The relationship between the soft-brush ground gear and dragis somewhat more complex. In addition to the longitudinal bot-tom contact by the 4-mm chains, there were hydrodynamic forcesacting on the floats; best illustrated by the large difference infrontal area of this configuration (0.44 m2) compared to the others(0.09–0.15 m2). Specifically, while the bottom contact was some 63% less than all the ground chains, the overall drag was comparable to the 8-mm chain. It might be possible to reduce the drag of the soft-brush to a level approaching the 6-mm or even lower, via different float sizes and/or a more streamlined design. For exam-ple, the floats might be turned 90o, or made thinner to present lessfrontal area
.Any changes to the floats of the soft-brush ground gear would require careful consideration in terms of their speciesspecific effects on catches. Specifically, the floats would have reduced theescape area below the foot rope, which probably explains the signif-icant increase in catches of forktail catfish by this design; either dueto added visual stimulus, or simply tactile responses if collisionsoccurred. Similar effects might also account for the significantlygreater catches of this species in trawls with the 10- rather thanthe 6-mm ground gears. Given the above, simply minimising float size could reduce the catches of fork tail catfish, but could similarly negatively impact on catches of school prawns, although the exact mechanisms probably differ (Watson, 1989).
The behavioural response of school prawns to trawls has not been studied, but work with other penaeids offers insight into theirlikely responses. For example, it is well established that during thedaylight, most penaeids remained burrowed or close to the sub-stratum (Ruello, 1973; Coles, 1979) and so their entry into trawlsrequires sufficient tactile stimuli to initiate an escape response(involving a contraction of the abdomen). Such stimuli usually isprovided by a tickler chain and/or a ground gear with continuousand sufficient bottom contact (Watson, 1989)—a requirement thatmight explain the slight reduction in catches (although not signif-icant and with no effect on sizes) of school prawns in the trawlswith the light 6-mm chain