Set-nets are long walls of netting which trap fish by the gills or by entanglement. The netting is mostly woven from fine nylon twine, which is practically invisible underwater. It is hung on ropes with those for demersal fish 1.5 - 6m deep and between 50 - 200m long. Plastic floats are attached to the headline to keep the netting upright.
Scottish vessels are not permitted to carry monofilament nets under 250 mm mesh size within six miles of the coast.
Floats need to be slightly larger than the mesh opening to prevent them from tangling the netting. Ring floats are convenient as the nets can then be mounted on a solid rod for shooting without snagging. A weighted line is attached to the footrope to ensure bottom contact. On large mesh nets for groundfish such as monkfish and skate that are found tight on the bottom, a polypropylene headline rope without floats provides sufficient uplift. In areas where currents are strong, additional weighting may be added to the footrope. Nets may be used singly or a number joined in fleets with suitable moorings to hold them in place.
In tightly hung gill nets the mesh size is chosen to allow only the head and gill covers of the targeted size of fish to pass through and be trapped. Mesh sizes range from 60 mm for bait nets, to over 200 mm for large bodied cod and saithe. Mesh size is less important in loosely hung tangle nets as most fish twist and turn when trying to escape and become very entangled, but a large proportion are still caught by the gills. Much larger mesh sizes over 250 mm are used in tangle nets for skate, crayfish and occasionally lobster.
In inshore waters set-nets are mainly fished seasonally by small boats for a variety of demersal species. Wrecks and other areas where fish concentrate are fished, and the nets left in the water overnight. If they are left much longer the catch attracts crabs which quickly destroy the trapped fish and are difficult to remove. Large vessels which fish offshore can work over 20 km of nets.
The main advantage of set nets over towed nets for demersal species is that when tightly hung, they are very size selective and retain few juveniles. If shot and hauled quickly the fish quality can be good. They can also be used on rough grounds inaccessible to towed gears.
However, by-catches of crabs can be a problem in some fisheries and there is also a risk of catching sea birds and large mammals in larger mesh nets in shallow water. If the nets are not hauled quickly, the fish die and spoil and/or are attacked by crabs which can lead to major quality losses.
|Potential Impacts (Biological/Environmental)||Gear Selectivity||Regulation|
• Some types of set net can attract unwanted crustacean bycatch
• Cetacean bycatch and occasional turtle bycatch can sometimes occur.
• Ghost fishing can be a major problem with lost gear
• Lost gear can pollute large areas of seabed
|• Mesh size||
• Minimum mesh sizes
• Minimum fish sizes
• Use of monofilament gill nets prohibited