Giant mud crab (Scylla serrata): relative efficiencies of common baited traps and impacts on discards

This study was initiated in response to a scarcity of data on the efficiency, selectivity and discard mortality of baited traps to target Scylla serrata. Five replicates of four traps, including hoop nets, rigid wire pots, and collapsible round and rectangular pots were deployed for 3, 6 and 24 h in two Australian estuaries. Trapped S. serrata were discarded into cages and monitored with controls over 3 d. All S. serrata were assessed for damage, while subsets of immediately caught and monitored individuals had haemolymph constituents quantified as stress indices. All traps retained similar-sized (8.1-19.1 cm carapace width) S. serrata, with catches positively correlated to deployment duration. Round pots were the most efficient for S. serrata and fish-mostly Acanthopagrus australis (3% mortality). Hoop nets were the least efficient and were often damaged. No S. serrata died, but 18% were wounded (biased towards hoop nets), typically involving a missing swimmeret. Physiological responses were mild and mostly affected by biological factors. The results validate discarding unwanted S. serrata for controlling exploitation, but larger mesh sizes or escape vents in pots and restrictions on hoop nets would minimise unnecessary catches, pollution and ghost fishing.