Single net demersal trawling is used in Scotland to fish for demersal whitefish and nephrops.
Trawlers operating demersal trawls have gantries or derricks to handle the trawl doors. The net hauling system varies depending on the size of the vessel and the type of trawl used. Most Scottish trawlers tow from the stern of the boat and range from small open boats to the largest stern trawlers with rear trawl ramps.
Demersal trawls are designed to have bottom contact during fishing and depending on the bottom substrate, may be equipped with rubber reinforced footropes or rock-hoppers (large rubber discs). These shield the lower leading edge of the trawl from ground damage, whilst maintaining ground contact and helping the trawl to move easily over the bottom. The harder and stonier the substrate, the larger the rock-hoppers needed to give the required protection.
|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
• 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 number 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.