Kotwicki et al,: Effect of autotrawl systems on the performance of a survey trawl 



45 



direction and velocity based on the current sensor in- 

 put. Because many surveys standardize to a 15-min 

 tow duration, a proportionally large percentage of tow 

 time may be involved with adjusting warp lengths in 

 pursuit of optimal symmetry and therefore vary trawl 

 catchability during the tow. Under commercial trawling 

 conditions, the manufacturer of the symmetry winch 

 control system recommends using a 2-min minimum 

 signal collection and analysis period between warp ad- 

 justments. After each adjustment, new sensor signals 

 would be received and evaluated, followed by another 

 warp adjustment, if necessary. We are uncertain as 

 to how many warp adjustments may be necessary to 

 orient the trawl in relation to crosscurrent flow, but 

 based on our experiences and the frequency with which 

 warp lengths changed during our experiment, it is con- 

 ceivable that several adjustment periods spanning the 

 majority of the survey tow may be necessary, leaving 

 minimal time for the net to actually fish symmetrically. 

 Furthermore, each 2-min warp adjustment would be 

 based on a maximum of only five current flow readings 

 because the current sensor refreshes data at 24-s inter- 

 vals. Should signal loss occur, then fewer data points 

 would be available. 



We recommend taking a cautious approach before 

 switching a trawl survey from locked winches to auto- 

 trawl. A change in survey method may require an ex- 

 tensive calibration experiment between the two trawl- 

 ing methods in order to maintain the continuity of a 

 survey time series. Furthermore, autotrawl systems, 

 like other mechanical devices, require service, appro- 

 priate inspections and periodic testing to ensure that 

 they are functioning correctly within manufacturer 

 specifications. Autotrawl calibration parameters are 

 dependent upon accurate measurements of the diameter 

 and length of each winch drum, warp diameter, and 

 construction (e.g., compacted vs. traditional or wire 

 core vs. fiber core), layers of warp, and the number of 

 windings per layer on each drum. Hydraulic pumps 

 and lines, electric motors, valves, solenoids, computer- 

 ized control panels, and geometric counters must all be 

 inspected to assure proper operation. Of course, should 

 surveys operate with a symmetry-style autotrawl sys- 

 tem, then all of the above procedures would hold true 

 in addition to the need for accurate calibration of the 

 current sensor and the proper mounting of the sensor 

 to the headrope. 



Acknowledgments 



The success of this project is due to the efforts of many. 

 We extend our gratitude to Tim Cosgrove, Brad Lougheed, 

 and the crew of the FV Vesteraalen. Our knowledge of 



winch control systems was greatly enhanced by the guid- 

 ance of Doug Dixon and Ed Ramberg, as was our ability 

 to communicate at-sea with a wide range of instru- 

 ments thanks to Scott Furnish, David Roetcisoender, 

 Jon French, and Dennis Benjamin. We are also grateful 

 to our reviewers including Captain John Gruver, Mark 

 Wilkins, Gary Walters, and Peter Munro. 



Literature cited 



Main, J., and G. I. Sangster. 



1981a. A study of the sand clouds produced by trawl 

 boards and their possible effect on fish capture. Scott. 

 Fish. Res. Rep. 20:1-20. 

 1981b. A study of fish capture process in a bottom trawl 

 by the direct observations from a towed underwater 

 vehicle. Scott. Fish. Res. Rep. 23:1-23. 

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



2002. Estimating net efficiency of a survey trawl for 

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



Neter, J., M. H. Kutner, C. J. Nachtsheim, and W. Wasserman. 

 1996. Applied linear regression models, third ed., 720 

 p. Irwin, Chicago, IL. 

 Rose, C. S., and E. P. Nunnallee. 



1998. A study of changes in groundfish trawl catch- 

 ing efficiency due to differences in operating width, 

 and measures to reduce width variation. Fish. Res. 

 36:139-147. 



Somerton. D. A. 



2003. Bridle efficiency of a survey trawl for flatfish: 

 measuring the length of the bridles in contact with the 

 bottom. Fish. Res. 60:273-279. 



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



2001. Bridle efficiency of a survey trawl for flatfish. Fish. 

 Bull. 99:641-652. 

 Somerton, D.A.. and R. S. Otto. 



1999. Net efficiency of a survey trawl for snow crab, 

 Chionoecetes opilio, and Tanner crab, C. bairdi. Fish. 

 Bull. 97:617-625. 



Somerton, D.A., and K. L. Weinberg. 



2001. The affect of speed through the water on footrope 

 contact of a survey trawl. Fish. Res. 53:1724. 



Stauffer, G. 



2004. NOAA protocols for groundfish bottom trawl sur- 

 veys of the nation's fishery resources. NOAA Tech. 

 Memo. NMFS-F/SPO-65, 205 p. Alaska Fish. Sci. 

 Center, 7600 Sand Point Way NE, Seattle, WA 98115. 



Weinberg, K. L., R. S. Otto, and D. A. Somerton. 



2004. Capture probability of a survey trawl for red 

 king crab iParalithodes camtschaticus). Fish. Bull. 

 102:740-749. 

 Weinberg, K. L., and D. A. Somerton. 



2006. Variation in trawl geometry due to unequal warp 

 length. Fish. Bull. 104:21-34. 

 Weinberg, K. L., D. A. Somerton. and P. T Munro. 



2002. The effect of trawl speed on the footrope capture 

 efficiency of a survey trawl. Fish. Res. 58:303-313. 



