Weinberg and Somerton: Variation in Irawl geometry due to unequal warp length 



23 



prior to the start of the experiment. Hauls were 

 made with the trawl codend open to eliminate any 

 catch effects on trawl geometry. 



Several measures of trawl geometry and per- 

 formance were taken during each treatment. The 

 distance between the doors (door spread), the wing 

 tips (wing spread), and the center of the headrope 

 to the sea floor were measured acoustically with 

 Scanmar sensors (Scanmar, Asgardstrand, Nor- 

 way) to 0.1 m at 4-s intervals. Water flow, both 

 perpendicular and tangential to the headrope, 

 was measured to 0.1 knot at 24-s intervals with a 

 Scanmar trawl speed sensor placed at the center 

 of the headrope. Vessel position was measured 

 with satellite navigation at 2-s intervals. Bridle 

 tension was measured in kilograms at 2-s inter- 

 vals using in-line tension recorders (Billings Ind. 

 TR-999, N. Falmouth, MA) attached behind the 

 door legs. Bottom current velocity in cm/s and 

 direction data were recorded at 10-s intervals. 



Footrope off-bottom distance was measured at 

 five positions simultaneously by placing bottom 

 contact sensors (BCS) at the center of the foo- 

 trope, at the corners located 3 m to either side 

 of the center, and on each wing 1 m behind the 

 wing tip (Fig. 1). These sensors are self-contained 

 units consisting of a tilt meter, which measured 

 angle to the nearest half degree at 0.5 s elapsed 

 time intervals, and a data logger housed in a wa- 

 tertight stainless steel container that fits inside 

 a steel sled (Somerton and Weinberg, 2001). One 

 side of this sled clips into a clamp on the footrope, 

 which allows that end of the sled to pivot freely 

 about the footrope while the other end drags along 

 the bottom (Fig. 2). In this way, changes in the 

 distance of the footrope from the bottom produced 

 changes in the recorded tilt angle. Conversion 

 from tilt angle to distance off-bottom was accom- 

 plished by applying a calibration function derived 

 for each BCS unit by fitting a quadratic function 

 to data from an experiment in which angles asso- 

 ciated with known distances from a hard surface 

 were measured. The BCS unit extended 44 cm 

 behind the footrope and weighed (BCS, sled, and 

 footrope clamp) 8.9 kg in seawater. The clamp ex- 

 tended beneath the footrope by 2 cm and, depend- 

 ing on the extent of penetration into the sediment, 

 could raise the footrope off the bottom (Fig. 2). 

 Because the degree of penetration is unknown, 

 no adjustments to our calibration functions were 

 made. 



Bridle off-bottom distance was measured at six posi- 

 tions simultaneously by placing BCS units on the lower 

 bridle at distances of 25, 40, and 50 m forward of the 

 wing tip on both sides of the trawl (Fig. 1). However, the 

 BCS units used on the bridles differed from those used 

 on the footrope. These units were mounted on a trian- 

 gular frame designed to hold the BCS perpendicular 

 to the bridle (Fig. 2; Somerton, 2003). The triangular 

 frame measured 49 cm in its longest dimension and was 



Figure 2 



Bottom contact sensors shown mounted to the footrope (upper) 

 and bridle (middle). The footrope shown in the "on-bottom" 

 position without lateral tension and on a hard surface is 

 elevated 2 cm by the footrope clamp (lower i. 



held in place by a cable stop that also extended beneath 

 the bridle by about 2 cm. The weight of a bridle BCS 

 unit and frame was 8.7 kg in seawater. 



Data analyses 



Three tilt angle measurements from each BCS unit were 

 averaged for each 1.5-s interval, converted to distance 

 off-bottom by applying the calibration function deter- 

 mined for that unit, and then the off-bottom distances 



