Broadhurst et al : Effects of twine diameter and mesfi size in tfie body of prawn trawls 



469 



1972), probably contributed to contact of both fish 

 and prawns with meshes in this section of the trawl. 

 A combination of the above effects in the larger-mesh 

 spectra-1-mm and momoi-1.7-mm trawl bodies may 

 have facilitated the escape of smaller individuals. 



It is also likely that a reduction in twine area 

 associated with the larger mesh sizes in both new 

 trawl bodies allowed a faster release of water than 

 did the control trawl body,' possibly contributing 

 to the effects discussed above. In partial support 

 of this hypothesis and although no significant dif- 

 ferences were detected between the spectra- 1-mm 

 and the momoi-1.7-mm trawl bodies for those vari- 

 ables examined (Table 3), the narrower-twine spec- 

 tra-l-mm trawl body consistently reduced a larger 

 percentage of bycatch across a greater range of vari- 

 ables. These variables included the weights of south- 

 ern sand flathead and red mullet and the numbers of 

 southern calamari (Table 2; Fig 3). Further investi- 

 gation, involving a greater number of replicate tows 

 would be required, however, to validate the potential 

 for this effect. 



A contributing operational factor towards the 

 escape offish from both large-mesh trawl bodies may 

 have been the period between the time when the 

 vessel was slowed and when the gear was hauled 

 to the surface (termed "haulback") (Watson, 1989; 

 Broadhurst et al., 1996). Watson (1989) showed that 

 differences in geometry of the trawl and associated 

 water flow during haulback caused fish (that were 

 still swimming in the trawl) to become disorien- 

 tated and to attempt escape through the surround- 

 ing meshes. More specifically. Workman and Taylor 

 ( 1989) observed that numerous small individuals of school- 

 ing species such as Carangidae (e.g. T}-achurus lathami 

 and Decapterus punctalus ) escaped during this period and 

 that larger individuals were often caught by their gills in 



K No of blue swimmer crab 



M Wt of souttiern calamari 



o 

 O 



o 

 O 



J Wt of red mullet 



oo-i^* 



L No of southern 



6 



4- 

 2 







I 



calamari 



N No of small-toothed 

 flounder 



03- 



0,1- 



I 1 

 -- o 

 ra O 



E o 



E 

 o 



Figure 3 (continued) 



Harrington, D. L., J. W. Watson, L. G. Parker. J. B. Rivers, and 

 C.W.Taylor. 1988. Shrimp trawl design and performance, 37 

 p. University of Georgia Marine Extension Ser\-ice, 715 Bay St., 

 Brunswick, Georgia 31520-4601. 



