Trawl nets are cone shaped nets ending in a codend, which are dragged through water column or over the seafloor. The trawls are towed on the sides or from the rear of the boat. This catching method is widely used in the fishery on whitefish (like cod, whiting and haddock) and flatfish (like sole and plaice). Bottom dwelling species often live in mixed groups, so not only the target species is caught but also other species. This method of fishery is also called mixed fishery. Trawl nets can be kept open in several ways. Several methods using trawl nets are listed below.
A beam trawl consists of a cone-shaped body ending in a bag or codend, which retains the catch. In these trawls the horizontal opening of the net is provided by a beam, made of wood or metal, which is up to 12 m long. This beam is supported by ‘shoes’ on the sides. These shoes run over the seabed while fishing. The fish are scared out of the seabed with steel ‘tickler chains’ that rake through the seabed. This method has much bycatch and considerable impact on the seabed and a high use of fuel.
A shrimp trawl is a lighter form of beam trawl. Instead of tickler chains, they use a rope with rubber bobbins that roll over the seabed to scare the shrimp. Often a ‘seaflap’ is used to reduce bycatch. It is an effective way of catching shrimp, and has less impact on the seabed as the beam trawl, but not negligible. There is also a lot of bycatch.
Pulse trawls have exchanged the tickler chains for light wires that give off electrical impulses. This makes the fish go out of the seabed. This gear uses considerably less fuel (20-40%) compared to a beamtrawl with tickler chains and is also less damaging to the seabed. It is also a more selective method, with less bycatch. Most pulse trawlers combine the pulse technique with the ‘sumwing’. This is called pulsewing. The hydrorig creates water swirls which makes the flatfish come out of the water. This reduces fuel use by 35% compared to the traditional beam trawl. This also has lower bycatch. The side-effects of the ‘pulses’ are however still unknown.
Bottom otter trawls
Bottom otter trawls are cone shaped nets with a body, generally made of two or four and sometimes more panels. The nets end in 1 or 2 codends. The net has lateral wings extending forward from the opening. Bottom otter trawls generally have a longer upper panel to prevent fish escaping over the net. The mouth of the net is kept open vertically by floaters and a weighted footrope. Bottom otter trawls owe their name to the large square ‘otter boards’. Otter boards are often made of wood or steel and are positioned so that hydrodynamic forces, which are present when they are dragged over the seafloor, push them outwards which keeps the net open. The boards also act as a plow which scares the fish into the net. Bottom otter trawls plow through the seabed and there is often bycatch of undersized and non-target species.
Pair trawls drag their nets through the water column. Pair trawls are dragged by two boats and in that way kept open horizontally. Long cables of steel or a combination of rope can be inserted to increase the width of the net. Cables of 4500 meters result in a width of 4500 meters; that is twice size of a conventional ottertrawl/bottomtrawl. This method enables the catch of whole schools of mackerel and herring. There is generally little by bycatch. There is relatively low fuel use and no impact on the seabed.
Midwater (pelagic) otter trawls
Midwater otter trawls are cone shaped nets which are dragged through the water column. An otter trawl consists of a cone-shaped body, normally made from two or four panels, ending aft in codend and with lateral wings extending forward from the opening. The net is opened horizontally by otter boards, and vertically by floaters and weights. Midwater otter trawls have no impact on the seabed and there is little bycatch.
Dredges are a catching method which uses steel frames with a net attached to it. On te front of this frame there are often steel spikes acting as a kind of rake. This method is used to catch for example mussels, oysters and scallops.
Mechanized dredges (including suction pumps)
Mechanized dredges (including suction pumps) are used to to dig and to wash out mussels and other shellfish that have buried themselves in the seabed. Some dredges are so improved that the prey is not only dug out, or stirred up and collected in a bag, but is also conveyed on board the vessel by the same gear. This can be done using suction pumps. There is impact on the seabed.
Twinrig of multirig
This special rigging system for this particular gear was developed to increase the horizontal fishing area of the trawl and comprises two identical trawls (“twin”) fixed together. The horizontal opening is provided by a single pair of otter boards, which are attached to the trawl close to the wings. Their inner wings are attached to a sledge towed simultaneously with the otter boards from a common crow foot. Another option used on modern vessels in the northern shrimp fishery is to tow the gear with three warps.
These are small, light dredges consisting of a mouth frame attached to a holding bag constructed of metal rings or meshes. They are operated by hand and the fishermen work at low tide.