Modelling capture processes in individual traps: entry, escapement and soak time

Abstract

Stationary fishing gear is employed in a diverse array of fisheries including those for many high unit-value species. The mode of operation of these gear types (traps, long-lines, gillnets, etc.) requires special consideration in the development of abundance indices because of gear saturation effects. Specifically, factors affecting the probability of capture and of escapement or other sources of loss must be addressed. Here, we describe two models of capture processes in traps that include existing models as special cases. Deterministic models are first developed to illustrate the basic principles. Stochastic analogues of two well-known models are then described using the theory of birth–death stochastic processes. It is shown that the mean of the stochastic models is identical to the deterministic case for models where closed form solutions are possible. The stochastic models provide additional diagnostic information in terms of the variance and the probability distribution of catch levels. Illustrations of this approach are provided for experimental observations on ingress and catch for the American lobster (Homarus americanus).