Estimating trapping areas from trap-catch data for lobsters and crabs

Abstract

Conversion of catch per unit effort data to density of an exploited species is fundamental to stock assessment. A vital component of this conversion for trap fisheries is the effective area fished by a trap. A conceptual model of interactions between traps set in strings that potentially allows estimation of trapping areas from trap-by-trap catch data predicted highest catch rates of lobsters and crabs in traps at the ends of strings. An individual-based simulation model developed to explore the effects of trap spacing, density, and agonistic behaviour at the trap shows that, in the absence of agonistic interactions at the trap (i.e., trap saturation), catch rates conform to those expected from the conceptual model, provided that trap exposure modifies the catchability at all traps equally. Trap saturation reduces the effect of trap interactions, particularly at high densities, and causes the relationship between capture probability and distance from the trap to depend on density. We discuss the application of the model to trap-by-trap data in practice and conclude that the approach may be valuable in an experimental context when direct methods of estimating effective area fished are not practicable.