Broadhurst et al : Effects of square-mesh panels in codends and of haulback delay on bycatch reduction 



419 



30^ 



25 



20 



10 





BPWL 



Control 



85 mm long 



Figure 5 



Differences in mean catch (± SE) 

 between the control and 85-mm- 

 long codend, combined across both 

 experiments: (A) the weight of dis- 

 carded bycatch; (B) the number of 

 discarded red spot whiting, Sillago 

 flindersi; (C) the weight of dis- 

 carded red spot whiting, Sillago 

 flindersi; and ID) the weight of 

 prawns Penaeus plebejus. * = P<0.05; 

 ** = P<0.01; unless stated otherwise 

 n = 18. 



| Control, r> = 1,061 

 □ 85 mm long, n= 1.128 



Carapace length (mm) 



too 

 80 

 60 



40- 



20 







B 



i 



 Control. n=789 

 [H 85 mm long, n= 317 



Itl 



1 1 — ^-n 



Length (cm) 



Figure 6 



Length -frequency distributions of (A) king prawn, Penaeus plebejus, 

 and (Bl red spot whiting, Sillago flindersi, from the control and 

 85-mm-long codends combined across experiments. 



species. Both the 85-mm-wide and 85-mm-long 

 codends performed similarly, significantly reducing 

 the weight of discarded bycatch (by 48% and 38% 

 respectively) with no significant reduction in the 

 weight of prawns or retained species (Fig. 2; Table 

 1 ). The 85-mm-wide codend also significantly reduced 

 the number and weight of red spot whiting by 71% 



and 75%, respectively (Fig. 2; Table 1), whereas, 

 based on the aggregated data, the 85-mm-long codend 

 reduced these by 68% and 58%, respectively (Fig. 5; 

 Table 4). 



Experiment 2 compared the differences between 

 the control and 85-mm-long codends with and with- 

 out a short ( 10-15 s) delay during haulback. The re- 

 sults suggest that the individuals comprising dis- 

 carded bycatch escaped continually throughout the 

 duration of the tow (Fig. 3A; Table 2). Their behav- 

 ior and escape from the square-mesh panel may be 

 explained, therefore, according to the theory dis- 

 cussed above. For red spot whiting, however, there 

 was no significant reduction in number or weight 

 when there was no delay in haulback (although the 

 means were reduced by 21% and 28%, respectively), 

 but there was a significant reduction of 64% and 

 56.6%, respectively, for the tows that included a 10- 

 15 s delay (see Figs. 3 and 6; Table 3). Red spot whit- 

 ing, therefore, did not appear to exhibit the same 

 behavior as the other species because the majority 

 of these fish appeared to escape during the 10-15 s 

 delay in haulback. 



