Abstract
In response to concerns over excessive discarding from Australian recreational round traps (with four funnel entrances) used to target giant mud crabs, Scylla serrata, an experiment was done to assess the independent and cumulative utility of paired, bottom-located horizontal escape gaps (46×120 mm) and increasing mesh size (from 51 to 101 mm). Compared to conventional traps comprising 51-mm mesh throughout, those with the same mesh size and escape gaps caught significantly fewer (by 95%) undersize (<85 mm carapace length – CL) crabs while maintaining legal catches. Traps made from 101-mm mesh (but with the same funnel entrances as conventional designs) and with and without escape gaps similarly retained fewer undersize crabs and also yellowfin bream Acanthopagrus australis (the key bycatch species) by up to 94%, but there were concomitant reductions in fishing power for legal sizes of S. serrata. Although there were no immediate mortalities among any discarded crabs, there was a greater bias towards wounding among post molts than late inter-molts and less damage to individuals in the 101-mm conventional than 51-mm conventional traps (without escape gaps). The results support retrospectively fitting escape gaps in conventional S. serrata traps as a means for reducing discarding, but additional work is required to determine appropriate mesh sizes/configurations that maximize species and size selectivity.
Highlights
The giant mud crab Scylla serrata is among the most popular recreationally targeted crustaceans in northern Australia, with an estimated total annual catch exceeding 2.5 million individuals; most (92%) of which are trapped [1,2]
Because all S. serrata were tagged and released, with many subsequently recaptured on multiple occasions, instead of using individuals as the experiment unit describing catches, we considered the individual crab per trap per day as the experimental and observational unit and the response variable given by the binary outcome of caught or not caught
The 101-mm traps had a significantly larger mesh than the 51mm designs (LM and false discovery rate (FDR), p,0.01) but there were no significant differences between traps with the same nominal mesh sizes, with means 6SE of 50.9260.07 and 101.3460.07 mm, respectively (LM and FDR, p.0.05)
Summary
The giant mud crab Scylla serrata is among the most popular recreationally targeted crustaceans in northern Australia (from Western Australia to New South Wales–NSW), with an estimated total annual catch exceeding 2.5 million individuals; most (92%) of which are trapped [1,2] This total catch is comparatively greater than that of many other local recreational species, owing to minimum legal sizes (MLS – interstate variations of ,130– 150 mm carapace width – CW), personal quotas (5–10 per day) and poor trap selectivities, only 32% of all S. serrata are harvested [1,2]. Beyond the perception of excessive discarding as being socially unacceptable, there are concerns regarding the so-called ‘ghost fishing’ of poorly selective traps, which if lost can perpetuate unaccounted fishing mortality [4] Such issues warrant investigating modifications to recreational S. serrata traps to improve their species and size selectivity [5,6]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
