468 



Fishery Bulletin 98(3) 



graded from the control and new trawl bodies ( Fig. 6 ). 

 Both trawls with the larger meshed bodies retained 

 proportionally fewer smaller prawns. 



Discussion 



The results from this study showed that both new 

 trawl bodies were effective in excluding under-size 

 prawns and large numbers of small fish (by up to 

 almost 60'r ) and in not reducing the weight of tar- 

 geted prawns. These results provide evidence to sug- 

 gest that simple changes to the body of prawn trawls 

 can have a contributing effect on overall trawl selec- 

 tivity. The escape of large numbers of small fish and 

 prawns from this area may be due to their behavior 

 in the trawl and the extent to which they were influ- 

 enced by the operational characteristics of the gear. 



It is well established that fish exhibit specific 

 responses to stimuli from trawls and attempt to avoid 

 contact with the trawl body by maintaining position 

 at the opening (Wardle, 1983; Watson, 1989). After 

 some period, depending on species- and size-specific 

 swimming abilities (Wardle, 1975), fish invariably 

 tire and turn towards the codend, allowing the trawl 

 to pass around them (Watson, 1989) or alternatively, 

 maintain swimming in the direction of the tow, but 

 gradually fall back along the taper of the body panel 

 towards the codend opening (Wardle, 1983). As the 

 taper of the trawl body narrows and the density 

 of fish increases, some fish may rise in the trawl 

 and attempt escape through the meshes or pass 

 into the codend and resume swimming immediately 

 anterior to the catch ( Wardle, 1983 ). In contrast, ben- 

 thic invertebrates like prawns tend to display lim- 

 ited responses during capture. SCUBA observations 

 by Watson ( 1976 1 showed that after contact with 

 the leading edge of the trawl, penaeid prawns con- 

 tracted their abdomens ventrally, propelling them- 

 selves backwards. This initial response was repeated 

 three to five times but because prawns are not capa- 

 ble of maintaining such activity, the speed of the 

 trawl through the water quickly forced them against 

 the meshes of the trawl body and they eventually 

 tumbled down the net into the codend. 



In view of these behavioral patterns, gear-related 

 factors, such as the fast towing speed ( 1.5 mJs) and 

 taper of the trawl body in our study, may have 

 increased the probability of small fish and prawns 

 being selected in this area. For example, the sizes of 

 most of the fish encountered, and particularly sand 

 trevally, leatherjackets, and small southern sand 

 flathead (5-15 cm, [Fig. 4]), means that they would 

 have been unable to maintain position in the moving 

 trawls. Studies quantifying the swimming speeds of tele- 

 ost fish suggest that although individuals 5 and 15 cm long 

 may be expected to have burst speeds of 0.5 m/s and 1.5 m/s 

 over very short periods, their normal maximum swim- 

 ming performance (or maximum cruising speed, Wardle, 

 1983) would be much less (Bainbridge, 1958; Wardle, 1975, 



A No of prawns 



B Wt of prawns 



E 600 - I H- 



In I 

 .JJL. 



C Wi of total discarded 



25 -| bycatch 



— 



E No of leatherjackets 

 800 1 



600- . rp 



400 -^H ^M 



:ll I II 



r+i 



D wt of discarded 



1 2-1 noncommercial bycatcfi 

 1 



08 



06 

 04 

 02 

 00 



300 



200 



F No of sand trevally 



c 



i 100-1 



' „]iLL 



G No of southern sand flathead 



I 



LLiU 



o E o 



»- o 



CO O 



E 

 E 



T^ O 



Figure 3 



Differences in mean catches (per 25 niin tow ±SE) between the 

 new trawl bodies and control for the (Ai numbers and (Bi weights 

 of king prawns iPenaeus latisulcatus); the weights of (C) total 

 discarded bycatch and (D) discarded noncommercial bycatch; the 

 numbers of lEi leatherjackets tThamnaconiif: degeni), (F) sand 

 trevally iPscudocaranx wrighti) and (G) southern sand flathead 

 iPlatycephalus bassensis); the weights of (Hi southern sand flat- 

 head; the numbers (I) and weights (Ji of red mullet iUpenewh- 

 thys porosus): the numbers of (K) blue swimmer crabs iPortunus 

 pelagicus); the numbers (L) and weights of (Ml southern calamari 

 ^Sepioteuthis australis); and the numbers of small-toothed floun- 

 der iPseudorhnmhus jenynsii). Significant differences are indi- 

 cated by shaded histograms. 



1983; Beamish, 1978). As a consequence, in our study some 

 fatigued individuals probably came in contact with meshes 

 in the body panels as they were herded towards the codend 

 opening. Further, the relatively steep tapers (i.e. 1N4B — 

 see Broadhurst et al., 1999) of the trawl bodies (compared 

 with those used in most other prawn-trawl fisheries, FAO, 



