44 



Fishery Bulletin 104(1) 



10 20 30 



Crosscurrent (cm/s) 



40 



Figure 10 



Comparison of regression lines illustrating the relation- 

 ship between bridle symmetry and crosscurrent for locked 

 winches (L). symmetry (S), and tension (T) treatments. 

 Treatments in which crosscurrent had a statistically sig- 

 nificant effect on bridle symmetry are identified with an 

 asterisk (*). 



procedures to include towing with an autotrawl system 

 would likely be beneficial. 



Our results showed that both autotrawl systems re- 

 duced the mean distance of the bridles off-bottom and 

 the standard deviation of this distance and increased 

 the symmetry between bridles (Fig. 4) over the locked- 

 winches treatment. Our experiment also demonstrated 

 that both autotrawl systems increased the stability of 

 lower bridle bottom contact in changing environmental 

 conditions, but we were unable to discern which of the 

 systems was better. The data indicate that bridle bot- 

 tom-tending performance was the least affected by en- 

 vironmental conditions during the symmetry treatment; 

 however the standard deviation in the bridle distance 

 off-bottom was almost always lower during the tension 

 treatment, the exception being under extreme heave 

 conditions (Fig. 9). The locked-winches treatment had 

 the highest standard deviation and was affected the 

 most by heave, crabbing, and crosscurrent velocity. 



Autotrawl systems counteract the effects of warp 

 length differential created by trawl crabbing. During 

 our locked-winches treatment crabbing likely caused 

 unequal tension in the warps similar to that seen while 

 towing straight behind the boat with unequal warp 

 lengths. Weinberg and Somerton (2006) reported that 

 warp tension changes significantly with offset in warp 

 lengths. To compensate for the crabbing angle and to 

 assure that the trawl is pulled square to the direction 

 of the tow during crabbing, one warp should be shorter 

 than the other. Tension-controlled autotrawl systems ad- 

 just the length of the warps to equalize the tension on 

 them. Symmetry-controlled autotrawl systems change 

 the length of the warps to minimize crosscurrent and 

 also increase the symmetry of the trawl in relation to 

 the tow direction. 



In summary, autotrawl systems proved to be effective 

 in decreasing some of the adverse effects of environ- 

 mental factors on some aspects of the 83-112 bottom 

 trawl performance and, as a result, have the potential 

 to reduce variance in among-haul catchability of the 

 survey trawl. For this trawl, footrope and bridle dis- 

 tances off-bottom were significantly different among the 

 three towing modes, albeit the differences in actual dis- 

 tances off-bottom were small. Trawls deploying heavier 

 groundgear, thus enabling more constant contact with 

 the bottom, may not be as affected. Further investiga- 

 tions are needed to assess the effects of the two types of 

 autotrawl systems on other types of survey trawl gear, 

 such as high-opening bottom trawls, trawls using differ- 

 ent footropes, shrimp trawls, and midwater trawls. The 

 effect of using autotrawls in areas with stronger current 

 and different depths also needs to be investigated for 

 the different types of trawl gear. 



For bottom trawl surveys, we are concerned with 

 two potential shortcomings in the symmetry-controlled 

 autotrawl system tested. First, videos of trawling in 

 rough terrain have revealed footrope distortion occur- 

 ring when the footrope or doors snag on the bottom 

 (Weinberg, unpublished data). Typically, the side op- 

 posite the snag is pulled forward while the snagged 

 side remains stationary or is pulled ahead at a slower 

 pace. This uneven pull on the net causes asymmetry in 

 the headrope shape, by skewing it from the general tow 

 direction (Weinberg and Somerton, 2006). Distortion 

 of the headrope introduces error in the current direc- 

 tion and velocity values obtained by the current sensor 

 mounted on the headrope from which warp length is 

 determined, and thus impacts trawl performance. Our 

 second concern involves the overall warp adjustment 

 period required for the trawl to equilibrate to current 



