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Fishery Bulletin 104(1) 



30 

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30 

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3 m offset 



5 m offset 



-15 -10 -5 5 10 15 -15 -10 -5 5 10 15 



7 m offset 



9 m offset 



-15 -10 -5 5 10 15 -15 -10 -5 5 10 15 



1 1 m offset 



-15 -10 -5 5 10 15 -15 -10 -5 5 10 15 



Meters 



Figure 10 



Estimated net shape (curve i and position of headrope center (O) 

 at varying levels of warp offset. The mean distance between wing 

 tips that was acoustically measured during the experimental tows 

 is indicated with a dashed line. The ma.ximum lateral dimensions 

 of the net are indicated with solid vertical lines. The distance 

 between these lines is the effective net width needed for estimat- 

 ing the area swept by the net. 



5 7 9 



Offset (m) 



Figure 11 



The mean distance between wing tips mea- 

 sured acoustically lOl during the experimental 

 tows (net width) and the calculated effective 

 net width (-t-) are shown plotted against the 

 offset increment in meters. Note that there 

 is little difference between the two measures 

 of net width until the offset increment is 

 increased to 9 m. 



curate measurement of swept area or because 

 it leads to a change in catch efficiency. 



Relative abundance indices produced for the 

 eastern Bering Sea shelf survey are based on 

 catch per area swept between the trawl wings. 

 As the net distorts on account of differential 

 warp length, the effective net width will become 

 increasing less than the width that is acousti- 

 cally measured during the survey. Thus, net 

 width will become increasingly overestimated 

 and relative abundance of fish species therefore 

 will be underestimated. At 7-m offset, however, 

 the measured net width differed from the ef- 

 fective net width by only 0.5%, and therefore 

 this source of error is unlikely to contribute to 

 bias in the swept area estimates. However, the 

 difference between measured and effective net 

 width increases rapidly at greater offsets and 

 could present a problem if a less restrictive 

 threshold value of offset were used. 



Catch efficiency of the 83-112 Eastern trawl 

 depends primarily on 1) herding by the bridles, 

 doors, and the mud clouds they create; 2) the 

 escapement under the footrope; and 3) the es- 

 capement through the mesh in the body of the 

 net. The relative importance of these three 

 processes, however, will vary for the major spe- 

 cies groups that are targeted in the surveys. 

 Gadoids (primarily walleye pollock [Theragra 

 chalcogramma] and Pacific cod [Gadus macro- 

 cephalus] appear to have little or no herding 

 response to the 83-112 Eastern trawl (Somer- 

 ton, 2004) and rarely pass under the footrope 

 (Somerton, unpubl. data). However, both Pa- 



