Somerton et al,: Whole-gear efficiency of a bentfiic survey trawl for flatfisfi 



285 



have been reported (Bublitz, 1996), our video recordings 

 were insufficiently clear for us to distinguish species 

 and therefore we could not determine the mechanism 

 leading to the decline in capture probability. Regard- 

 less of the mechanism, a decline in net efficiency with 

 increasing size has been previously reported for another 

 flatfish (i.e., yellowtail flounder, Limanda ferruginea; 

 Walsh, 1992). 



Assumptions with the net efficiency experiment 



The auxiliary net did not extend across the full width 

 of the trawl net, but only across the center 78% of the 

 width where the mesh of the trawl net attaches to the 

 footrope. Our use of the estimated net efficiency in this 

 section as a proxy for that of the total net spread, which 

 is measured at the junction of the upper bridle and wing 

 tip, is based on the assumption that the average escape- 

 ment in the unsampled section is the same as that in the 

 sampled section. From video observations (K. Weinberg, 



unpubl. data), it is evident that flatfish encountering 

 these outer portions of the footrope tend to be herded 

 toward the center of the footrope rather than pass under 

 or over the footrope. Although we have no quantitative 

 data on the change in escapement rate across the width 

 of the trawl, we believe that our extrapolation of the 

 measured escapement rate into the unsampled portion 

 of the net spread potentially results in a slight under- 

 estimate of net efficiency. 



Herding 



One would expect herding to be a length-dependent 

 process because the swimming endurance of fish, and 

 therefore their ability to maintain position in front 

 of the bridle, increases with body size (Winger et al., 

 1999). Our evidence for length-dependent herding, 

 however, is equivocal. All four of the species had a 

 positive slope in the regression of ^^ on body length 

 (Table 2), but in only one case, arrowtooth flounder, was 



