Sex-specific vulnerability of Portunus armatus to capture in round traps with traditional and novel fish baits

In response to few data describing the effects of bait on trap efficiencies for blue swimmer crabs, Portunus armatus in an Australian fishery and an impetus to reduce costs, traditional baits (sea mullet, Mugil cephalus and luderick, Girella tricuspidata) were compared against a novel, less-expensive bait (European carp, Cyprinus carpio). Eight replicate traps with each bait type were fished over four days at one location in the fishery for a total of 96 trap lifts. Traps baited with sea mullet caught significantly more (by >1.5×) total blue swimmer crabs and at a greater net-profit trap deployment (>3.7×) than traps with the other two baits, which produced similar catches and profits. But, the superior performance of sea mullet as bait was due to greater catches of female blue swimmer crabs (which were larger and are more valuable owing to ovarian development), with no significant difference in catches of males among bait types. Such sex specificity was attributed to possible divergent, temporal nutritional requirements including a dietary bias among mature females preparing to migrate to oceanic areas prior to reproduction. Additional data are required across larger spatio-temporal scales to further elucidate patterns; however there are clear implications for fishers that seek to maximise catches of females in the studied fishery or potentially minimise female catches which are prohibited in other Australian fisheries. Further, it is also clear that sex-specific catch data acquired during surveys should be considered for possible confounding effects of bait (and other technical factors) prior to extrapolating relative abundances