ 
periment shows that the herding of 
yellowfin sole (Limanda aspera ) by 
the survey trawl decreases with an 
increase in wave height. A proposed 
mathematical model of flatfish herd¬ 
ing is used to provide a hypotheti¬ 
cal mechanism describing the effect 
of surface waves on flatfish herding. 
In addition, the annual biomass es¬ 
timates of yellowfin sole determined 
from the bottom trawl survey data 
are shown to decrease with increas¬ 
ing wave height. The potential im¬ 
pacts of these relationships on the 
yellowfin sole assessment model are 
then considered. 
Manuscript submitted 9 May 2017. 
Manuscript accepted 18 October 2017. 
Fish. Bull. 116:21-33 (2018). 
Online publication date: 9 November 2017. 
doi: 10.7755/FB.116.1.3 
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. 
David Somerton (contact author) 
Kenneth Weinberg 
Peter Munro 
Louis Rugolo 
Thomas Wilderbuer 
Uncertainty in estimates of relative 
stock abundance determined from bot¬ 
tom trawl survey data has long been 
recognized to be a function of sample 
size, survey design, and species patch¬ 
iness (Kimura and Somerton, 2006). 
However, the relative-abundance es¬ 
timate at each sampling location de¬ 
pends on trawl sampling efficiency 
(i.e., the proportion of animals that 
are captured within the trawl path); 
consequently, uncertainty in survey¬ 
wide estimates of relative abundance 
must also reflect spatial and tempo¬ 
ral variability in sampling efficiency. 
Despite efforts to reduce the influence 
of this variability on NOAA bottom 
trawl surveys through the standard¬ 
ization of survey gear and operational 
protocols (Stauffer, 2004), biotic and 
abiotic factors remain that cannot 
be controlled. We examined the in¬ 
fluence of one such factor: that is, 
surface waves, which affect, in turn, 
vessel motion, trawl geometry, and 
sampling efficiency (Politis et al., 
2012). Here we consider the effects 
of wave height on the performance of 
the bottom trawl used by the Alaska 
Fisheries Science Center, National 
Marine Fisheries Service, to conduct 
its annual eastern Bering Sea (EBS) 
trawl survey (henceforth referred to 
as “the survey trawl”; Conner and 
Lauth, 2016). 
Although the connection between 
surface waves and vessel motion is 
readily apparent, the connection be¬ 
tween vessel motion and trawl per¬ 
formance is not. Previous studies, 
however, have shown that effects of 
vessel motion can be pronounced. For 
example, O’Neill et al. (2003) consid¬ 
ered the increase in size selectivity 
observed by trawl fishermen during 
stormy periods, focused on the os¬ 
cillation in trawl speed caused by 
vessel motion, and investigated its 
consequent effects on the opening 
of codend meshes and fish escape¬ 
ment. Likewise, Politis et al. (2012) 
examined how vessel motion can be 
transmitted down the towing warps 
to impact many aspects of trawl per¬ 
formance, especially bottom contact 
of the trawl bridles (which influences 
fish herding) and bottom contact of 
the footrope (which influences fish 
escapement) (Dickson, 1993; Somer¬ 
ton et al., 2007). 
