An angle to address benthic-trawl impacts

Benthic trawls are spread by rectangular hydro-vanes (called ‘otter boards’) which are heavy (up to 75% of the total system weight) and leave discernible marks on the substrate. Lowering the otter-board base-plate angle... [ view full abstract ]

Benthic trawls are spread by rectangular hydro-vanes (called ‘otter boards’) which are heavy (up to 75% of the total system weight) and leave discernible marks on the substrate. Lowering the otter-board base-plate angle of attack (AOA; conventionally between 30 and 40o) is one mechanism for potentially reducing habitat impacts among demersal trawls, and with concomitant drag (and fuel) benefits, but there are few quantitative assessments. To address this information deficit, we assessed a novel prawn-trawling otter board, termed the ‘batwing’ (rigged with a base plate at 0o AOA) against a conventional flat-rectangular design (base plate at 35o AOA) for relative drag, catchability and habitat impacts. The batwing maintained spreading force with no significant effects on catches, but had up to 18% less drag and mobilised significantly less benthic material (empty shells, Anadara trapezia and Spisula trigonella and timber fragments) at rates closely correlated to the reduction in base-plate bottom contact (by 87%). Further, of the shells that were displaced, the batwing damaged proportionally fewer. The latter results have encouraging implications for habitat preservation. Specifically, if a typical Australian prawn trawler towing two trawls had its four conventional otter boards (i.e. each with a length of 2.13 m at 35o AOA) replaced with batwings, then the bottom contact of heavy steel (>250 kg being dragged obliquely) would be reduced by ∼19 ha (>10 times the ANZ Stadium oval) over one fishing night (8-hours trawling). If only 110 vessels (∼10% of the national prawn trawling fleet) used the batwings for 120 nights, the total reduction in bottom contact would amount to an area greater than the Australian Capital Territory (2,358 km2). The associated fuel savings (up to 18%) would also be considerable, considering that a prawn trawler can use in excess of 60 l per hour while fishing. While further refinements are required, reducing otter-board AOA via designs like the batwing demonstrates the utility of simple modifications to prawn-trawling systems for holistically addressing environmental inefficiencies.