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



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Figure 7 



Plots of the deviance residuals from selection curves for the (A) composite-square-2 

 codend and (Bl composite-square-3 codend. 



One hypothesis to explain this result is that the 

 large, open, square meshes in the composite square- 

 mesh codends, combined with the overall size of the 

 codends (approx. 4 m in length) allowed a faster re- 

 lease of water from the body of the trawl than did 

 the control. Such an increase in flow may have re- 

 sulted in prawns quickly passing into the codend af- 

 ter initial contact with the footrope, with less chance 

 of randomly escaping over the headrope or out through 

 the mouth of the trawl. In support of this, Walsh et al., 

 (1992) provided evidence of similar flow-related ef- 

 fects on the catches offish (American plaice, Hippo- 

 glossoides platessoides) from trawls fitted with 

 square-mesh codends. In the present study, however, 

 any potential effects of increased flow did not appear 

 to influence fish such as small-toothed flounder (the 



only species that could not pass through the square 

 mesh), because there were no significant differences 

 in the catches between the composite square-mesh 

 and control codends ( Table 1 ). An alternative hj^joth- 

 esis proposed by Broadhurst and Kennelly (1997) to 

 describe a similar increase in catches of prawns from 

 NSW oceanic prawn trawls containing codends with 

 composite square-mesh panels is that by reducing 

 the weight of bycatch and therefore the drag in the 

 codend, trawls with the composite square-mesh 

 achieved slightly more swept area than did controls, 

 thereby covering more of the sea bed and capturing 

 more prawns. 



Regardless of the underlying mechanisms, the in- 

 crease in catches of target-size prawns, combined 

 with the substantial reductions in the bycatch offish, 



