Weinberg and Kotwicki: Reducing variability in bottom contact and net width of a survey trawl 
189 
spread is constant. One explanation for bottom contact 
sensor angles (our proxy for increasing distances of the 
footrope off bottom) increasing with increasing depth 
when the restrictor limited spread could be that the 
sensor housing penetrated the softer bottom substrate. 
If the edge of the bottom contact sensor had been sink- 
ing into the seabed, recorded angles would have been 
higher and our function for transforming angles into 
distances off bottom, which was derived from a sepa- 
rate experiment performed on a hard surface, would be 
incorrect. In retrospect, we compared the locations of 
our towing sites with locations from a sediment map 
of the Bering Sea (Smith and McConnaughey, 1999) 
and observed that our deepest site occurred in a soft, 
muddy region of the Bering Sea where the sensor could 
possibly penetrate the substrate. Use of this tool for 
predicting footrope performance may best be limited to 
areas of comparable substrate, and data collected over 
an entire survey area should be used cautiously. 
In conclusion, reduced variability in net spread and 
bottom contact of our survey trawl was best achieved 
by affixing a restrictor line between the trawl warps to 
constrain the spread of the otter doors in conjunction 
with the use of a 3.8:1 ratio of warp length to depth. 
The stabilization of trawl shape across a survey area 
will reduce the variability in horizontal herding and 
fish escapement, leading to more precise estimates of 
abundance for stock assessments. Before inclusion of 
a restrictor line as part of standardized survey equip- 
ment, we recommend that further experimentation be 
conducted with retention of the catch to quantify the 
effect of the restrictor line on horizontal and vertical 
herding. These studies would be most meaningful for 
the eastern Bering Sea semipelagic gadoids walleye 
pollock and Pacific cod. With a clearer understanding, 
catch coefficients could be developed and applied to his- 
torical survey data. 
Acknowledgments 
We would like to thank E. Ona and A. Engas for their 
inspirational work in the development of the restric- 
tor concept and Arill for his guidance to us during the 
2004 International Council for the Exploration of the 
Sea, Working Group on Fishing Technology and Fish 
Behaviour meeting in Gydnia, Poland. The manuscript 
was improved through comments from various review- 
ers, including D. Somerton of the AFSC, A. Aglen of 
IMR, and anonymous reviewers. 
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