21 



Abstract — Survey standardization 

 procedures can reduce the variabil- 

 ity in trawl catch efficiency thus 

 producing more precise estimates 

 of biomass. One such procedure, 

 towing with equal amounts of trawl 

 warp on both sides of the net, was 

 experimentally investigated for its 

 importance in determining optimal 

 trawl geometry and for evaluating the 

 effectiveness of the recent National 

 Oceanic and Atmospheric Adminis- 

 tration (NOAA) national protocol on 

 accurate measurement of trawl warps. 

 This recent standard for measuring 

 warp length requires that the differ- 

 ence between warp lengths can be no 

 more than 49f of the distance between 

 the otter doors measured along the 

 bridles and footrope. Trawl perfor- 

 mance data from repetitive towing 

 with warp differentials of 0. 3, 5, 7, 

 9, 11, and 20 m were analyzed for 

 their effect on three determinants 

 of flatfish catch efficiency: footrope 

 distance off-bottom, bridle length in 

 contact with the bottom, and area 

 swept by the net. Our results indi- 

 cated that the distortion of the trawl 

 caused by asymmetry in trawl warp 

 length could have a negative influ- 

 ence on flatfish catch efficiency. At a 

 difference of 7 m in warp length, the 

 NOAA 4'7f threshold value for the 83- 

 112 Eastern survey trawl used in our 

 study, we found no effect on the acous- 

 tic-based measures of door spread, 

 wing spread, and headrope height off- 

 bottom. However, the sensitivity of the 

 trawl to 7 m of warp offset could be 

 seen as footrope distances off-bottom 

 increased slightly (particularly in the 

 center region of the net where flatfish 

 escapement is highest), and as the 

 width of the bridle path responsible 

 for flatfish herding, together with the 

 effective net width, was reduced. For 

 this survey trawl, a NOAA threshold 

 value of 4% should be considered a 

 maximum. A more conservative value 

 (less than 4%) would likely reduce 

 potential bias in estimates of relative 

 abundance caused by large differences 

 in warp length approaching 7 m. 



Variation in trawl geometry due to 

 unequal warp length 



Kenneth L. Weinberg 



David A. Somerton 



National Marine Fisheries Service 



Alaska Fisheries Science Center 



7600 Sand Point Way N E 



Seattle, Washington 98115 0070 



E-mail address (for K L, Weinberg): l<en-weinberg(gJnoaa gov 



Manuscript submitted 20 October 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 6 June 2005 by the Scientific Editor. 



Fish. Bull. 104:21-34(20061. 



Standardization of trawl survey pro- 

 cedures can reduce the variability in 

 abundance indices between samples, 

 survey vessels, and over time by reduc- 

 ing the variability in trawl catch effi- 

 ciency. Such standardization was the 

 focus of the recently developed U.S. 

 National Oceanic and Atmospheric 

 Administration (NOAA) protocols for 

 the operation of its groundfish bottom 

 trawl surveys (Stauffer, 2004). The 

 first of these protocols concerns the 

 measurement of towing cables or 

 warps. For vessels towing with two 

 warps, the NOAA protocols specify 

 that the difference in length between 

 port and starboard warps may not 

 exceed 4% of the wire length between 

 otter doors measured along the bridles 

 and footrope. The need for adopting 

 such a critical value was considered 

 essential because of the belief that 

 unequal warp lengths — from inac- 

 curate measurement or subsequent 

 stretching — would lead to distortion of 

 trawl geometry and a change in catch 

 efficiency, particularly for operations 

 that use trawl winches with the brakes 

 set or locked. The adopted value, how- 

 ever, was chosen somewhat arbitrarily 

 because experimental data showing the 

 dependency of trawl geometry or fish- 

 ing performance on warp symmetry 

 was lacking for any of the bottom trawl 

 surveys subject to the protocols. 



In this study, we examine the ef- 

 fect of unequal warp lengths on the 

 geometry of the 83-112 Eastern trawl 

 which is used by the Alaska Fisher- 

 ies Science Center (AFSC) to conduct 

 the annual eastern Bering Sea shelf 

 survey. Although we monitor a full 

 suite of trawl dimensions, such as 



door spread, wing spread, and hea- 

 drope height that are typically mea- 

 sured on trawl surveys, our attention 

 was primarily focused on the distance 

 between the footrope and lower bri- 

 dles with the sea floor. Prior studies 

 with this trawl have demonstrated 

 that escapement under the footrope 

 (Somerton and Otto, 1999; Munro 

 and Somerton, 2002; Weinberg et 

 al., 2004) and herding by the bridles 

 (Somerton and Munro, 2001) are the 

 most important determinants of catch 

 efficiency for flatfishes and other 

 benthic species. Although we under- 

 stand that catch efficiency depends 

 on animal behavior as well as trawl 

 geometry, a goal of our study was to 

 assess whether the 4% critical value 

 is appropriate to prevent an appre- 

 ciable degradation of catch efficiency 

 due to warp asymmetry, the result of 

 unequal trawl warp lengths. 



Materials and methods 



Experimental design 



The experiment was conducted during 

 14-17 September 2003 along the Alaska 

 Peninsula in Bristol Bay approximately 

 85 km NE of Amak Island (55°58'N, 

 162°55'W) on smooth, relatively level 

 bottom at a depth of 82 m. Trawling 

 was performed with the chartered 

 38-m stern trawler FV Vesteraalen. 

 The Vesteraalen is powered by a single 

 1725-hp engine and is equipped with 

 split Rapp Hydema (Rapp Hydema AS, 

 Bodo, Norway) trawl winches carrying 

 2.5 cm (1") diameter, compacted, solid- 

 core trawl warp. 



