Twin Rig Trawling is used by demersal trawlers as a modern variation and expansion of the single demersal trawl. Generally speaking, twin rig gear operates with a lower headline height, so is best suited for species which stay close to or on the seabed. In Scotland, twin rig gear is used to target species such as nephrops, flat fish and monkfish.
The gear is rigged in a similar way to a single demersal trawl, in that trawl doors are used to provide the spread on the outer wings of the pair of nets. The main difference is use of a central wire from the boat to a large weight (the clump) located on shorter bridles between the two nets. This weight is often fitted with rollers or wheels to prevent unnecessary digging in on the seabed, and to help reduce drag.
a. Trawl door
b. Central warp
|Potential Impacts (Biological/Environmental)||Gear Selectivity||Regulation|
• Removal of and damage to sedentary marine organisms such as seaweed/corals
• Capture and removal of small sized marine organisms and non-target species
• Capture/discarding of undersized target species
• Damage to seabed strata
• Disturbance of bottom sediments
• Additional damage though the use of weighted clump between trawl nets.
• Mesh size
• Use of square mesh panels
• Trawl door design
• Control of headline height
• Design/construction of footrope materials
• Minimum mesh size
• Minimum fish sizes
• Inclusion of square mesh panels
• Maximum twine thickness
• Net attachments must meet specific regulation conditions to prevent obstruction to net meshes, eg use of bag strengtheners
• Maximum _ of meshes in bag circumference
• % of catch mix retained on board
Improvement measures and initiatives taken by Scottish Fishermen
• Progressive increases to mesh size has reduced undersized capture and discards. Cod end mesh size has increased to 120mm for basic towed gear. This has increased the age of capture of most demersal species of fish.
• Introduction through regulation of compulsory square mesh panels (SMPs) has greatly improved selectivity. Further research into different mesh size SMPs and alternative positions in net configuration were carried out in 2006 with Scottish industry and are ongoing.
• Towed gear SMPs have a minimum dimension of 80mm or 90mm depending on trawl type. Many Scots fishermen now operate voluntarily with SMPs of up to 120mm, which increases the escape capacity of the panel.
• Twine materials have improved, allowing net manufacturers to construct trawls from thinner twine, which in turn makes them lighter and gives less drag under tow, hence interaction and damage to the seabed is reduced.
• Many vessels now use footropes equipped with larger diameter discs, which raise the footrope from the seabed. This reduces seabed damage and can acts as a selectivity aid to allow bottom-dwelling fish species to avoid capture.
• Many Scottish nephrops boats operating twin rig gear have reduced the overall size of the nets by 30-35%. Smaller nets also require lighter warps and sweeps and smaller trawl doors. Lightening gear reduces drag and consequent damage to the seabed and with less weight to tow through the sea, boats save energy (fuel) and reduces carbon emissions.
• Scottish or British boats operating in Scottish zoned waters are prohibited from using multi rig trawls (more than 2 nets)
• The use of a solid weighted clump was acknowledged to excessively damage the seabed. Scottish boats have moved to spherical or drum roller designs which reduce seabed interaction by rolling over the seabed, thus reducing substrate damage and drag.