A method for estimating dredge catching efficiency for blue crabs, Callinectes sapidus, in Chesapeake Bay

A winter dredge survey of blue crab (Callinectes sapidus Rathbun) is conduc ted annually in Chesapeake Bay as a key element of a longterm, bay-wide pop ulation dynamics study. Removal experiments are performed routinely as part of this stratified random survey of the blue crab population. We present a method for estimating the catching efficiency of the standard Virginia cra b dredge used in the winter survey. Data from 88 experiments conducted betw een November 1992 and March 1995 were analyzed; up to 10 removals were comp leted in each experiment. Two models were used to estimate catching efficie ncy for each experiment: 1) the Leslie model, and 2) a log-linear model in which it is assumed that a fixed proportion of crabs is removed in each swe ep of the experimental area, allowing for an error term epsilon. We estimat ed the catchability coefficient ((q) over bar) as a weighted mean of the po int estimates from each experiment; its standard error was estimated with t he jackknife method. The average catchability coefficients across years wer e 0.16 (SE=0.01) for model 1, and 0.15 (SE=0.02) for model 2. There were no significant differences in yearly estimates of dredge efficiency for the p eriod investigated in our study. We show how the estimated catching efficie ncy can be used to calibrate catch per unit of effort in a dredge survey. T he precision of estimates of absolute abundance could be improved significa ntly by increasing the precision of the estimates of catchability. Similar improvements of estimates 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.