Somerton et al.: The effects of wave-induced vessel motion on the herding of Limanda aspera 
27 
Net Birdies 
center 25 m 
The standard deviation of off-bottom distance, in centimeters, plotted against the standard 
deviation of vessel heave, in centimeters, at the starboard trawl block for each of 6 positions 
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 standard deviation of 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. 
relatively insensitive to changes in SDH; however at 
the 40-m bridle position the constraints on vertical 
motion are less and the mean off-bottom distance in¬ 
creases with wave height. At the 50-m bridle position, 
the vertical oscillations are apparently unconstrained 
by the bottom, so that the mean off-bottom distance is 
independent of SDH. 
One consequence of the progressive sensitivity of 
mean off-bottom distance to SDH with increasing dis¬ 
tance from the wing tips (at least out to 40 m) is that 
the length of the bridle in contact with the bottom 
must decrease with SDH. To show this, we evaluated 
equations predicting off-bottom distance (Table 3) at 
the wing, and at the 25 m, 40 m, and 50 m bridle posi- 
