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Fishery Bulletin 102(4) 



80% to 93% Our estimated capture probability for the 

 survey trawl on the female portion of the spawning 

 RKC population ranged from 70% to 77% for crab up 

 to 162 mm. A review of the AFSC database for EBS 

 crab surveys showed that the largest male and female 

 crabs taken were 200 mm and 172 mm. Corresponding 

 capture probabilities estimated by the model for these 

 size crabs were 94% and 78%, respectively. 



Two main factors affect the overall capture efficiency 

 of epibenthic species by a bottom trawl: 1) horizontal 

 herding, defined as movement into the path of the trawl 

 between the wingtips in response to stimuli produced by 

 the doors or bridles; and 2) escapement, defined as the 

 avoidance of capture once the crab is within the path of 

 the trawl. We believe herding is negligible because our 

 observations of crab movement, which were consistent 

 with those reported by Rose (1999), indicated that RKCs 

 are slow-moving animals that can travel only slight dis- 

 tances before being overtaken by a trawl approaching 



at 1.5 m/sec. Our video observations of the trawl bridle 

 revealed that RKCs consistently passed over the top of 

 the bare cable, with one exception — where a few crabs 

 were seen sliding along the bridle, legs entangled, to the 

 wingtip before being cast outside the path of the trawl. 

 Escapement is likely restricted to footrope escapement 

 because mesh escapement is impeded by the spiny sur- 

 face and long legs of the crab and could only occur for 

 the smallest individuals, which we encountered in low 

 numbers and which could not be predicted reliably by 

 our model. 



We recognize from the analysis of our in situ data 

 that capture probability is influenced not only by trawl 

 performance but also by crab behavior. For instance, 

 crabs standing upright, such as moving or migrating 

 individuals, are more susceptible to capture than those 

 with their bodies resting on the substrate. Crab density 

 could also affect capture probability as seen for some 

 species offish (Godo et al., 1999). The crabs we observed 

 with our video cameras were fairly dispersed and 

 the maximum number of crabs seen in any single 

 video frame was two (twice observed). Crabs in 

 relatively low abundance are likely to react di- 

 rectly to the gear, but in areas of high abun- 

 dance, crabs may react to each other in response 

 to the stimuli from the approaching gear, causing 

 them to crouch or conversely move away from 

 perceived danger. Both of these responses would 

 result in a different capture probability. 



Our estimates of capture probability apply 

 to the conditions in which the EBS survey is 

 conducted; that is, relatively disperse offshore 

 populations encountered during daylight hours on 

 sandy bottom during the summer months. There 

 are other behavioral factors or environmental 

 conditions that we did not consider in the present 

 study but which could affect the efficiency of the 

 survey trawl. These include, but are not limited 

 to, the following: trawling where the substrate is 

 substantially different; crabs that are either ag- 

 gregated into pods or are buried (Dew 3 ); and tem- 

 peratures or tidal currents that would affect the 

 migratory or feeding behavior, and therefore the 

 body height of crab (Dew, 1990). Our estimates 

 of capture probability are also based on the as- 

 sumption that the auxiliary net is 100% efficient 

 at capturing crab escaping beneath the footrope 

 of the survey trawl. We have no direct evidence to 

 believe otherwise. However, if crabs also escaped 

 the auxiliary net, then our estimates of capture 

 probability would be too large. 



In conclusion, we wish to clarify to users of 

 our findings that, although these experimen- 

 tally determined selectivity models indicate an 

 upward correction in spawning biomass of red 

 king crab may be in order, we find no reason 



3 Dew. C. B. 2003. Personal commun. Alaska 

 Fisheries Science Center, 7600 Sand Point Wav NE, 

 Seattle, WA 98115. 



