30 
Fishery Bulletin 11 6(1) 
Table 3 
Slope, intercept, and P-value for linear regressions of 
the mean distance off-bottom, in centimeters at various 
positions along the lower bridles and footrope, against 
vessel motion expressed as the standard deviation of 
vessel heave at the trawl block from an experiment 
conducted in the eastern Bering Sea during September 
2003. 
Position 
Intercept 
Slope 
P-value 
Center 
2.537 
0.0111 
0.197 
Corner 
2.664 
0.0106 
0.101 
Wing 
1.136 
0.0039 
0.223 
Bridle—25 m 
2.133 
0.0013 
0.665 
Bridle—40 m 
2.642 
0.0165 
0.017 
Bridle—50 m 
11.661 
-0.0063 
0.722 
12 -f 
0 10 20 30 40 50 
Distance from wingtip (m) 
Figure 5 
Predicted off-bottom distance at the 0-, 25-, 40-, 
and 50-m positions on the lower bridles by us¬ 
ing the parameters in Table 3. Predictions were 
made at 5 levels of vessel motion (standard de¬ 
viation of vessel heave [SDH] at the starboard 
trawl block: 0, 20, 40, 60, 80 cm) spanning the 
values observed during the experiment conduct¬ 
ed in August 2016 to examine the effect of wave 
height on herding of yellowfin sole (Limanda 
aspera). In the region between 25 and 40 m, 
the top line corresponds with an SDH of 80 cm, 
and the bottom- line corresponds with an SDH 
of 0. The horizontal line represents a hypotheti¬ 
cal reference level of off-bottom distance, 2.5 cm, 
above which the bridle no longer elicits herding 
of yellowfin sole. As SDH increases, the point 
along the bridle where herding ceases (i.e., the 
intersection point of the horizontal and diagonal 
lines) moves progressively toward the wings. 
o 
o 
2.0 - 
o 1.8 - 
2 
° 
o 
Q 
IS 1.6 - 
o 
1.4 - 
O O 
O 
0 ° 
O 
1.2 - 
/• 
o 
o 
o 
o 
1 1 1 1 1 1 
1.5 2.0 2.5 3.0 3.5 4.0 
Wave height (m) 
Figure 6 
Ratio of the standardized catch from the long bridle 
tow to that from the short bridle tow in the experi¬ 
ment conducted in August 2016 to examine the effect 
of wave height on herding of yellowfin sole (Limanda 
aspera), plotted against the sum of the estimated wave 
and swell heights in meters. The slope of the regression 
line is significant (P=0.038). 
For the 83-112 Eastern trawl, one important effect 
of the oscillations is that they result in an increase in 
the mean off-bottom distance at the bridle position that 
is most influential in determining the size of the bridle 
contact area. Prior studies of herding by the survey 
trawl conducted in calm conditions indicate that the 
off-bottom distance of the lower bridle increases for¬ 
ward of the wing tips and at ~31 m reaches an off- 
bottom distance sufficiently large that the bridle no 
longer elicits herding behavior of flatfish (Somerton 
and Munro, 2001). Our study on the change in mean 
off-bottom distance with SDH along the lower bridles 
indicates that at 25 m the downward forces are suf¬ 
ficient to inhibit a change in off-bottom distance with 
increasing SDH; however at 40 m the change is the 
highest of all measured positions. As a consequence, 
the length of the bridle in contact with the bottom, and 
the size of the area exposed to herding stimuli, will 
decrease with increasing SDH (Fig. 5). 
Another likely effect of SDH is on escapement under 
the footrope. Studies show that the central portion of 
the footrope is the aggregation point of herded flatfish 
and where most escapement occurs (Main and Sang- 
ster, 1981). In addition, escapement of flatfishes un¬ 
der the footrope of the survey trawl has been shown 
to increase as the off-bottom distance increases at the 
center of the footrope (Weinberg et al., 2002). Conse¬ 
quently, any factor that increases footrope off-bottom 
distance should increase escapement, and although not 
