740 



Abstract— The relative abundance 

 of Bristol Bay red king crab (Para- 

 lithodes camtschaticus) is estimated 

 each year for stock assessment by 

 using catch-per-swept-area data col- 

 lected on the Alaska Fisheries Sci- 

 ence Center's annual eastern Bering 

 Sea bottom trawl survey. To estimate 

 survey trawl capture efficiency for red 

 king crab, an experiment was con- 

 ducted with an auxiliary net (fitted 

 with its own heavy chain-link foot- 

 rope I that was attached beneath the 

 trawl to capture crabs escaping under 

 the survey trawl footrope. Capture 

 probability was then estimated by 

 fitting a model to the proportion of 

 crabs captured and crab size data. 

 For males, mean capture probability 

 was 727 at 95 mm (carapace length), 

 the size at which full vulnerability to 

 the survey trawl is assigned in the 

 current management model: 84. 1^ at 

 135 mm, the legal size for the fish- 

 ery; and 939c at 184 mm, the maxi- 

 mum size observed in this study. For 

 females, mean capture probability was 

 707c at 90 mm, the size at which full 

 vulnerability to the survey trawl is 

 assigned in the current manage- 

 ment model, and 777 at 162 mm, 

 the maximum size observed in this 

 study. The precision of our estimates 

 for each sex decreased for juveniles 

 under 60 mm and for the largest 

 crab because of small sample sizes. 

 In situ data collected from trawl- 

 mounted video cameras were used to 

 determine the importance of various 

 factors associated with the capture of 

 individual crabs. Capture probabil- 

 ity was significantly higher when a 

 crab was standing when struck by the 

 footrope, rather than crouching, and 

 higher when a crab was hit along its 

 body axis, rather than from the side. 

 Capture probability also increased as 

 a function of increasing crab size but 

 decreased with increasing footrope 

 distance from the bottom and when 

 artificial light was provided for the 

 video camera. 



Capture probability of a survey trawl for 

 red king crab (Paralithodes camtschaticus) 



Kenneth L. Weinberg 



Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way N.E 



Seattle, Washington 98115 



E-mail address ken Weinberg gnoaa gov 



Robert S. Otto 



Kodiak Fisheries Research Center 

 National Marine Fisheries Service, NOAA 

 301 Research Court 

 Kodiak, Alaska 99615 



David A. Somerton 



Alaska Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 7600 Sand Point Way N.E. 

 Seattle, Washington 98115 



Manuscript submitted 5 September 2003 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 28 April 2004 by the Scientific Editor. 



Fish. Bull. 102:740-749(2004) 



Regulations limit the annual har- 

 vest of Bristol Bay red king crab 

 (RKC; Paralithodes camtschaticus) to 

 males >135 mm in carapace length 1 

 (6.5 inches carapace width), and the 

 size of the harvest is dependent upon 

 the estimated biomasses of mature 

 males and females. For stock assess- 

 ments of RKC, area-swept abundance 

 estimates are determined from the 

 data from annual eastern Bering Sea 

 (EBS) bottom trawl surveys conducted 

 by the National Marine Fisheries Ser- 

 vice, Alaska Fisheries Science Center 

 (AFSC), and these estimates are used 

 as input into a length-based assess- 

 ment model (Zheng et al., 1995) to 

 compute the total allowable catch for 

 each annual fishing season. 



It is assumed with the current as- 

 sessment model that all male RKC 

 >95 mm and all female RKC >90 mm 

 within the path of the survey trawl 

 (wingtip to wingtip) are captured. 

 This assumption seems reasonable 

 because the survey trawl uses a small 

 diameter footrope designed to stay 

 close to the bottom and red king crab 

 are quite large. However, video pho- 

 tography taken following the 2000 

 EBS survey revealed that a consider- 

 able number of large (>90 rami RKC 



pass under the footrope of the survey 

 trawl. 



To assess the potential impact of 

 escaping crab on the calculation of 

 crab biomass, we conducted an ex- 

 periment to estimate the size-related 

 capture efficiency of the standard 

 survey bottom trawl for Bristol Bay 

 RKC. In this experiment, crab pass- 

 ing beneath the survey trawl were 

 subsequently captured with an aux- 

 iliary net that was attached under- 

 neath and behind the footrope of 

 the survey trawl (Engas and Godo, 

 1989; Walsh, 1992). Experimental 

 nets like the one used for this study 

 have been used previously in trawl 

 efficiency studies for flatfish (Munro 

 and Somerton, 2002), as well as for 

 snow iChionoecetes opilio) and Tan- 

 ner (C. bairdi) crabs (Somerton and 

 Otto, 1999). Trawl catch data alone, 

 however, tell little about the details 

 involved with escapement. Therefore, 

 we deployed a video camera on the 

 trawl to observe crab behavior and 

 analyzed a combination of trawl-per- 



1 All references to measured crab lengths 

 are carapace length. 



