Capture Probability of Blue Swimmer Crab (Portunus armatus) in Baited Traps and Implications for Trap Survey Design

Fishery independent surveys provide valuable data for fisheries assessments that predominantly rely on trends in catch per unit effort (CPUE). Many crustacean fisheries are harvested using traps, so this gear is often used for fishery independent surveys. The areal sampling unit for baited traps is normally poorly understood, so surveys can be biased if sampling areas overlap. The blue swimmer crab (Portunus armatus) is an important commercial and recreational species distributed around the warm temperate and tropical coast of Australia, and stock management typically relies on catch data supplemented by independent surveys. Acoustic telemetry was used to track fine-scale movements of 37 crabs in response to deployment of baited-traps and to estimate probability of capture relative to distance from the trap. Traps attracted crabs from an initial distance up to 70 m, with most movements towards traps after dark. Capture probability increased with decreasing initial distance from a deployed trap. Despite this, the probability of capture was < 50% for crabs positioned adjacent to a deployed trap, which could lead to a mismatch between catch and crab abundance if the proportion of catchable crabs varied with abundance. Our findings reinforced that nocturnal sampling was more appropriate for the species, and we recommend that traps be no closer than 140 m apart. Our findings also demonstrated the utility of fine-scale acoustic telemetry for informing survey designs and may serve as a template for other trap surveys and fisheries.