Weinberg et al.: Capture probablity of a survey trawl for Paralnhodes camtschat/cus 



747 



conditions and when crab are oriented sideways 

 to the oncoming footrope, was predicted as a func- 

 tion of footrope distance off the bottom for each 

 size class, and for both standing and crouching 

 crab (Fig. 6). For all size groups, capture prob- 

 ability decreased with increasing footrope height 

 from the bottom. The importance of whether a 

 crab was standing or crouching diminishes with 

 decreasing crab size because the footrope is more 

 likely to pass completely over smaller crab. In 

 contrast, the importance of standing was higher 

 for large crab because they were more likely to be 

 undercut by the footrope and captured, whereas 

 crouching crab were more susceptible to hav- 

 ing their legs first pinned down by the footrope, 

 which exerted a downward pressure on their 

 carapace and allowed the footrope to pass over 

 the crab. Capture probability of medium-size in- 

 dividuals, which included a large proportion of 

 egg-bearing females, was more dependent upon 

 the body height of the crab. Footrope contact be- 

 low the carapace typically resulted in capture; 

 however contact above the legs often forced the 

 crab's carapace down, causing the crab to roll 

 forward and pass beneath the footrope. 



In the second case, capture probability was 

 predicted for natural light conditions when the 

 footrope is 1 cm off-bottom, as a function of crab 

 size, by body orientation to the footrope, and 

 for standing and crouching individuals (Fig. 7). 

 Under these conditions, capture probability was 

 greater for crab contacted along their body ax- 

 is than for crab hit from the side. In addition, 

 capture probability increased with crab size for 

 both standing and crouching crab, regardless 

 of whether the footrope first contacted the crab 

 along their body axis or from the side. When the 

 footrope was 1 cm off bottom, the difference in 

 the body-orientation effect on capture probability 

 for standing crab was greater for smaller crab 

 than for medium and larger individuals, but rela- 

 tively equal for crouching crab of all sizes. 



Discussion 



Our observations confirmed that adult Bristol Bay red 

 king crab can escape beneath the footrope of the AFSC's 

 83/112 Eastern survey bottom trawl under normal 

 towing conditions. Capture probability increased with 

 size but did not reach 100% for the largest crab caught. 

 For the current management model used for RKC stock 

 assessments, 100% capture probability is assumed for 

 adult crabs and should be revised. A recruitment ogive 

 is used in the calculation of the total spawning bio- 

 mass for defining overfishing under the Magnuson- 

 Stevens Fishery Conservation and Management Act 

 (Stevens 2 ). Revised computations of vulnerability will be 

 required for this purpose as well. Survey trawl selectiv- 

 ity, although similar between the two sexes at prerecruit 



sizes, was generally 15% higher for legal-size males 

 than for equal-size females. This between-sex difference 

 in capture probability may be explained by behavioral 

 differences (for instance, egg-bearing females stand dif- 

 ferently from large males). Unfortunately, crab, when 

 viewed from above, mask their gender; thus sex was 

 excluded from our modeling exercise of video data. 



Survey catch statistics for RKC are routinely included 

 in the management modeling procedure to estimate the 

 abundance of legal-size males (>135 mm), male prere- 

 cruits (95-134 mm), the effective spawning biomass of 

 males (>120 mm), and the spawning biomass of females 

 (>90 mm, as determined from size at 50% maturity). We 

 estimated capture probability for legal-size males (up 

 to 184 mm) to range from 84% to 93%, for prerecruit 

 males from 73% to 84%, and for the mature portion 

 of the male spawning population (up to 184 mm) from 



