Broadhurst et al.: Effects of twine diameter and meshi size in the body of prawn trawls 



465 



104-mm-mesh panel 



A ^ 



Lastridge ropes 



Figure 2 



Diagi'ammatic representation of (Al panels and sections of the composite square-mesh codend 

 tB=bars), (B) completed composite square-mesh codend, and (C) prawn-trawl body and codend used 

 in the study- B = bars. 



Experimental procedure 



Before starting the experiment, all trawls were towed for 

 a short period to allow knots and bindings to stretch. To 

 obtain accurate information on mesh sizes, a set of dial 

 calipers was used to measure 30 randomly located meshes 

 (stretched length in mm, between the inside knots I at four 

 separate locations (starboard wing, footrope, headrope, 

 and posterior body) in each trawl body. 



The spectra- 1-mm and momoi-1.7-mm trawls were com- 

 pared separately against the control trawl. In each paired 

 comparison, the trawls were shackled to the outside sweeps 

 of the sleds and otterboards of the triple rigged gear and 

 towed simultaneously. A conventional trawl body was used 

 as the center net in all comparisons, but because it was 

 not rigged in exactly the same manner as the outside nets, 

 its catch was excluded from analysis. The triple gear was 

 towed in normal commercial tows of 25-min duration at 

 3 knots (1.5 m/s) over a combination of sandy and light 

 coral bottoms. Each of the outside trawls were randomly 

 assigned after each tow ( to eliminate any potential biases i, 

 so that three paired comparisons of each new trawl against 

 the control trawl were made on each night (i.e. a total of 

 six tows per night ). Over five nights, we completed a total 

 of 15 replicate comparisons of each configuration. 



After each tow, the catches from the two outside trawls 

 were emptied onto a partitioned tray. Data collected from 

 each tow were the following: the total weight of western 

 king prawns and a subsample ( 100 prawns from each trawl ) 

 of their carapace lengths (to the nearest 1 mm); the total 

 number of prawns (estimated from the weight of the sub- 

 sample ); the total weight of discarded bycatch and discarded 

 noncommercial bycatch; the weights and numbers of com- 

 mercially or recreationally (or both) important bycatch spe- 

 cies; and the sizes (to the nearest 0.5 cm ) of commercially or 

 recreationally (or both) important fish. A random sample of 

 prawns (approximately 12 kg) from each trawl in each tow 

 was separated and sent to A. Raptis & Sons PTY LTD ( sea- 

 food processing plant ) in Adelaide for grading into "commer- 

 cial categories" ( based on a system of numbers per pound ) 

 with a locally built "dynamic grading machine." 



Several commercially important species were caught in 

 sufficient quantities to enable meaningful comparisons. 

 These were western king prawns ^Penaeug lafisulcatus), 

 blue swimmer crabs iPortunus pelagicusK sand trevally 

 (Pseudocaranx icrighti), red mullet iUpeneichthyf: poro- 

 sus), leatherjacket iThamnaconus degeni), southern sand 

 flathead iPlatycephalus bassensis). small-toothed flounder 

 {Pseudorhombusjenynsii ), and southern calamari iSeploteii- 

 this australis}. With the exception of western king prawns 



