26 



Fishery Bulletin 104(1) 



center increased (Fig. 5). We interpret this as 

 indication that the speed sensor at the headrope 

 was rotated in relation to the direction of travel 

 as the warp offset was increased. Calculating the 

 angle between the perpendicular at the center 

 of the headrope and the direction of travel as 

 tan"'(U/V), the angle increased from 11° at 3 m 

 offset to 61° at 20 m offset. Although the absolute 

 value of the tangential velocity was measured at 

 values >0 at zero offset, the tangential velocity 

 was not significantly different than zero it-tesi. 

 P=0.71). This result indicates that the alignment 

 of the experimental tows in relation to the pre- 

 vailing current was sufficient to reduce the cross 

 current to negligible levels. 



Net and door measurements 



At zero offset, the mean door spread obtained 

 was 61.9 m. The mean wing spread was 17.1 m, 

 and the mean headrope height was 2.0 m. Differ- 

 ences in the means of all three quantities were 

 not apparent at lower offsets, however headrope 

 height and wing spread were more sensitive to 

 changes in large offsets because both were signifi- 

 cantly (P<0.05l greater than the zero offset means 

 when offset was increased to 11 m, whereas door 

 spread did not differ significantly until the offset 

 was approximately 14 m (Fig. 6). 



Bridle and footrope distance off-bottom 

 by position 



Bridle and footrope off-bottom distance varied consider- 

 ably with position, not only with respect to the mean 

 value but also with respect to the sensitivity of the mean 

 to changes in offset. At zero offset, the mean off-bottom 

 distance of the bridle declined from 12.3 cm at 50 m from 

 the wing tips, to 3.2 cm at 40 m and 2.0 cm at 25 m (Fig. 

 7). Mean off-bottom distance remained small along the 

 footrope, varying from 1.7 cm at 1 m behind the wing tip 

 to 2.5 cm at the corner and 1.9 cm at the center. 



The mean response to changes in offset varied greatly 

 by position. Along the bridles, the most sensitive location 

 was at 50 m, where off-bottom distance increased on the 

 short side and decreased on the long side with increasing 

 offset (Fig. 7). At 40 m. a similar pattern was repeated, 

 but for most offsets on the long side, the off-bottom dis- 

 tance was near the minimum recorded, indicating that 

 the bridle was resting on the bottom. At 25 m, the bridle 

 was nearly always in contact with the bottom and off- 

 bottom distance was insensitive to variations in warp 

 offset. Along the footrope, the most sensitive position was 

 the corner where off-bottom distance increased greatly 

 with offset, particularly with positive offsets due to the 

 relaxation in warp tension. At the center of the footrope, 

 off-bottom distance was also sensitive to warp offset, re- 

 sponding almost identically on the long and short sides. 

 At 1 m behind the wing tip, sensitivity to warp offset 

 was quite low and the off-bottom distance indicated that 



Figure 5 



Current velocity (in m/sec) measured at the center of the hea- 

 drope for each tow is shown separated into the component per- 

 pendicular to the headrope (O) and the component tangential to 

 the headrope ( + ). The solid and dashed lines connect the means 

 at each offset increment. Note, for clarity, that the offsets are 

 incremented by plus or minus 0.1 m for the tangential and 

 perpendicular components. 



the footrope was in contact with the bottom except for 

 large offsets on the long side. 



An alternate method of assessing the sensitivity of 

 geometry of the 83-112 Eastern trawl to changes in 

 offset is to determine if the mean off-bottom distance 

 at 7 m, the maximum offset allowed under the NOAA 

 protocols for the 83-112 Eastern trawl, differs statisti- 

 cally from the mean off-bottom distance at zero offset. 

 Based on the bootstrapped confidence intervals (Fig. 7), 

 off-bottom distance is significantly different from what 

 it is at zero offset at the 50-m and 40-m bridle positions 

 and at the center and corner footrope positions but is 

 not significantly different at the wing and the 25-m 

 bridle position. 



Bridle shape and herding area 



To understand better how the change in tension that 

 accompanies offsets in warp leads to changes in bridle 

 shape, we show the mean off-bottom distances plotted 

 against the BCS positions on the wing and bridles for 

 both the short side and long side of the trawl. From this 

 perspective it is clear that as the tension is increased, 

 off-bottom distance increases on the forward part of the 

 bridle. Likewise, as the tension is reduced, the off-bottom 

 distance decreases (Fig. 8). For flatfish, the effect of 

 these changes in off-bottom distance is a change in the 

 area subjected to herding stimuli. For the case where 

 the reaction height is 1 cm, the bridle contact length 

 is determined by the intersection of the line depicting 



