Evaluation of a novel research trap for surveys of blue swimmer crab populations

Abstract

Surveying free-ranging crab populations is important for monitoring the health of exploited stocks and predicting future productivity. Here, we present a novel research trap design for use in fisheries-independent surveys of blue swimmer crab (Portunus armatus) populations, and evaluate the trap against some existing approaches within an independent survey framework. Specifically, the trap design aimed to improve efficiency for capturing smaller crabs, without adversely affecting catches of larger crabs. We quantify and report the abundance and selectivity of these traps, relative to co-located samples obtained using beam trawls and standard commercial round traps, to establish whether these small-mesh traps may offer any improvements over existing survey methods. Comparison and evaluation of these small-mesh traps against other existing survey gear, in different places and at different times, showed that the traps are more effective at catching smaller crabs when they are present, and equally or more effective at catching larger size classes of crabs. The beam trawl appeared to be effective at capturing a reasonable size range of crabs; however, the number of crabs caught (using a similar investment of time) was substantially lower than that caught in traps. This novel small-mesh research trap appears suitable for fisheries-independent surveys of portunid crab species.