Somerton et al.: The effects of wave-induced vessel motion on the herding of Limanda aspera 
29 
Net Birdies 
center 25 m 
SD (heave, cm) SD (heave, cm) 
Figure 4 
Mean off-bottom distance, in centimeters, plotted against the standard deviation of vessel heave, 
in centimeters, at the starboard trawl block for each position along the lower bridles and footrope, 
from the footrope center to the 50-m bridle position, from an experiment conducted in the eastern 
Bering Sea during September 2003. The line on each plot is the regression of the mean off-bottom 
distance on the standard deviation of heave. Slope of the line indicates the relative sensitivity to 
vessel motion of the footrope and bridles at each position. 
such as vessel size and trawling procedures. As in pre¬ 
vious studies of the influence of sea-state on bottom 
trawl surveys (Stewart et ah, 2010; Politis et al., 2012), 
we emphasize the need to measure vessel motion di¬ 
rectly on all trawl survey vessels. 
The coherence between the vertical oscillations of 
the vessel and the vertical oscillations at all measured 
positions along the bridles and footrope of the trawl 
indicate that, at least in high waves, vessel motion is 
transmitted down the warps and throughout the trawl. 
O’Neill et al. (2003) and Politis et al. (2012) used simi¬ 
lar methods to ours and obtained similar results. For 
bottom trawl surveys, however, the important question 
is how these oscillations influence sampling efficiency. 
