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



41 



Table 3 



Multiple regression model slope coefficients with P-value <0.05. for locked winches, symmetry-controlled, and tension-controlled 

 towing modes (y=bottom current parallel to the direction of the tow, AbsX=absolute value of the crosscurrent). 



Statistic 



AbsX 



ff2 



P-value 



Footrope 



Bndle 



footrope SD during the tension mode. The sym- 

 metry in footrope distance off-bottom was sig- 

 nificantly affected by the current parallel to the 

 direction of the tow during the locked-winches 

 treatment only (Table 3, Fig. 7). 



Bridle distance off-bottom Mean bridle distance 

 off-bottom was affected by heave only during 

 some of the treatments (Table 3, Fig. 8). Heave 

 had a greater impact on the mean bridle distance 

 off-bottom during the locked-winches treatment 

 (slope = 0.0680) than during the tension treat- 

 ment (slope = 0.0491). The standard deviation of 

 the bridle distance off-bottom was affected by 

 heave and crabbing during two of the treatments 

 (Table 3, Fig. 9). Heave had an effect on the SD of 

 the bridle distance off-bottom during both locked 

 (slope = 0.0796) and tension (slope = 0.0411) treat- 

 ments. A significant effect due to crabbing was 

 also detected during these same two treatments 

 (slopes 0.9536 and 0.3023, respectively). During 

 the symmetry mode no effect due to any of the envi- 

 ronmental conditions was detected, even though 

 SD was almost always higher than that observed in 

 the tension mode, with the exception of extreme heave 

 conditions. Symmetry in the bridle distance off-bottom 

 was affected only by crosscurrent (Table 3, Fig. 10). 

 Crosscurrent had an effect on the symmetry during 

 the locked {slope = 0.0556) and tension (slope=0.0361) 

 treatments. 







20 



40 



60 80 



100 



Heave (cm) 

 Figure 5 



Comparison of regression lines illustrating the relationship 

 between the mean footrope distance off-bottom and heave for 

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

 Treatments in which heave had a statistically significant effect 

 on the mean are identified with an asterisk (*). 



Discussion 



The objective of this study was to identify towing modes 

 that could potentially reduce variance in the catchability 

 of the 83-112 Eastern bottom trawl among hauls. Three 

 separate aspects of trawl performance affecting catch- 

 ability were investigated: trawl geometry, bottom tend- 



