A mark-recapture approach to estimating population density from continuous trapping data: application to edible crabs, Cancer pagurus, on the east coast of England

Abstract We develop a mark-recapture approach to estimate population density from continuous trapping data. The method incorporates flexibility to allow for sampling difficulties encountered in real field experiments, such as unequal sampling intervals and incomplete sampling of the gear. A short-term mark-recapture experiment is described, aimed at estimating density of edible crabs, Cancer pagurus , on an offshore bank off the English east coast. More than 3000 crabs were marked with paint and sampled using baited traps of standard commercial design. We apply the mark-recapture model to estimate probabilities of capture and fidelity to the experimental capture area, and thereby estimate the size of the local population from the total catch data. A local population of 4800 crabs is estimated to exist in a capture area of 2.3 km 2 , giving an estimated density of 2100 crabs/km 2 . Bootstrapping is used to show that temporal variation is much more important than spatial variation as a component of uncertainty in density estimates. Experimental catch data are used to draw inferences about the relationship between trap soak time and effective fishing effort. Density estimates are shown to be robust to uncertainty about this relationship. Problems of interpretation of density estimates at the population level are discussed.