32 
Fishery Bulletin 11 6(1) 
may create a greater herding stimulus than a similar 
length of bridle in calm water. If so, then the efficiency 
of herding within the bridle contact area, which was 
assumed to be constant, would also be influenced by 
wave height. Further research is needed to better de¬ 
fine these parameters and determine how they might 
change at higher sea states. 
Estimated abundance of yellowfin sole and its relation to 
wave height 
The abundance of yellowfin sole estimated by the an¬ 
nual bottom trawl survey clearly declines with an in¬ 
crease in mean wave height (Fig. 7)—a finding that 
is consistent with wave height effects reported for 
the NOAA West Coast bottom trawl survey (Stewart 
et al., 2010). In addition, the deviations between the 
survey estimates and assessment model estimates of 
abundance also decline with an increase in mean wave 
height (Fig. 7). The question is whether these associa¬ 
tions establish causation. We have shown that increased 
wave height results in a reduction in the bridle contact 
area, which should reduce the herding of flatfish. We 
also confirmed this notion by providing evidence that 
increased wave height reduces the sampling efficiency 
of yellowfin sole, presumably owing to the reduction in 
herding. However, a decrease in herding by itself may 
not exert a sufficiently strong effect to account for the 
observed decline in estimated yellowfin sole biomass 
because during normal survey conditions only -30% of 
a flatfish catch comprises herded fish (Somerton et al., 
2007). However, the potential effects of wave height on 
sampling efficiency could be much larger if escapement 
under the footrope is considered. 
Although our experimental evidence showed that the 
mean off-bottom distance at all points on the footrope 
did not increase significantly with SDH, the magnitude 
of the change was still strong enough to potentially al¬ 
low greater escapement. Coupled with both the oscilla¬ 
tions in off-bottom distance and, likely, trawl speed, we 
suspect that footrope escapement increases with SDH, 
but we have no experimental evidence to prove this. 
Exactly how the vertical and horizontal oscillatory 
motion of the footrope and bridles influences fish be¬ 
havior is unknown. However, the study of O’Neill et 
al. (2003) on codend mesh escapement has shown that 
oscillations in trawl speed produce changes in fish 
behavior, as well as changes in net geometry. Conse¬ 
quently, footrope escapement and bridle herding may 
be quite different in an elevated sea state from what 
has been reported for relatively calm water (Somerton 
et al., 2007) and this difference may be the primary 
reason why we obtained such a poor fit of our model to 
the experimental catch ratio data. 
Even with the potential for increased footrope es¬ 
capement to compound the apparent influence wave 
height has on sampling efficiency of the survey trawl, 
the observed decrease in yellowfin sole biomass esti¬ 
mates with increased wave height may also reflect oth¬ 
er influences. Wilderbuer et al. 1 , in a stock assessment 
model for EBS yellowfin sole, determined that the trawl 
survey catchability function is temperature dependent 
and that the estimated stock abundance declines at 
colder temperatures. Two hypotheses were proposed 
for this: 1) sampling efficiency of the survey trawl is 
temperature dependent (i.e., yellowfin sole herd less or 
escape more in cold water) and 2) availability of this 
species to the survey is temperature dependent (i.e., 
more yellowfin sole remain in their shallow spawning 
areas, which are outside the survey area, during colder 
years). To complicate these hypotheses further, annual 
mean wave height and bottom temperatures in yellow¬ 
fin sole areas are negatively correlated (Nichol 3 ), and 
therefore perhaps both sampling efficiency and avail¬ 
ability are involved. Regardless of which mechanism is 
ultimately responsible for the correlation between yel¬ 
lowfin sole abundance and wave height, the inclusion 
of either variable in the survey catchability function 
used in the yellowfin sole assessment model of the Na¬ 
tional Marine Fisheries Service will have a significant 
effect on the estimates of total allowable catch for this 
species. Therefore further research to clarify this issue 
is critically needed. 
Acknowledgments 
We thank B. Lauth, D. Nichol, and W. Palsson for re¬ 
viewing the manuscript, D. Nichol for providing the 
EBS survey wave height and yellowfin catch data, and 
the captains and crew of the FV Vesteraalen and FV 
Cape Flattery for their help and expertise during the 
experiments. This research was funded, in part, by 
NOAA, National Marine Fisheries Service, Office of 
Sustainable Fisheries from their National Catch Share 
and Magnuson-Steven Act Implementation Funds. 
Literature cited 
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2016. Results of the 2013 eastern Bering Sea continental 
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Dickson, W. 
1993. Estimation of the capture efficiency of trawl gear. 
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Kimura, D. K., and D. A. Somerton. 
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