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



279 



Auxiliary net 



Figure 1 



A diagram of the Poly Nor'eastern trawl net and the auxiliary net that was attached beneath the 

 trawl net to capture fish escaping under the trawl footrope. Details on the construction of these 

 nets can be found in the Appendix. 



lengths of 13-mm long-link chain are joined to a single 

 19-mni diameter steel cable, known as the tailchain 

 extension, that is varied in length to suit the needs of 

 each vessel. Tailchain length is the combined length 

 of the chain and extensions. Tailchain extensions are 

 connected to the leading edge of each wing with three 

 bridles measuring 54.9 m in length and constructed of 

 16-mm steel cable. The net mesh in the lower section 

 of each wing ends 6.1 m behind the end of the footrope 

 (Fig. 1; Appendix); the footrope in this section will be 

 referred to as the wing extension. 



Trawl efficiency model for flatfish 



For flatfish, which are unlikely to escape through the 

 small-mesh, codend liner, or over the headrope, the 

 catch of a trawl (A'^) can be expressed as the sum of the 

 catches offish originating from the net and bridle paths 

 (Dickson, 1993a; Somerton and Munro, 2001)i: 



h (herding coefficient) = the proportion of fish within 



the bridle contact path that 

 are herded into the net path. 



Because h is relative to W^^, which will vary with trawl 

 design, for comparative purposes a more convenient 

 measure of herding is the bridle efficiency (kf^) or the 

 proportion of fish that are herded from the entire bridle 

 path (Dickson, 1993a). Bridle efficiency can be calculated 

 from the herding coefficient by using 



^W„, 



** = w.-w 



(2) 



N = kDLW+khDLW„. 



(1) 



where W^ = the width of the door path. 



k)^ = the average bridle efficiency in the area 

 swept by the entire bridle (i.e., wing tip to 

 door); and 

 h = the bridle efficiency only in the area actually 

 contacted by the lower bridle. 



where D = fish density; 



L = tow length; 

 W„ = the width of the net path; 

 W^^ = the width of the bridle path 

 contacted by the bridle; 

 /e„ (net efficiency) = the proportion of fish within 

 net path that are captured; 

 and 



' In Somerton and Munro (20011 the parameter W^,^ in Equa- 

 tion 1 was instead represented by the equivalent, but more 

 complex, expression: Wj- W„ 



of the bridle path not contacted by the bridle 



W^^^ where W^^-f is the width 



Trawl efficiency (£) can then be derived from Equa- 

 tion 1 by dividing by the total number of fish within the 

 door path (i.e., DLW^). 



E 





(3) 



Of the five quantities needed to evaluate this expres- 

 sion, two (W„, and W^) are routinely measured on bot- 

 tom trawl surveys, but three (^„, h, and W^„) require 

 separate field experiments for their estimation. In this 

 study, /2„ was estimated with the data obtained from a 

 net efficiency experiment which consisted of attaching 



