463 



Abstract.— The influences on catches 

 ;ind bvcatches due to D an increase in 

 size of mesh and 21 a reduction in twine 

 diameter in the body of prawn trawls 

 were investigated in Gulf St. Vincent. 

 Australia. Compared with a conven- 

 tional trawl body (mesh size 45 mm) 

 attached to a composite square-mesh 

 codend. two new trawl bodies, made 

 with 53-mm mesh but with different 

 twine diameters ( 1 and 1.7 mm. respec- 

 tively) and each attached to identical 

 composite square-mesh codends. were 

 equally effective in significantly reduc- 

 ing the numbers of a range of small 

 fish (by between 23.79?- and 67%) and 

 in not significantly reducing the weight 

 of targeted prawns. Because there were 

 no significant differences in the selec- 

 tivity parameters of both new trawl 

 bodies, these results indicated that the 

 main cause of bycatch reduction was 

 the increase in size of mesh in the body 

 of the trawl. The escape of large num- 

 bers of unwanted small fish and prawns 

 is discussed in terms of their probable 

 behavior in the body of the trawl and 

 the extent to which this behavior was 

 influenced by the operational charac- 

 teristics of the gear 



Effects of twine diameter and mesh size 

 in the body of prawn trawls on bycatch 

 in Gulf St. Vincent^ Australia 



Matt K. Broadhurst 



Departamento de Pesca, Laboratorio de Oceanografia Pesqueira 



Universidade Federal Rural de Pernambuco-UFRPE 



Av Dom Manuel de Medeiros, s/n 



Dois Irmaos, Recife-PE, Brasil, CEP: 52 171-900 



E-mail address fhvha21nalruenetcom.br 



Paul E. McShane 



South Australian Aquatic Sciences Centre 



PO. Box 120 



Henley Beach, South Australia 5022, Australia 



Roger B. Larsen 



University of Tromse 

 Breivika N-9037 

 Troms0, Norway 



Manuscript accepted 1 December 1999. 

 Fish. Bull. 98:463-473 (2000). 



In many of the world's prawn-trawl fish- 

 eries, large numbers of organisms are 

 captured incidentally with the targeted 

 catch (collectively termed "bycatch" 

 sensu Saila, 1983). This bycatch com- 

 prises a diverse assemblage of small fish, 

 cephalopods, and crustaceans, includ- 

 ing prawns that are smaller than opti- 

 mum size (for reviews see Saila, 1983: 

 Andrew and Pepperell. 1992; Alverson 

 et al., 1994). Concerns over the negative 

 effects that the mortality of these fish 

 and small organisms may cause on the 

 yield of stocks has led to the applica- 

 tion of various management strategies 

 designed to minimize bycatch (Andrew 

 and Pepperell, 1992 1. The most common 

 approach has been to change the con- 

 ventional trawls to improve overall 

 selectivity of the gear. Depending on 

 the sizes and species to be targeted and 

 excluded, these changes have involved 

 the sizes and types of mesh used, and 

 the application of physical modifica- 

 tions, collectively termed bycatch reduc- 

 ing devices (BRDs) (see Broadhurst. 

 2000. for review). 



Inherent variations among the char- 

 acteristics of different prawn-trawl fish- 

 eries have resulted in a range of modi- 

 fications to conventional trawls. Regard- 

 less of design, however, the majority of 

 functional modifications have been ap- 



plied within or immediately anterior to 

 the codend (e.g. Isaksen et al., 1992; 

 Thorsteinsson, 1992; Broadhurst and 

 Kennelly, 1997; Rogers et al., 1997) 

 mainly because observations suggest 

 that most of the selection process for 

 many species occurs in this area (Arm- 

 strong et al., 1990; MacLennan, 1992; 

 Wileman et al., 1996). Moreover, be- 

 cause codends often are similar among 

 the different trawl designs within a par- 

 ticular fishery (e.g. Broadhurst and 

 Kennelly, 1997), any modifications to 

 improve selectivity are more easily imple- 

 mented, adopted, and regulated through- 

 out the fishery. 



Although alterations to codends have 

 been successful in reducing various sub- 

 sets of bycatch, there is evidence sug- 

 gesting that individuals of some species, 

 particularly those of prawns, escape 

 from the bodies of prawn trawls ( High et 

 al.. 1969; Sumpton et al, 1989; Vendev- 

 ille, 1990). For example. High et al. 

 (1969) attached various covers to the 

 upper, lateral, and posterior bodies of 

 trawls to isolate areas that could be 

 modified to passively separate fish from 

 prawns iPandalus sp. ). Although there 

 was little evidence of fish escaping, 

 large numbers of small prawns were 

 retained in the covers, particularly in 

 the posterior sections. Similarly, in one 



