35 



Abstract — Three aspects of a survey 

 bdttdin trawl performance — 1) trawl 

 geometry (i.e., net spread, door spread, 

 and headrope height); 2) footrope dis- 

 tance off-bottom; and 3) bridle dis- 

 tance off-bottom — were compared 

 among hauls by using either of two 

 autotrawl systems (equal tension and 

 net symmetry) and hauls conducted 

 with towing cables of equal length 

 and locked winches. The effects of 

 environmental conditions, vessel 

 heave, crabbing (i.e., the difference 

 between vessel heading and actual 

 vessel course over ground), and 

 bottom current on trawl performance 

 with three trawling modes were 

 investigated. Means and standard 

 deviations of trawl geometry mea- 

 sures were not significantly different 

 between autotrawl and locked-winch 

 systems. Bottom trawls performed 

 better with either autotrawl system 

 as compared to trawling with locked 

 winches by reducing the variance 

 and increasing the symmetry of the 

 footrope contact with the bottom. 

 The equal tension autotrawl system 

 was most effective in counteracting 

 effects of environmental conditions 

 on footrope bottom contact. Footrope 

 bottom contact was most influenced 

 by environmental conditions during 

 tows with locked winches. Both of 

 the autotrawl systems also reduced 

 the variance and increased the 

 symmetry of bridle bottom contact. 

 Autotrawl systems proved to be 

 effective in decreasing the effects of 

 environmental factors on some aspects 

 of trawl performance and, as a result. 

 have the potential to reduce among- 

 haul variance in catchability of survey 

 trawls. Therefore, by incorporating 

 an autotrawl system into standard 

 survey procedures, precision of survey 

 estimates of relative abundance may 

 be improved. 



The effect of autotrawl systems on 

 the performance of a survey trawl 



Stan Kotwicki* 



Kenneth L. Weinberg* 



David A. Somerton 



National Marine Fisheries Service 



Alaska Fisheries Science Center, 



7600 Sand Point Way N.E. 



Seattle, WA 98115 



E-mail address (for K L Weinberg, contact author) ken weinbergig'noaa gov 



'Equal authorship 



Manuscript submitted 27 October 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 6 June 2005 by the Scientific Editor. 



Fish. Bull. 104:35-45 12006). 



Bottom trawl survey operating pro- 

 cedures are standardized in order to 

 reduce the variability of catch per unit 

 of effort (CPUE) estimates. Many of 

 the current standardization proce- 

 dures address the efficiency of the 

 trawl gear and the maintenance of 

 constant catchability among samples 

 and over time. Despite these efforts, 

 variability in trawl catchability can 

 be exacerbated by uncontrollable envi- 

 ronmental conditions. Variables such 

 as surface and bottom currents, sea 

 state, wind direction, varying sub- 

 strate types and inclinations, and 

 depth of tow may all contribute to dif- 

 ferences in gear efficiency by influenc- 

 ing the area swept by the net (Rose 

 and Nunnallee, 1998), the herding 

 efficiency of the bridles (Somerton 

 and Munro, 2001; Somerton, 2003), 

 and escapement beneath the footrope 

 (Weinberg et al., 2002). 



Many bottom trawl surveys con- 

 ducted by the National Marine Fish- 

 eries Service, such as the Alaska 

 Fisheries Science Center's (AFSC) 

 eastern Bering Sea (EBS) shelf sur- 

 vey, operate with trawl winch brakes, 

 set or locked, and tows are made with 

 equal amounts of towing cable (warp) 

 on both sides of the vessel. Other 

 than by controlling towing speed and 

 direction, these surveys are unable 

 to compensate for changing environ- 

 mental conditions. In contrast, auto- 

 trawl systems are widely used by the 

 commercial fleet and are purported 

 to improve fishing performance by 

 stabilizing trawl geometry over vary- 

 ing environmental conditions, such 



as rough weather when vessel heave 

 produces an upward lift on the trawl 

 door resulting in loss of ground shear 

 and wing spread, or over rough bot- 

 tom when doors and nets have a 

 greater probability of snagging. If 

 autotrawl systems are able to reduce 

 some of the variability in gear effi- 

 ciency that is due to environmental 

 variability, such as sea state and cur- 

 rents, then including the use of auto- 

 trawl systems as a standard survey 

 bottom trawl procedure may improve 

 the precision of survey results. 



In simple terms, autotrawls are dy- 

 namic systems that operate on the 

 principle of ensuring that the trawl 

 is being towed in a direction perpen- 

 dicular to the center of the footrope 

 and headrope in order to optimize its 

 performance. We are aware of two 

 styles of autotrawl systems current- 

 ly marketed. The first is a tension- 

 controlled system that reacts to the 

 difference in warp tension between 

 winches by equalizing hydraulic pres- 

 sure (equal tension). When the ten- 

 sion on either side exceeds that of the 

 other side (a user-defined threshold) 

 due to factors such as increased drag, 

 currents, sediments, or steep slopes, 

 the system lengthens that warp to 

 equalize the pressure between the 

 two winches. Conversely, when the 

 tension decreases on one warp, the 

 system compensates by shortening 

 that warp to equalize pressure be- 

 tween the two winches. The second 

 autotrawl style is a symmetry-con- 

 trolled system that actively adjusts 

 warp length in response to cross flow 



