Broadhurst et aL: Use and success of square-mesh codends in reducing bycatch and improving size-selectivity of prawns 



435 



and Larsen, 1989; Tokai et al., 1990; Thorsteinsson. 

 1992;Tokai and Sakaji, 1993). However, to minimize 

 biological impacts on bycatch and to promote ecologi- 

 cally sustainable fisheries, prawn trawls should be 

 modified to address both these issues. Furthermore, 

 it is essential that the prawn industry actively con- 

 tributes towards the development of such designs so 

 that they will be more readily accepted and adopted 

 as part of commercial operations (Kennelly and 

 Broadhurst, 1995). 



In some prawn-trawl fisheries, industry has re- 

 sisted gear modifications because of negative effects 

 on gear performance (e.g. Renaud et al., 1993; Rob- 

 ins-Troeger et al., 1995). However, an inexpensive 

 and simple modification that has been successfully 

 tested and adopted in fish trawls in the North At- 

 lantic involves changing only the configuration of 

 mesh in the codend by hanging conventional dia- 

 mond-shaped mesh on the bar (termed "square 

 mesh") (e.g. Robertson and Stewart, 1988; Fonteyne 

 and M'Rabet, 1992; Suuronen and Millar, 1992). Pan- 

 els of square mesh have also been used successfully 

 in prawn trawls to improve target-selectivity and to 

 reduce the bycatch of juvenile fish. For example, in 

 the Icelandic shrimp, Pandalus borealis. fishery, 

 Thorsteinsson (1992) found that codends made en- 

 tirely from square mesh effectively reduced the 

 catches of small prawns (10-20^^ of catch) and 0- 

 group gadoids. In New South Wales (NSW), Austra- 

 lia, Broadhurst and Kennelly (1996; 1997) showed 

 that strategically located composite panels of square 

 mesh (BRDs) in diamond-mesh codends effectively 

 excluded large quantities of unwanted fish and there 

 was no concomitant reduction in catches of the tar- 

 geted prawns, Penaeus plebejus. 



Through industry involvement in research, these 

 sorts of results have facilitated the rapid endorse- 

 ment and acceptance of designs in the respective fish- 

 eries and in some cases, in other fisheries sharing 

 similar characteristics (e.g. similar gear configura- 

 tions, target species, etc). An example of this type of 

 transfer of technology recently occurred in the Gulf 

 St. Vincent prawn-trawl fishery in South Australia. 

 This fishery currently involves ten vessels targeting 

 western king prawns, Penaeus latisiilcatits; some le- 

 gally retained bycatch comprising balmain bugs, 

 Ibacus spp., and southern calamari, Sepioteuthis 

 australis are also taken. Operators are restricted by 

 means of a series of controls that include a minimum 

 mesh opening of 45 mm in the codend and a limit of 

 34 nights of trawling each year. Although no formal 

 studies have been undertaken to determine the se- 

 lectivity of the conventional gear or to quantify 

 bycatches, in recent years fishermen have suggested 

 that the mesh size is too small and retains unaccept- 



Lenqth in 

 meshes 



70 



150 



30 



Length in 

 meshes 



111 



160 



Total Total 



81 B 



ION 



61 B 



ION 



i 



120 8 



120, 



120 



121 



30 ply. 52 mm 

 mesh on the bar 



48 ply. 45 mm 

 mesh 



30 ply, 52 mm 

 mesh on the bar 



48 ply, 45 mm 

 mesh 



Figure 1 



Diagrammatic representation of a "Gundry prawn trawl" 

 and the industry developed "square-mesh- 1 codend". T = 

 transversals; N = normals; and B = bars. 



able proportions of juvenile prawns and fish. During 

 the 1996-97 season, in an attempt to increase the 

 mean sizes of prawns caught, fishermen designed and 

 incorporated full square-mesh codends in all their 

 trawls (see Fig. 1). 



Anecdotal reports from industry suggested that 

 this codend (termed the "square-mesh- 1 codend") re- 



