Demersal Pair Trawling operates with similar nets to a single trawl, but they are generally much larger. As two vessels tow the gear, spread can be maintained by the distance between them when towing, which eliminates the need for trawl doors and reduces drag. It also allows two boats to tow a large net with relatively modest power usage.
Rock-hoppers or rubber reinforced footropes are used to protect the net from bottom damage and the gear is usually shot and hauled over the stern using a net drum. A length of wire and/or chain is included in the rig between warp and bridles to make good bottom contact. After one vessel shoots its net, the bridles are passed across to the partner with the aid of a messenger and connected to the heavy sweep wire. Both boats pay out the warps as they steam ahead to take up towing positions. Scottish pair trawlers generally tow between 0.15 and 0.4 nautical miles apart, depending on water depth and bottom conditions. At the end of the haul, both boats come together again and the previously transferred bridle is passed back to allow the first vessel to complete hauling operations.
|Potential Impacts (Biological/Environmental)||Gear Selectivity||Regulation|
Biological• 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
• Less overall disturbance due to the omission of trawl doors.
• 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 have 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.
• Pair trawling does not require trawl doors to be rigged into the trawl gear. This lightens the demersal trawl considerably and reduces drag. It also means less interaction with and consequent damage to the seabed. Less drag also means that boats save energy (fuel) and reduce carbon emissions.