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

Abstract

A winter dredge survey of blue crab (Callinectes sapidus Rathbun) is conducted annually in Chesapeake Bay as a key element of a longterm, bay-wide population 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 crab dredge used in the winter survey. Data from 88 experiments 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 assumed that a fixed proportion of crabs is removed in each sweep of the experimental area, allowing for an error term ε. We estimated the catchability coefficient (q) as a weighted mean of the point estimates from each experiment; its standard error was estimated with the jackknife method. The average catchability 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 estimates 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 signifi cantly by increasing the precision of the estimates of catchability. Similar improvements of estimates of absolute abundance are expected for analogous dredging surveys of slowmoving or sedentary benthic species buried in the sediment, such as scallops and clams.