488 



Abstract— Dungeness crabs {Cancer 

 magister) were sampled with commer- 

 cial pots and counted by scuba divers 

 on benthic transects at eight sites 

 near Glacier Bay, Alaska. Catch per 

 unit of effort (CPUE) from pots was 

 compared to the density estimates 

 from dives to evaluate the bias and 

 power of the two techniques. Yearly 

 sampling was conducted in two sea- 

 sons: April and September, from 1992 

 to 2000. Male CPUE estimates from 

 pots were significantly lower in April 

 than in the following September; a 

 step-wise regression demonstrated 

 that season accounted for more of 

 the variation in male CPUE than 

 did temperature. In both April and 

 September, pot sampling was signifi- 

 cantly biased against females. When 

 females were categorized as oviger- 

 ous and nonovigerous, it was clear 

 that ovigerous females accounted for 

 the majority of the bias because pots 

 were not biased against nonovigerous 

 females. We compared the power of 

 pots and dive transects in detecting 

 trends in populations and found that 

 pots had much higher power than dive 

 transects. Despite their low power, the 

 dive transects were very useful for 

 detecting bias in our pot sampling and 

 in identifying the optimal times of 

 year to sample so that pot bias could 

 be avoided. 



Estimating Dungeness crab (Cancer magister) 

 abundance: crab pots and dive transects compared 



S. James Taggart 



Glacier Bay Field Station 



Alaska Science Center 



U.S. Geological Survey 



3100 National Park Rd. 



Juneau, Alaska 99801 



E-mail address |im_taggart(S ! usgsgov 



Charles E. O'Clair 



National Marine Fisheries Service 

 Auke Bay Laboratory 

 11305 Glacier Highway 

 Juneau, Alaska 99801 



Thomas C. Shirley 



Juneau Center, School of Fisheries & Ocean Sciences 

 University of Alaska Fairbanks 

 11120 Glacier Highway 

 Juneau, Alaska 99801 



Jennifer Mondragon 



Glacier Bay Field Station 

 Alaska Science Center 

 U.S. Geological Survey 

 3100 National Park Rd. 

 Juneau, AK 99801 



Manuscript submitted 13 March 2000 

 to Scient ific Editor's Office. 



Manuscript approved for publication 

 25 March 2004 by the Scientific Kditor. 



Fish Bull. 102:488-497 (2004) 



Reliable population assessments are 

 fundamental to the management and 

 conservation of commercially har- 

 vested crabs. Many crab populations 

 are sampled with commercial crab 

 pots to estimate population trends, to 

 set harvest quotas, or to differentiate 

 natural population fluctuations caused 

 by anthropogenic changes to the eco- 

 system. Pots are used, for example, to 

 assess the population status of blue 

 crabs, Callinectes sapidus (Abbe and 

 Stagg, 1996), red king crabs, Para- 

 lithodes camtschaticus (Zheng et al., 

 1993), snow crabs, Chionoecetes opilio 

 (Dawe et al., 1996), and southern king 

 crabs, Lithodes santolla i Wyngaard 

 and Iorio. 1996). 



The Dungeness crab (Cancer ma- 

 gister) fishery began in southeastern 

 Alaska in 1916 and has been charac- 

 terized by large fluctuations on an- 



nual and decadal scales tOrensanz 

 et al., 1998). Large variation in the 

 Dungeness crab harvest is not unique 

 to Alaska; similar fluctuations have 

 been documented in California and 

 their causes are the subject of an on- 

 going debate (Higgins et al., 1997a. 

 1997b). It is not clear whether the 

 processes that cause fluctuations in 

 California are the same as those re- 

 sponsible for oscillations in Dunge- 

 ness crab abundance in Alaska. 



Most of the Dungeness crab fisher- 

 ies in Alaska are managed by regu- 

 lating the size and sex of the crabs 

 caught, and, in some places, the sea- 

 son of the harvest. In southeastern 

 Alaska, legal harvest is restricted to 

 males with a carapace width greater 

 than or equal to 165 mm (excluding 

 the 10 th anteriolateral spines) and the 

 season is timed to avoid sensitive life 



