Relative engineering and catching performances of paired penaeid-trawling systems

Two paired-trawl spreading mechanisms were compared, with a view towards progressing more environmentally benign penaeid-trawling systems. Identical twin trawls (7.35-m headline) were alternately towed in a “double rig” (four otter boards) with either 18- or 36-m bridles (“short-” and “long-” double rigs) and in a “dual rig” (36-m bridles with two otter boards and a sled). At similar speeds (∼1.31 m s−1), wing-end spread per trawl was significantly greatest in the long-double (mean ± SE: 4.50 ± 0.05 m), followed by the short-double (4.27 ± 0.05 m) and dual (3.86 ± 0.05 m) rigs. The dual rig required 24% and 20% less towing force and fuel and caught significantly fewer (by ∼60%) small mulloway, Argyrosomus japonicus per ha of foot-rope contact than both double rigs. Other standardised catches were not significantly different among configurations, although many, including school prawns, Metapenaeus macleayi were lower (by 25–39%) in the dual rig – which manifested as a relatively greater quantity of fuel required per kg caught (up to ∼1.5 times more). When school-prawn catches were combined across double rigs and compared against the dual rig, the latter caught significantly larger individuals, but fewer per ha of foot-rope contact (by 35%); an effect that was negated when catches were re-standardised to the contact of the foot-rope and otter boards. The results show that removing otter boards reduces benthic contact, towing force, fuel consumption and bycatch, but there are negative implications for penaeid catches that may require compensatory offsets.

► Dual- and double-rigged penaeid trawls were compared for engineering and catching performances.
► The dual rig required 20% less fuel and caught 60% fewer small Argyrosomus japonicas per ha trawled than the double rig.
► Mean standardised catches of the targeted Metapenaeus macleayi were also lower (by 25–39%) in the dual-rigged trawls.
► The results were attributed to the removal of two otter boards (dual rig) and their effects on herding organisms.