464 



Fishery Bulletin 98(3) 



of the few studies on the effects of changes in the types of 

 twine used in the body of trawls on catches, Sumpton et 

 al. ( 1989) showed that compared with muUifilament twine 

 (1.1 mm diameter), significantly more smaller-size prawns 

 (Penaeus spp. ) and squid (Loligo spp. ) escaped through the 

 bodies of trawls made from monofilament twine (0.9 mm 

 diameter). 



Despite these results and although nearly all prawn- 

 trawl fisheries are regulated by means of legally defined 

 minimum mesh sizes (typically ranging from 40 to 50 

 mm stretched mesh, Vendeville, 1990), there is a paucity 

 of information on the effects of different sizes of mesh 

 used in the bodies of trawls on selectivity. This is particu- 

 larly the case in Australian prawn-trawl fisheries, where 

 despite extensive research to develop BRDs (see Broad- 

 hurst, 2000), no published studies have quantified the 

 effects of different sizes of mesh. In a recent experiment in 

 Gulf St. Vincent, South Australia (Broadhurst et al., 1999), 

 we provided evidence to suggest, however, that the mini- 

 mum size mesh (45 mm) throughout the trawls was too 

 small. As a first step in addressing this issue, we investi- 

 gated modifications to the codends and demonstrated that 

 new designs comprising composite panels of different-size 

 square-shape mesh (52 and 80 mm mesh hung on the bar) 

 were effective in significantly reducing bycatches of under- 

 size western king prawns i Penaeus latisulcatus ) and small 

 fish without reducing the weight of the commercial catch. 



These results, combined with the simplicity of the new 

 codend, led to the immediate and unanimous adoption 

 of a design, based on those tested, by Gulf St. Vincent 

 prawn trawlers. Encouraged by the performance of simple 

 changes in mesh type and size to improve selectivity, com- 

 mercial fishermen sought assistance to examine other 

 refinements to their trawls. Given evidence to suggest that 

 the mesh size used in Gulf St. Vincent was inappropriate 

 and that selection for some species may occur in the body 

 of prawn trawls, our aims in the present study were to 

 quantify the influences on selection in this area due to 1 ) 

 increasing the mesh size (from approx. 45 mm to 53 mm) 

 and 2) reducing the diameter of twine used. 



i 



34P(X)'S - 



Adelaide 



Figure 1 



Location of study at Gulf St. Vincent, South Australia. 



Materials and methods 



Our work was done in Gulf St. Vincent, South Australia 

 (Fig. 1), in October, 1998, with a chartered commercial prawn 

 trawler rigged to tow three trawls in a standard triple gear 

 configuration (see Andrew et al., 1991, for details). Three 

 different trawl bodies were examined. The first (termed the 

 "control") represented conventional trawl bodies ( see Broad- 

 hurst et al., 1999, for specifications) and was constructed 

 of 1.7-mm-diameter, 24-strand, polyethylene twisted twine 

 with a mean mesh size (stretched distance between the 

 knots) of 44.42 mm (see "Results" section). The second and 

 third trawl bodies (termed "spectra- 1-mm" and "momoi- 

 1.7-mm") were identical in design, in headrope and footrope 

 length, in rigging and tapers to the control trawl body but 

 were made from 1-mm-diameter, polyethylene cabled twine 

 (brand name "spectra") and 1.7-mm-diameter, .30-ply, poly- 

 ethylene twisted twine ( brand name "momoi" i. respectively. 



with mean mesh sizes of 52.43 and 52.96 mm, respectively 

 (see "Results" section). 



All three trawl bodies were attached to identical com- 

 posite square-mesh codends (Broadhurst et al., 1999) 

 consisting of two sections, each measuring 80 bars in cir- 

 cumference, 70 bars in length, and constructed of 52-mm 

 mesh (3-mm-diameter, braided polyethylene twine) cut on 

 the bar (Fig. 2, A-C). Each square-mesh section consisted 

 of an upper and lower panel sewn together so that the 

 direction of the knots on the upper panel were opposite 

 those on the lower panel (Fig. 2A). A panel of 104-mm net- 

 ting, measuring 6 bars x 10 bars, was inserted into the 

 tops of the posterior sections of each square-mesh codend, 

 starting at the leading edge and ending about 1.3 m ante- 

 rior to the last row of meshes ( Fig. 2, A and B ). Lengths of 

 12-mm-diameter, polyethylene 3-strand rope ( termed "last- 

 ridge ropes") were firmly laced at a hanging ratio of 0.9 to 

 each of the two lateral seams of the codends (to provide 

 length-wise strength ) ( Fig. 2B ). 



