Kotwicki et al.: Effect of autotrawl systems on tfie performance of a survey trawl 



37 



Prior to experimental towing, trawl warps were 

 measured and marked at 366 m, the amount of 

 warp used on the EBS survey when stations are 

 fished at a depth of 115 m, the depth of our study 

 site. Warps were measured and marked in ac- 

 cordance with AFSC protocol (Stauffer, 2004) 

 by using in-line wire counters (Olympic 750-N, 

 Vashon, WA) while at the same time, calibration 

 of the geometric winch counters associated with 

 the autotrawl system was performed. 



A haul consisted of towing at a vessel speed 

 of 3 knots while steering a steady course over 

 ground. Tow direction was selected by attempt- 

 ing to expose the trawl to the maximum amount 

 of side current. Vessel speed and position were 

 measured at 2-sec intervals with satellite navi- 

 gation. Each haul consisted of two treatment 

 sets in which three 15-min towing treatments 

 (locked winches [locked], tension-controlled au- 

 totrawl [tension], symmetry-controlled autotrawl 

 [symmetry]) were conducted, allowing at least 

 two minutes between treatments for the net to 

 equilibrate. Randomizing the order of treatments 

 within each treatment set reduced the influence 

 of treatment order on trawl performance intro- 

 duced by sea state, wind, and tidal currents. 

 Likewise, towing at the same site for the dura- 

 tion of the experiment eliminated bias that could 

 be attributed to varying substrate. 



Wing spread, door spread, and headrope height 

 were measured acoustically with Scanmar sensors 

 at 4-sec intervals to the nearest 0.1 m. Footrope 

 distance from the sea floor (cm) was measured 

 at 0.5-sec intervals and averaged over 1.5-sec 

 periods at five positions along the footrope si- 

 multaneously by placing bottom contact sensors 

 (BCS) at the center, at both trawl corners (located 

 3 m to either side of the center), and on each wing 

 1 m aft of the wing tips (Fig. 1). These sensors 

 are self-contained units consisting of a tilt meter 

 capable of measuring angle to the nearest 0.5° 

 and a data logger housed in a watertight stainless steel 

 container that fits inside a steel sled (Somerton and 

 Weinberg, 2001). One side of the sled clips into a clamp 

 that pivots freely on the trawl footrope and the other end 

 drags along the bottom (Fig. 2). Changes in the distance 

 of the footrope from the bottom produce changes in the 

 recorded tilt angle. Conversion from tilt angle to distance 

 off-bottom was accomplished with a calibration function 

 determined for each BCS unit by fitting a quadratic 

 function to data derived from a separate calibration 

 experiment in which tilt angles were recorded when the 

 footrope clamp was elevated set distances from a hard 

 surface. The BCS unit extended out from the footrope 44 

 cm and its combined weight (consisting of BCS, sled, and 

 footrope clamp) was 8.9 kg in seawater. The thickness 

 of the clamp beneath the footrope was 2 cm. Because 

 underwater video equipment was unavailable for this 

 experiment, the extent to which this clamp penetrates 

 into variable substrates was not estimated. 







Figure 2 



Bottom contact sensors mounted to the footrope (top) and the 

 bridle (bottom). 



Bridle distance from the bottom was measured at 

 three positions simultaneously on both port and star- 

 board sides by placing BCS units 25, 40, and 50 m 

 forward of each wing tip. The BCS units and sled were 

 mounted on triangular frames designed to hold them 

 perpendicular to the bridle (Fig. 2, Somerton, 2003). 

 The triangular frame measured 49 cm in its longest 

 dimension. The combined weight of a BCS unit and 

 frame was 8.7 kg in seawater. 



In addition to trawl mensuration, data were also 

 collected on certain environmental variables during the 

 different towing modes. Three variables were studied; 1) 

 vessel heave measured at the trawl block; 2 ) the relative 

 degree of offset of the warps from the heading of the ves- 

 sel (crabbing); and 3) bottom current velocity both paral- 

 lel and perpendicular to the direction of the vessel. 



The effect of sea state on the vertical displacement 

 and attitude of the vessel is transmitted to the footrope 

 from the trawl blocks through the trawl warps and 



