Broadhurst et al : Flow-related effects on prawn-trawl codends 



Codend 



Composite square-mesh panel 



Semiporous panel of mesh 



Figure 4 



Diagrammatic representation of proposed modification to codends ( 

 taining the composite square-mesh panel. 



posite square-mesh panel. It is apparent that to 

 maximize the effects of anteriorly displaced water 

 in the posterior section of codends, the composite 

 square-mesh panel should be located as close as pos- 

 sible to the end of the codend, but sufficiently in front 

 of the anticipated build-up of catch to prevent prawns 

 from accumulating past the square meshes and escap- 

 ing through them. A solution to this problem would be 

 to increase the codend mesh circumference in the pos- 

 terior panel (i.e. the 200 panel codend), causing the 

 catch to spread laterally in the back of the codend, 

 rather than to accumulate in front. Although such a 

 modification would also increase surface area, displace- 

 ment of water forwards, and probably enhance fish es- 

 cape through the square-mesh panel, it would also re- 

 duce mesh openings and the selectivity of the codend 

 itself (Broadhurst and Kennelly, 1996). 



An alternative modification that may increase dis- 

 placement of water forwards (other than increasing 

 codend circumference and catch) is to move the com- 

 posite square-mesh panel forward in the codend and 

 create areas of "artificial catch" by using semiporous 

 panels (e.g. those with fine mesh). These could be 

 positioned on the bottom of the codend, behind the 

 panel and at an angle to the direction of tow (e.g. see 

 Fig. 4). Such modifications would produce similar 

 flow-related effects as those observed in our study, 

 i.e. they would displace water anteriorly, directing 

 fish towards the panel. Future research into the re- 

 finement of square-mesh panels and other BRDs in 

 prawn-trawls that exploit the behavioral differences 



of fish in trawls, may benefit from these 

 or similar modifications. 



Acknowledgments 



This work was funded by the Australian 

 Fishing Industry Research and Develop- 

 ment Council (grant no. 93/180). Thanks 

 are extended to Bryan MacDonald, Garry 

 Day, and Peter Cover for their assistance 

 in conducting the experiments and to 

 Takafumi Arimoto, John Watson, Jack 

 Forrester, and Will Seidel for providing 

 helpful discussions. 



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