22 



Fishery Bulletin 104(1) 



door legs 

 K extension 



■l52m ~T 



bridle 

 * 55 rn* 





Figure 1 



Schematic diagram of the 83-112 Eastern survey trawl and rigging shown 

 from the side (upper panel) and from above (lower panel). The experimentally 

 determined mean door spread and wing spread dimensions shown apply when 

 towing at a depth of 82 m with 274 m of trawl warp on each side. Bottom 

 contact sensor units, shown as oversized triangles along the bridles and 

 footrope, are labeled by position as discussed in the text. 



The 83-112 Eastern is a low-rise, 2-seam, flatfish 

 trawl designed for use on smooth, soft bottom. The 

 nylon net is constructed of 10.1-cm stretch mesh in 

 the wing and body, 8.9 cm in the intermediate, 

 and double 8.9-cm mesh lined with a 3.1-cm mesh 

 in the codend. It is towed behind a pair of 1.8x2.7 

 m steel "V" doors, weighing approximately 816 kg 

 apiece, which are attached to the net by two 3-m- 

 long door legs (consisting of 1.6-cm long-link chain); 

 a 12.2-m-long door leg extension (consisting of 1.9-cm 

 diameter stranded wire); and a pair of 55-m-long, 

 bridles (consisting of 1.6-cm diameter bare stranded 

 wire) on each side of the net (Fig. 1). The 25.5-m-long 

 (83 ft) headrope has 41 evenly spaced, 20.3-cm diameter 

 floats that provide 116.4 kg of total lift. The 34.1-m-long 

 (112 ft), 5.2-cm diameter footrope is constructed of 1.6-cm 

 diameter stranded-wire rope protected by a single wrap 

 of both 1.3-cm diameter polypropylene line and split 

 rubber hose. The footrope is weighted with 51.8 m of 

 chain (0.8-cm proof-coil) attached at every tenth link, 

 forming 168 loops to which the netting is hung. An ad- 

 ditional 0.6-m-long, 1.3-cm long-link chain extension 

 connects each lower bridle to the trawl wing tips to 

 help keep the footrope close to the bottom. Because the 

 wire length between otter doors measured along the 

 bridles and footrope is 175.6 m, the critical value for 

 differential warp length for this trawl established by 

 the NOAA 4% rule is 7 m. 



Prior to the experiment, the warp length to be used 

 was measured to the nearest 0.1 m with a calibrated, 

 in-line wire counter (Olympic 750-N, Vashon, WA). A 



zero reference point was painted on each trawl warp, 

 as determined by first setting the wire counter to zero 

 with the trawl door just beneath the water surface and 

 then measuring out 274 m (150 fml, the survey stan- 

 dard warp length for a fishing depth of 82 m. To verify 

 this benchmark measurement, the process was repeated 

 and all replicate measurements were found to be within 

 0.3 m, which is less than the 1 m specified by the NO- 

 AA trawl survey protocols for replicate measurements. 

 Subsequent reference marks, measured by tape, were 

 then placed along each warp at 3, 5, 7, 9, 11, and 20 m 

 from the zero mark. 



The experiment consisted of examining the effect of 

 seven differences in warp length, henceforth referred to 

 as "offsets," where one warp was positioned at values 

 of 0, 3, 5, 7, 9, 11, or 20 m longer. An experimental set 

 consisted of all seven offsets, chosen in random order. A 

 treatment consisted of towing the trawl, with the winch 

 brakes locked, at the specified offset for 5 minutes at 

 3 knots while maintaining a constant vessel heading. 

 Treatments were preceded by a 2-min equalization pe- 

 riod at the specified offset. A haul consisted of two 

 treatment sets: one with a port offset and the other 

 with a starboard offset — the order having been chosen 

 randomly for each haul. To minimize the effect of bot- 

 tom currents on trawl symmetry, hauls were made in 

 pairs along the axis of the dominant current direction, 

 either with or against the current — again, the order 

 having been chosen randomly. This direction was deter- 

 mined by deploying a current meter (Nobska MAVS-3, 

 Woods Hole, MA) 3 m above the bottom for one day 



