36 



Fishery Bulletin 104(1) 



signals from a sensor mounted on the trawl headrope. 

 This system operates on the principle that net skewing 

 can be caused by a crosscurrent. If the net is pulled 

 square to the direction of flow then, its geometry will be 

 symmetrical and trawl performance will be optimized. 

 In the late summer of 2003, the AFSC conducted an 

 experiment to examine the effect of these two types of 

 autotrawl systems on the geometry of a survey trawl, 

 comparing them to towing with equal amounts of warp 

 on each side with the winches locked. The study consid- 

 ers three aspects of trawl performance: 1) the factors of 

 trawl geometry influencing the area and volume swept 

 by the trawl (door spread, wing spread, and headrope 

 height); 2) the bottom-tending performance of the foo- 

 trope; and 3) the bottom-tending performance of the 

 lower bridles. 



Materials and methods 



Operations and instrumentation 



The experiment was conducted during 19-25 September 

 2003 aboard the chartered 38-m-long commercial stern 

 trawler FV Vesteraalen on smooth, relatively level bottom 

 in 115 m of water at a site approximately 70 km north 

 of Unimak Pass in the Bering Sea (55°10'N, 166°15'W). 

 The Vesteraalen is powered by a single 1725-hp engine 

 and is equipped with split Rapp Hydema (Rapp Hydema 

 AS, Bod0, Norway) trawl winches carrying 2.5 cm (1") 

 diameter, compacted, solid-core trawl warp. The winches 

 are controlled by a Scantrol 2000 (Scantrol, Bergen, 

 Norway) winch control system capable of quickly switch- 

 ing to different towing modes as requested by the vessel 

 operator. For this experiment towing was performed 

 with the codend open and with three different winch 

 control modes; locked winches with equal amounts of 

 warp on the port and starboard side (locked); a tension- 

 controlled autotrawl, which maintains equal tension on 

 both warps by adjusting warp length based on the drag 

 forces on each side (tension); and a symmetry-controlled 

 autotrawl, which adjusts warp length according to side 

 current forces in order to "optimize" water flow through 

 the net (symmetry). The symmetry-controlled system 

 requires a real-time speed sensor capable of detecting 

 the direction of water flow across the headrope. We used 

 an acoustically linked Scanmar (Scanmar, Asgardstrand, 

 Norway) trawlspeed sensor that transmits flow data at 

 24-sec intervals both perpendicular and tangential to 

 the headrope at its center. For this experiment and 

 when in symmetry mode, the Scantrol system adjusted 

 warp length at 30-sec intervals in response to changes 

 in tangential velocity. 



The experiment was conducted with the AFSC stan- 

 dardized trawl for the BBS shelf survey, the 83-112 

 Eastern bottom trawl. The 83-112 Eastern is a low- 

 rise, 2-seam flatfish trawl designed to fish on smooth, 

 soft bottom. The nylon net is constructed of 10.1-cm 

 stretch mesh in the wing and body, 8.9-cm mesh in 

 the intermediate, and double 8.9-cm mesh lined with 



Figure 1 



Schematic diagram of the 83-112 Eastern bottom 

 trawl and rigging shown from above. Mean door 

 spread (68.0 m) and mean wing spread (17.8 ml 

 were calculated from e.xperimental tows made at 

 a depth of 115 m with the winches locked and 366 

 m of trawl warp out on each side. Bottom contact 

 sensor units, shown as oversized triangles along 

 the bridle and footrope, are labeled by position, 

 as discussed in the text. 



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, 1.6-cm long-link chain door legs, a 12.2-m- 

 long, 1.9-cm diameter stranded-wire door leg exten- 

 sion, and a pair of 55-m-long, 1.6-cm diameter bare 

 stranded-wire bridles on each side (Fig. 1). The 25.5- 

 m-long (83 feet) headrope has forty-one evenly spaced, 

 20.3-cm diameter floats providing 116.4 kg of total lift. 

 The 34.1-m-long (112 feet), 5.2-cm diameter footrope is 

 constructed of 1.6-cm diameter stranded-wire rope that 

 is protected with 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) at- 

 tached at every tenth link, forming 168 loops to which 

 the netting is hung. An additional 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. 



