Trawls

Trawl nets are cone shaped nets that end in codends, which are dragged through the water column and over the sea floor. The trawls are towed from the sides or 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 are caught but also other kinds of species end up in the net. When this occurs in a fishery, it is called a mixed fishery as not only the one target species is caught. The opening of trawl nets can be held open in many different ways. Several methods using trawl nets are listed below.

Beam trawls

A beam trawl net 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 either side. These shoes glide over the sea floor while fishing. The fish are startled up from the sea floor with steel ‘tickler chains’ that rake through the sea floor. This method has a large amount of bycatch, a considerable impact on the sea floor, and a high fuel usage.

Shrimp trawl

A shrimp trawl is a light-weight form of the beam trawl. Instead of tickler chains, they use a rope with rubber bobbins that roll over the sea floor to scare the shrimp. This method of fishing has a lot of by-catch and a ‘seaflap’ is often used to reduce this amount of by-catch. This is an effective way of catching shrimp and has less impact on the sea floor than the beam trawl.

Pulse trawl

Pulse trawls have substituted the tickler chains for light weight wires that give off electrical impulses, which makes the fish swim away from  the sea floor. This type of gear uses a considerably lower amount of fuel than the beamtrawl that uses the tickler chains (around 20-40%). In addition, the pulse trawl is less damaging to the sea floor and a more selective method with less by-catch. Most pulse trawlers combine the pulse technique with the ‘sumwing’. This is called the pulsewing. A hydrorig creates water eddies which make the flatfish swim up from the bottom. This reduces fuel use by 35% in comparison to the beam trawl and has less by-catch. However, the effects of the ‘pulses’ in and on the sea floor have yet to be researched.

Bottom otter trawls

Bottom otter trawls are cone shaped nets consisting of a body, generally made of two or four and sometimes more panels, and usually 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 out of wood or steel and are positioned so that hydrodynamic forces, which are present when they are dragged over the sea floor, push them outwards which keeps the net open. The boards also act as a plough which startles the fish into the net. As bottom otter trawls plough through the sea floor, a lot of by-catch of undersized and non-target species is caught.

Pair trawls

Pair trawl nets are dragged through the water column by two boats that keep the net open horizontally. Long cables of steel or a combination of steel and rope can be inserted to increase the width of the net. Cables of 4500 meters result in a width of 4500 meters; this is twice the size of a conventional ottertrawl/bottomtrawl. This method enables the catch of whole schools of mackerel and herring. There is generally little by bycatch, no impact on the sea floor, and the fuel usage is relatively low.

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 in a codend 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 sea floor and there is little bycatch.

Dredges

Dredging is a fishing method that uses a net which is attached to steel frames, often containing steel spikes to acts as a rake. This method is used to catch mussels, oysters, and scallops.

Mechanized dredges (including suction pumps)

Mechanized dredges (including suction pumps) are used to dig and wash out mussels and other shellfish that have buried themselves in the sea floor. Some dredges have been made so that they not only dig out or stir up the catch, but also convey it onto to the vessel. This is done by using suction pumps. This method of fishing has some impact on the sea floor.

Twinrig or multirig

This rigging system was developed especially for this particular gear in order to increase the horizontal fishing radius. The system is comprised of two identical trawls, thus the ‘twin’, that have been fused together. The horizontal opening in the net is provided by a single pair of otter boards, which are attached to the trawl, close to the wings. In the North Sea, twinrig is used to fish for langoustine, cod, whiting, haddock in the summer and for plaice, dab, and mullet in the winter. This method of fishing uses a low amount of fuel and has little impact on the sea floor. However, there is a lot of by-catch when fishing for small crustaceans.

Hand dredge

Hand dredges are small, light dredges consisting of a mouth frame that is attached to a holding bag which is constructed of metal rings or meshes. They are operated manually at low tide.