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



39 



standard deviations of the wing spread, door spread, 

 and net height). No significant differences were detected 

 among the three towing modes (Table 1); consequently, 

 no environmental variables were tested for their influ- 

 ence on trawl geometry. 



Footrope distance off-bottom Analyses based on 33 suc- 

 cessful treatment sets produced varying results among 

 the three measures describing the bottom tending perfor- 

 mance of the footrope. Mean footrope distance off-bottom 

 differed significantly among the three towing modes 

 (Table 2, Fig. 3). Footrope distance was lowest for the 

 tension treatment followed by the symmetry treatment 

 and locked winches. The greatest observed difference 

 occurred between the locked (13.98 cm) and the tension 

 treatment (12.08 cm). Standard deviation in footrope 

 distance off-bottom also differed significantly among 

 treatments. Differences were similar to those observed 

 for the mean, in that the lowest SD was observed for 

 the tension treatment (5.24 cm) and the highest SD was 

 observed for the locked treatment (6.39 cm). Because the 

 variance equals the square of the standard deviation, 

 this seemingly small reduction in standard deviation 

 corresponds to a fairly large reduction in the variance 

 (-30%). Symmetry in the footrope off-bottom distance 

 did not differ significantly (P=0.0554) among the three 

 towing modes. 



Bridle distance off-bottom A total of 34 successful 

 treatment sets were used to analyze the bridle distance 

 off-bottom data. Mean bridle distance off-bottom differed 

 significantly among treatments (Table 2, Fig. 4); it was 

 lowest for the tension treatment (6.28 cm) and highest 

 with the winches locked (7.50 cm). Standard deviation 

 of bridle distance off-bottom also differed significantly 

 among the three towing modes. SD values were signifi- 

 cantly lower in the autotrawl towing modes, being lowest 

 in the tension treatment (3.37 cm) and highest in the 

 locked treatment (4.91 cm). Bridle distance off-bottom 

 was most symmetrical in the symmetry and tension 

 towing modes. Symmetry and tension means were simi- 

 lar (0.93 cm and 0.88 cm, respectively) and significantly 

 lower than that observed for the locked-winches treat- 

 ment (1.59 cm). 



Assessment of the effect of environmental factors 



Footrope distance off-bottom The effect of environmen- 

 tal factors on our three measures describing the bottom 

 tending performance of the footrope produced varying 

 results. Mean distance off-bottom was significantly 

 affected by heave only, during all treatments (Table 3, 

 Fig. 5). The standard deviation of the footrope distance 

 off-bottom was also significantly affected by heave, crab- 

 bing, and bottom current parallel to the direction of the 



