Escape Gaps as a Management Strategy for Reducing Bycatch in Net-Covered Traps for the Giant Mud Crab Scylla serrata

In response to experimental evidence and anecdotal concern about bycatch in the net-covered traps used increasingly by recreational and commercial fishers to target giant mud crabs Scylla serrata in New South Wales, Australia, experiments were done to examine the utility of escape gaps in reducing the bycatch of Yellowfin Bream Acanthopagrus australis (also known as Surf Bream) and undersized giant mud crabs. In each of two rivers, 10 different treatments comprising four different sizes of rectangular, horizontal escape gaps (85 x 45mm, 85 x 55mm, 95 x 45mm, and 95 x 55mm) and a control (no escape gaps) applied to both two-entrance and four-entrance traps were tested separately during the day and night. Traps fitted with escape gaps reduced the mean numbers of Yellowfin Bream and undersized giant mud crabs by 53-78% and 58-84%, respectively. Despite some inconsistent results, the effect of escape gaps was statistically significant in four of the seven analyses. There were, however, no significant differences in the mean numbers of Yellowfin Bream and undersized giant mud crabs among the different sizes of escape gap. By contrast, the largest escape gap treatment (i.e., 95 x 55mm) reduced the mean number of legal-size giant mud crabs by 35-41%, which was significant in one of four analyses. Moreover, size-selectivity analyses indicated that the largest escape gap also allowed a comparatively greater proportion of undersized mud crabs to escape. Our findings show that implementing an 85-mm x 55-mm escape gap could substantially reduce bycatch without affecting catches of the target organism. Further reductions in catches of undersized giant mud crabs is possible with the 95-mm x 55-mm treatment, but at the cost of some reduction in the catch target.