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Trawling: Demersal Species

Demersal trawlBottom Trawling (Single Boat)

The most widely used of the towed gears, this method is also known as otter trawling. Trawl nets are funnel shaped, with sides extended forward to form wings to guide fish into the funnel. In single boat trawling otterboards spread the towing wires (warps and bridles) to hold the net open horizontally.

Floats attached to the upper edge of the net mouth (headline) provide vertical lift. Weight distributed along the lower edge (ground rope) ensures good contact with the sea bottom. Demersal industrial species i.e. non-food fish such as Norway pout and Sandeel (Ammodytes marinus), are also taken by this method.

Trawlers range in size from small inshore vessels of less than 10 metres to large factory ships of 60 metres or more.

As roundfish stocks have declined, considerable fishing effort has been expended on the exploitation of high value species such as Monkfish (Lophius piscatorius, Lophius budegassa) and flatfish using scraper trawls. These nets are designed to maximise groundfish catches by increasing area swept i.e. they have very long wings and do not require a high headline.

Bottom Trawling (Pair)

In two-boat trawling, the wires that connect each vessel to the net are held open horizontally by the vessels keeping station some distance apart while towing. Consequently, otterboards are not required to provide horizontal spreading forces, and this enables vessels of modest horsepower to tow a relatively large gear between them.

Scottish demersal pair trawlers range from 15 to 30 metres with a typical combined horsepower of around 1,000 hp.

Seine Netting

Scottish seining, or fly dragging, depends on the long lengths of rope used (up to 3 km per side) to 'herd' fish into the path of the net as the gear is hauled. The gear is set roughly in the shape of an isosceles triangle with the dan, which marks the end of rope first shot and to which the vessel returns to complete the set, as the apex and the net as the centre of the base.

Having picked up the dan, the vessel steams slowly ahead while heaving in both ropes, gradually advancing winch speed as the gear closes to keep the net moving forward at a steadily increasing rate.

Seine net vessels range from 12 to 30 metres and have traditionally been fitted with much lower powered engines than trawlers of comparable size because of the differences in fishing techniques. New vessels are usually built as dual-purpose seiner/trawlers with engines developing 500 hp or above.

Twin Beam Trawling

beam trawlA beam trawl, as the name implies, is a trawl where the mouth is held open by a metal beam up to 12 metres in length. The beam is mounted on trawl heads or skids, one at each side of the net. This gear is used primarily for flatfish and, on the larger vessels, several tonnes of tickler chains can be used ahead of the ground rope to raise fish which may otherwise be over-run. Two beam trawls are used, deployed from outrigger booms, one on each side of the vessel.

This fishing method is widely used by Dutch, Belgian and English fishermen for species such as Sole and Plaice, but in recent years a growing interest has developed in north and east Scotland. Vessels of 20 metres or more can be rigged for either beam trawling or scallop dredging.

Longlining

In longline fishing a number of strings, each consisting of a main line with baited hooks on branch lines called snoods, are connected end-to-end and placed on or just off the seabed with an anchor and dan (marker buoy) at each end.

Scottish vessels engaged in this fishery are typically small inshore vessels of 10 metres or less, operating on grounds near their home port. With a limited resurgence of longlining in north west Scottish waters, several larger vessels (of 15 to 30 metres) are now using automated baiting, shooting and hauling systems.

Set Nets

Set nets are walls of netting up to 3 metres high and 70 metres long used singly, or a number may be joined end-to-end with suitable moorings to hold them in place on the sea bottom. Fish are caught either by gilling or entanglement, depending on the size of mesh and the hang of the netting.

As with longlining, set netting in Scotland is confined mostly to inshore vessels. One important advantage of both these static fishing methods is the ability to fish grounds too rough for towed gears such as trawls and seines.