Broadhurst et a\ Effects of twine diameter and mesfi size in tfie body of prawn trawls 



471 



45 



Carapace lengtfi (mm) 



Figure 5 



Plots of the selection curves of western king prawns ^Penaeus latisulca- 

 tus) for the spectra-1-mm and momoi-1.7-mm trawl bodies. 



trawl comprising a body of 45-mm mesh attached to a similar 

 composite square-mesh codend. During this previous study, 

 the codend that showed the most appropriate size-selectivity 

 for prawns tapered ft-om a circumference of 70 bars at the 

 start of the anterior section to 58 bars at the end of the pos- 

 terior section. After extensive commercial testing, however, 

 it was reported that the slight taper in this codend occasion- 

 ally "wedged" the catch in the posterior section, making it dif- 

 ficult to remove the catch when the codend was retrieved and 

 the draw-strings were opened, lb address this problem, fish- 

 ermen widened the circumference to 80 bars throughout the 

 codend. Although such an alteration solved the problems asso- 

 ciated with buildup of catch in the codend ( by allowing it to 

 distribute more horizontally I, it probably lowered the selectiv- 

 ity for prawns ( Broadhurst and Kennelly, 1996 ). Any contribut- 

 ing effects on trawl selectivity due to the larger mesh in the 

 spectra-1-mm and momoi-1.7-mm trawl bodies may have been 

 slightly negated by a reduction in overall selectivity of the com- 

 posite square-mesh codends. This result illustrates the need for 

 ongoing assessments of influences on selectivity of trawls due 

 to subtle modifications to facilitate operational procedures. 



Like the results from other related studies, the signifi- 

 cant reduction in numbers of prawns caught, but not in 

 their weights, has shown that increases in mesh sizes or in 

 openings ( or in both ) can facilitate an increase in catches of 

 target-size individuals (Walsh et al., 1992; Broadhurst and 

 Kennelly, 1997; Broadhurst et al., 1999 1. Possible hypothe- 

 ses to explain this effect include 1) a faster release of water 

 from the larger-mesh trawls resulted in prawns quickly 

 passing into the trawl after initial contact, with less chance 

 of escaping over the head rope and out through the mouth 

 of the trawl or alternatively, 2) less drag in the larger- 

 mesh trawls, due to less twine area and amount of bycatch, 

 allowed the mouth of the trawl to spread wider, thereby 



covering more of the sea bed and capturing more prawns. 

 Of these two hypotheses, the potential for an increase in 

 spread is less likely because there were no significant dif- 

 ferences in the catches of blue swimmer crabs or small- 

 toothed flounder (species that could not pass through 

 either the trawl bodies or composite square-mesh codends) 

 between any of the trawls examined. 



Because there were no significant differences detected in 

 the weights of prawns captured between the various trawls, 

 a further increase in the size of mesh in the body of the trawl 

 (e.g. 60 mm) warrants investigation. Given the results pre- 

 sented in our study, this increase in mesh size could facili- 

 tate a greater release of small fish and further improve size 

 selectivity for the targeted prawns. Alternatively, it may be 

 feasible to examine the utility of trawl bodies comprising 

 composite panels of larger mesh, particularly in the poste- 

 rior section, because it is apparent that as fish are herded 

 together in this area their densities increase, resulting in 

 random attempts at escape through the sides of the trawl 

 (Wardle, 1983). In addition, any fatigued fish still swimming 

 in the posterior section of the trawl during haulback may 

 have an opportunity to escape through these larger meshes. 



Although not mandated, the results from our study have 

 led to the use of large-mesh trawl bodies by many of 

 the fishermen operating in Gulf St. Vincent. Combined 

 with the contributing effects on bycatch reduction due 

 to the composite square-mesh codend, this modification 

 should result in a comparatively more selective prawn- 

 trawl fishery. For example, in a review of literature quan- 

 tifying bycatches from prawn-trawl fisheries throughout 

 the world, Andrew and Pepperell ( 1992 ) reported that typ- 

 ical bycatch-to-prawn ratios in similar temperate fisher- 

 ies were in the order of 5:1. From the mean catch per tow 

 from the control and the new trawl bodies in our study, 



