410 



Abstract.— A winter dredge survey of 

 blue crab iCallinectes sapidus Rath- 

 bun) is conducted annually in Chesa- 

 peake Bay as a key element of a long- 

 term, bay-wide population dynamics 

 study. Removal experiments are perfor- 

 med routinely as part of this stratified 

 random survey of the blue crab popu- 

 lation. We present a method for esti- 

 mating the catching efficiency of the 

 standard Virginia crab dredge used in 

 the winter survey. Data from 88 expe- 

 riments conducted between November 

 1992 and March 1995 were analyzed; 

 up to 10 removals were completed in 

 each experiment. Two models were used 

 to estimate catching efficiency for each 

 experiment: 1) the Leslie model, and 2) 

 a log-linear model in which it is assu- 

 med that a fixed proportion of crabs is 

 removed in each sweep of the experi- 

 mental area, allowing for an error term 

 E. We estimated the catchability coef- 

 ficient ( g ) as a weighted mean of the 

 point estimates from each experiment; 

 its standard error was estimated with 

 the jackknife method. The average cat- 

 chability coefficients across years were 

 0.16 (SE=0.01) for model 1, and 0.15 

 (SE=0.02) for model 2. There were no 

 significant differences in yearly estima- 

 tes of dredge efficiency for the period 

 investigated in our study. We show how 

 the estimated catching efficiency can be 

 used to calibrate catch per unit of effort 

 in a dredge survey. The precision of 

 estimates of absolute abundance could 

 be improved significantly by increasing 

 the precision of the estimates of catcha- 

 bility. Similar improvements of estima- 

 tes of absolute abundance are expected 

 for analogous dredging surveys of slow- 

 moving or sedentary benthic species 

 buried in the sediment, such as scallops 

 and clams. 



A method for estimating dredge catching 

 efficiency for blue crabs, Callinectes sapidus, 

 in Chesapeake Bay 



Jon H. Velstad 



Versar, Inc 

 9200 Rumsey Road 

 Columbia, Maryland 21045 

 E-mail address VolstadioniS versar com 



Alexei F. Sharov 



Maryland Department of Natural Resources 

 Tawes State Office Building B-2 

 580 Taylor Avenue 

 Annapolis, Maryland 21401 



Glenn Davis 



Brenda Davis 



Maryland Department of Natural Resources 

 301 Marine Academy Drive 

 Stevensville, Maryland 21666 



Manuscript accepted 29 November 1999, 

 Kish. Bull. 98;410-420 (2000), 



Stratified random dredge surveys 

 have been conducted in Chesapeake 

 Bay yearly since 1989 during the 

 coldest winter months to estimate 

 abundance and other key statistics 

 for the blue crab (Callinectes sapi- 

 dus Rathbun), The survey design 

 implemented during the winter of 

 1992-1993 became the standard. 

 Three geographic strata were sam- 

 pled every year thereafter: upper 

 bay and rivers (619f of the total 

 area), middle bay (2T7c of the total 

 area), and lower bay (12% of the 

 total area ), The number of randomly 

 selected stations in each stratum 

 was proportional to the area of the 

 stratum. The strata were designed 

 to encompass major areas of habi- 

 tat and to account for differences 

 in spatial distribution of crabs by 

 size and sex. Details of the design 

 of the Chesapeake Bay blue crab 

 winter survey, its characteristics, 

 and history can be found in Vol- 

 stad et al,' and Rothschild and 

 Sharov,^ Survey results indicated 



that the distribution of blue crabs is 

 highly patchy, and the coefficient of 

 variation (CV) of average crab den- 

 sity is usually large. Nevertheless, 

 catch per unit of effort (CPUE ) from 

 the annual dredge surveys gene- 

 rally provides accurate estimates of 

 relative abundance because of effici- 

 ent stratification and large sample 

 sizes. 



1 Volstad, J. H., B. J. Rothschild, and T. 

 Maurer. 1994. Abundance estimation 

 and population dynamics of the blue crab in 

 the Chesapeake Bay. Ref No. UMCEES 

 ICBLj 94-014. Final report to the Mary- 

 land Department of Natural Resources, the 

 Che.sapeake Bay Stock Assessment Commit- 

 tee, and the National Oceanic and Atmosphe- 

 ric Administration iNOAAi. Chesapeake 

 Biological Laboratory, P,0. Box 38, Solo- 

 mons, MD 21236, 



- Rothschild, B. J„ and A. F. Sharov. 1997. 

 Abundance estimation and population 

 dynamics of the blue crab in the Ches- 

 apeake Bay. Final report to the Mary- 

 land Department of Natural Resources and 

 Chesapeake Bay Stock Assessment Com- 

 mittee. Center for Marine Science and 

 Technology, University of Massachu.setts, 

 North Dartmouth, MA 02747-2.300. 



