32 



Fishery Bulletin 104(1) 



Chionoecetes species (Somerton and Otto, 19991 and red 

 king crab (Weinberg et al., 20041 do escape under the 

 footrope. Flatfishes (including yellowfin sole [Limanda 

 aspera], flathead sole [Hippoglossoides elassodon], and 

 rock sole [Lepidopsetta biliueata] display a strong herd- 

 ing response to the 83-112 Eastern trawl (Somerton 

 and Munro, 2001); as much as 49% of the catch con- 

 sisted of fish that were herded by the bridles into the 

 net path. Likewise, flatfishes are readily capable of 

 escaping under the footrope and, for species such as 

 yellowfin sole, at least 25% of the largest individuals 

 escape in this manner (Munro and Somerton, 2002). 

 Thus, the capture efficiency of the trawl is species- 

 specific. The change in catch efficiency due to warp 

 offset is likely minimal for species not captured as a 

 function of herding and footrope escapement behaviors; 

 however, because flatfish are susceptible to herding and 

 are adept at footrope escapement, their catch rate could 

 potentially be affected most by warp offsets. 



Bridle efficiency (i.e., the fraction of fish in the area 

 between the wing tips and doors that are herded into 

 the path of the net) for flatfish catch is strongly influ- 

 enced by the size of the herding area or area swept by 

 the bridles because flatfish are stimulated to herd by 

 the close approach or direct contact of the lower bridle. 

 Although we were able to measure the off-bottom dis- 

 tance along the bridle and thereby predict the shape of 

 the bridle, there is still considerable uncertainty as to 

 the exact size of the herding area because the reaction 

 height of a fish will vary with species, size, physiological 

 state, state of arousal to the approaching bridle, viewing 

 conditions for the fish, and, perhaps, other variables. 

 Additionally, there is uncertainty in the estimate of the 

 size of the herding area because it is based on the as- 

 sumption of symmetry in the bridle angle-of-attack — a 

 symmetry that is increasingly untenable with increasing 

 offset. Despite this uncertainty, it is likely that the loss 

 of herding area on one side of the trawl is not countered 

 by an increase on the other side; thus some overall loss 

 of herding efficiency is to be expected. In the hypotheti- 

 cal case chosen in our study, the reduction in herded 

 area was 10.3%, which when applied to a strong herding 

 flatfish such as rock sole (the herded component of the 

 catch has been estimated to be about 49%, Somerton 

 and Munro, 2001), the expected reduction in catch with 

 an 8-m offset would be roughly 5%. 



Flatfish escapement under the footrope will be influ- 

 enced not only by the increase in off-bottom distance 

 but also by the location along the footrope where the 

 increase occurs. At a 7-m offset, the footrope off-bottom 

 distance is about 2 cm higher in the footrope corner 

 on the long side of the trawl and approximately 1 cm 

 higher at the center and opposing corner than at zero 

 offset (Fig. 12). Footrope off-bottom distances increased 

 appreciably with greater offsets. Weinberg et al. (2002) 

 demonstrated that flatfish escapement can increase 

 with similar increases in footrope off-bottom distance; 

 however their study focused on a different trawl and 

 considered escapement for the entire footrope rather 

 than by position along the footrope. Although we are 



unaware of any studies that quantify escapement rate 

 by position along the footrope, our video observations 

 indicate that fiatfish are less likely to escape under the 

 footrope near the wings than in the center (Somerton, 

 unpubl. data). Because it is the center portion of the foo- 

 trope where most of the increases in off-bottom distance 

 occur in the 83-112 trawl, flatfish escapement is likely 

 increased and potentially could represent a significant, 

 but presently unquantifiable, loss in catch and a source 

 of bias in estimates of relative abundance. 



In conclusion, most aspects of the 83-112 Eastern 

 trawl geometry were significantly degraded by warp 

 offset differences equal to or greater than 7 m compared 

 to zero offset. More importantly, the locations where the 

 detectable differences occurred could affect catch effi- 

 ciency; therefore a NOAA threshold value of 4% should 

 be considered a maximum value for the 83-112 Eastern 

 trawl and perhaps a more conservative value (less than 

 4%) would be prudent. However, given today's standard- 

 ized survey procedures for measuring warp and for real- 

 time monitoring of warp offset, the probability of warp 

 offsets even approaching 7 m is highly unlikely on our 

 surveys when locked-winches are used. Likewise, we 

 argue that any appreciable differences in warp lengths 

 between sides due to stretching are unrealistic because 

 AFSC charter vessels use large diameter, compressed, 

 solid-core wire. In fact, a review of the 413 hauls made 

 during the 2004 EBS survey revealed that only three 

 tows had a recorded maximum 1-m length difference 

 between sides (Weinberg, unpubl. data). 



Acknowledgments 



We would like to thank Captain Brad Lougheed and the 

 FV Vesteraalen crew, Niles Griffen, Waldemar Janezak, 

 Van Ngo, and Todd Becker for their outstanding profes- 

 sionalism, positive attitudes, and constant attention to 

 detail; AFSC scientists Stan Kotwicki and Dennis Benja- 

 min for their assistance at sea; reviewers Guy Fleischer 

 and Henry Milliken for their helpful comments; and the 

 AFSC editorial staff, including Gary Duker, Jim Lee, 

 and Kama McKinney for their help during the in-house 

 review and manuscript preparation process. 



Literature cited 



Efron, B., and R. Tibshirani. 



1993. An introduction to the bootstrap, 436 p. Chapman 

 and Hall, New York, NY. 

 Fridman, A. L. 



1969. Theory and design of commercial fishing gear. 

 (Transl. from Russian by Israel Program Sci. Transl., 

 Jerusalem) 1973, 489 p. [Available as TT 71-50129 

 from Natl. Tech. Inf. Serv., Springfield, VA.] 

 Munro, P. T, and D. A. Somerton. 



2002. Estimating net efficiency of a survey trawl for 

 flatfishes. Fish. Res. 55:267-279. 

 Somerton, D. A. 



2004. Do Pacific cod (Gadus macrocephalus) and wall- 



