Weinberg and Somerton Variation in trawl geometry due to unequal warp length 



29 



10 



20 



30 40 



10 20 30 40 



Distance from wing tip (m) 



50 



50 



Figure 8 



Mean off-bottom distance is shown plotted against the 

 distance measured from the wing tip to the positions of 

 the wing and the three bridle bottom contact sensors. 

 This approximation to the shape of the bridle when 

 viewed laterally is shown for each of the offset incre- 

 ments for both the side with the longer warp and. the side 

 with the shorter warp. The dashed line represents the 

 hypothetical reaction height of a fish. The intersection 

 of the dashed line with the solid line for each configura- 

 tion defines the bridle length that is sufficiently close 

 to the bottom to elicit a herding response. 



Discussion 



There are two distinct approaches forjudging whether a 

 difference in warp length between the sides of a survey 

 trawl will lead to a significant bias in estimates of rela- 

 tive abundance. In both approaches we focused on the 

 adequacy of the maximum 7-m offset allowed for the 

 83-112 Eastern trawl under NOAA trawl survey pro- 

 tocols. In the first approach, we simply asked whether, 

 given the sampling effort used in the experiment, any 

 of the measured dimensions at 7-m offset were statis- 

 tically different from zero offset. In our experiment, 

 none of the three standard measures of trawl geometry 

 (i.e., door spread, wing spread, and headrope height) 



differed from mean values at zero offset. This finding 

 indicates that either these dimensions are fairly robust 

 to changes in warp offset or that the acoustic measure- 

 ment of these dimensions was insufficiently precise to 

 detect a difference. Off-bottom distance, however, was 

 significantly different at the two forward positions on 

 the bridles and along the footrope at the center and 

 corner positions. 



From the perspective of trawl survey standardiza- 

 tion, however, the detectability of changes in geometry 

 is not of primary importance; these changes, however, 

 may produce a significant effect on estimates of relative 

 abundance. Bias in these estimates could result either 

 because the change in trawl geometry leads to an inac- 



