Somerton and Munro: Bridle efficiency of a survey trawl for flatfish 
651 
frequently at longer bridle lengths and would thereby 
diminish herding efficiency. This assumption was tested by 
comparing the fit of a herding model with a bridle-length 
dependent h to that of an unmodified model. The modi- 
fied model provided a better fit for only one species, Pacific 
sanddab. Because this species is the smallest of the seven 
(Fig. 3), it is likely that its swimming endurance might 
have been exceeded at the longest bridle length. However, 
for the remaining species the assumption is valid. 
Second, we assumed that none of the fish in the area 
spanned by W o ^ are herded into the net path. Although we 
recognize that the doors and the mudclouds immediately 
behind the doors must herd some fish toward the net path, 
such fish would be herded into the section of the bridle un- 
obscured by the mudcloud and likely escape beneath the 
lower bridle. We attempted to confirm this possibility by 
examining video observations from this area of the bridle 
to see if flatfish escaped or showed any herding behavior, 
but the results were equivocal because few fish, and prob- 
ably only moving fish, were seen. 
Third, we assumed that in changing the length of the 
bridles we did not alter trawl geometry in any way other 
than by increasing the area exposed to herding. As in 
the studies of Engas and Godp (1989a) and Ramm and 
Xiao ( 1995), increased bridle length in our study produced 
a small, but significant, reduction in wing spread (Table 
1). If this change produced a change in footrope contact 
and net efficiency, then it would lead to a biased estimate 
of herding efficiency (net efficiency is explicitly assumed 
to be independent of bridle length in Eq. 4). Other pos- 
sible changes in trawl geometry are that 1) at the longest 
length the upper bridle may sag sufficiently to touch the 
bottom within L a ^ and 2) at the shortest length the bridle 
tension may be sufficient to lift the wingtip off the bot- 
tom. With such changes, and perhaps even without, L o ^ 
may not be constant at all bridle lengths. Although we ex- 
amined the performance of the trawl along the entire dis- 
tance from the wing tips to the doors at the standard bri- 
dle length, we had insufficient ship-time to examine the 
performance at all bridle lengths to verify that the bridles 
performed as intended. We recommend that further stud- 
ies of herding efficiency include additional research to ver- 
ify that standard trawl performance is maintained for all 
experimental configurations. One approach for doing this 
is to simultaneously estimate escapement under the foot- 
rope, perhaps by using an auxiliary net attached beneath 
the trawl net as in Engas and Godp (1989b), so that the 
assumed independence of net efficiency and bridle length 
could be tested. 
Fourth, we have conducted these experiments in rela- 
tively small areas that do not encompass all of the bot- 
tom conditions found over the areas covered by the bottom 
trawl surveys; therefore the results are potentially biased 
and certainly less variable than if they were conducted 
over such a range of conditions. 
Bridle efficiency is only one component needed to deter- 
mine trawl efficiency (i.e. the proportion of fish within the 
door path that are caught). The remaining component, for 
flatfish at least, is net efficiency (i.e. the proportion of fish 
within the net path that are caught). If estimates of net ef- 
ficiency are also available, perhaps obtained with the use 
of an auxiliary net to capture escaping fish (Engas and 
Godp, 1989b), they can be included with estimates of bri- 
dle efficiency in Dickson’s (1993a) model of the trawl fish- 
ing process to produce estimates of trawl efficiency. Such 
estimates could then be used to convert relative indices of 
fish abundance from trawl survey to absolute estimates of 
abundance. 
Acknowledgments 
We thank Asgeir Aglen, Martin Dorn, Olav Rune Godp, 
Jack Turnock, and Gary Stauffer for reviewing the manu- 
script and for making helpful suggestions. 
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