Modify the design or configuration of trawl doors
Overall effectiveness category Awaiting assessment
Number of studies: 3
Background information and definitions
Trawling is a method of fishing that involves pulling a fishing net (trawl) through the water behind one or more vessels. Trawl doors are large wooden boards or metal plates (otter boards) attached to the mouth of the trawl net by “bridles” and keep the mouth of the trawl net open as it is pulled through the water. As they are towed through the water, trawl doors and the attached bridle system cause physical damage to the seabed and herd fish into the path of the trawl (Dickson 1993). The design of trawl doors can be modified, for instance by using smaller otter boards, mounting otter boards on a sled, or changing the angle of towing attack, to reduce sediment penetration. Reducing physical contact with the seabed, and therefore sediment penetration, may reduce the herding effect of the doors and bridles and the amount of unwanted fish that move into the path of the trawl gear.
For related interventions describing other modifications to the overall design of different types or parts of trawl gear is summarized under ‘Fishing gear modification - Modify the design or configuration of trawl gear (mixed measures)’, ‘Change the size of the main body of a trawl net’, ‘Decrease the circumference or diameter of the codend of a trawl net’, ‘Modify a bottom trawl to raise parts of the gear off the seabed during fishing’ and ‘Modify design or arrangement of tickler chains/chain mats in a bottom trawl’.
Dickson W. (1993) Estimation of the capture efficiency of trawl gear. II: Testing a theoretical model. Fisheries Research, 16, 255–272.
Supporting evidence from individual studies
A replicated, controlled study in 2014 of an area of seabed in the Tasman Sea off New South Wales, Australia (Broadhurst et al. 2015) found that triple rigged otter trawls with different otter board designs (two types) caught higher amounts of one of six discarded fish species, and similar amounts of discarded catch overall (fish and invertebrates combined), compared to a conventional door type. For unwanted fish, catch rates of only one of six species, spiky flathead Ratabulus diversidens, differed between door designs and was lower with the conventional doors than either of the non-conventional designs (restrained: 0.03, batwing: 0.07, conventional: 0.01 kg/ha). Catches of long spine flathead Platycephalus longispinis, small-toothed flounder Pseudorhombus jenynsii, red bigeye Priacanthus macracanthus, long-finned gurnard Lepidotrigla argus and eastern bluespotted flathead Platycephalus caeruleopunctatus were similar between door designs (data reported as statistical model results). The average catch rates of all unwanted catch (fish and invertebrates combined) was not significantly different between “restrained” flat, rectangular doors (2.6–3.2 kg/ha), “batwing” doors (3.2–3.5 kg/ha) and the conventional flat, rectangular doors (4.1–4.6 kg/ha). In addition, catches of commercial target eastern king prawns Penaeus plebejus were similar across trawl designs (see paper for data). Fifteen trawl deployments/door type were made in July 2014 using a triple-rigged trawl fitted with either conventional flat-rectangular otter doors (2.0 × 0.8 m, angle of 42°), “restrained” flat-rectangular doors (same doors as conventional with a line attached to the central sleds to minimise door spread), or “batwing” doors, comprising a plastic “sail” mounted on a main stainless steel sled at an angle of 20°. Trawls were towed for an average of 8.6 km at an average depth of 55 m. Full details of trawl design are provided in the original study.Study and other actions tested
A replicated, paired, controlled study in 2013 in an area of seabed in the Clarence Estuary, New South Wales, Australia (McHugh et al. 2015) found that using a novel “batwing” design of otter boards on a shrimp trawl did not reduce the unwanted catch of four of four fish species or the overall discarded catch (fish and invertebrates), compared to three different conventional otter board designs. Average catch number of four of four unwanted fish species was similar between the batwing design and the three other conventional designs: forktail catfish Neoarius graeffei (batwing: 6.4, flat-rectangular: 7.7, kilofoil: 6.7, cambered: 5.6 fish), southern herring Herklotsichthys castelnaui (batwing: 2.7, flat-rectangular: 1.6, kilofoil: 1.1, cambered: 1.9 fish) mulloway Argyrosomus japonicas (batwing: 1.9, flat-rectangular: 1.8, kilofoil: 2.1, cambered: 1.7 fish) and yellowfin bream Acanthopagrus australis (date reported as model results). Average catch weight of all unwanted catch (fish and invertebrates combined) was similar with the batwing design (0.5 kg) compared to the flat-rectangular (0.4 kg), kilofoil (0.3 kg) and cambered board designs (0.4 kg). The batwing design comprised a sled and sail on a steel boom and mast (61 kg, 1.1 × 1.2 m) set to remain at a constant angle of 20° from the towing direction. The conventional designs were standard flat-rectangular boards (52 kg, 1.4 × 0.6 m), steel kilofoil boards with three vertical foils in a rectangular frame (63 kg, 1.3 × 0.6 m) and cambered boards with a single cambered foil over the boards length (53 kg, 1.1 × 0.7 m). Twenty-four 30-min paired trawl deployments (blocks of two door types towed from each side of the vessel) were performed with each board design on a 10 m trawler in depths of 4–18 m during May 2013 using a 41 mm mesh.Study and other actions tested
A replicated, controlled study in 2014 in lagoon waters in Lake Wooloweyah, New South Wales, Australia (McHugh et al. 2015) found that using an alternative “batwing” trawl door design reduced the number of one of five unwanted fish species caught, but not the overall amount of unwanted fish caught, compared to a conventional door type. Compared to the conventional door design, the batwing design reduced the number of bridled gobies Arenigobius bifrenatus caught/500 m by 25%, but catches of unwanted southern herring Herklotsichthys castelnaui, pink-breasted siphonfish Siphamia roseigaster and Australian anchovy Engraulis australis were similar (data reported as statistical model results). Whitebait Hyperlophus vittatus catch increased by 91%. The total unwanted fish catch was similar between batwing and conventional designs (data reported as statistical model results). In addition, catch weight/500 m of the commercial target school prawns Metapenaeus macleayi was reduced by 72%. During the Australian autumn in 2014, two pairs of otter boards (batwing and conventional) were deployed one at a time on a 6 m beam trawl from a 10 m trawler in 1-2 m of water. Batwing boards had a main sled at 0° angle of attack and a sail offset by 20° attached to a boom and mast, with an area of 0.74 m2. The conventional design was flat-rectangular otter boards with an area of 0.77 m2 and a 35° angle of attack (see original paper for gear specifications). Thirty-six replicate trawls were completed using each pair of boards.Study and other actions tested