Bottom trawling (known in the scientific community as Benthic trawling) is a fishing method which involves towing trawl nets along the sea floor, as opposed to pelagic trawling , where a net is towed higher in the water column. Bottom trawling can be carried out from one vessel or two vessels fishing cooperatively. It is practiced from a very wide range of fishing vessels, starting with small motor boats powered by engines of several tens of horsepower and up to large ocean-going trawlers, up to 100 m long, and powered by engines of several thousand horsepower.

Fishing Gear

The design requirements of a bottom trawl are relatively simple, a mechanism for keeping the mouth of the net open in horizontal and vertical dimensions, a "body" of net which guides fish inwards, and a "cod-end" of a suitable mesh size, where the fish are collected. The size and design of net used is determined by the species being targeted, the engine power and design of the fishing vessel and locally enforced regulations.

Beam Trawling

The simplest method of bottom trawling, the mouth of the net is held open by a solid metal beam, attached to two "shoes", which are solid metal plates, welded to the ends of the beam, which slide over and disturb the seabed. This method is mainly used on smaller vessels, fishing for flatfish or prawns, relatively close inshore.

Otter trawling

Otter trawling derives its name from the "trawl doors" or "otters" which are used to keep the mouth of the net open. As these are towed along the seabed, hydrodynamic pressure pushes them outwards, preventing the mouth of the net closing. They also act like a plough, digging up to 15cm into the seabed, creating a turbid cloud, and attracting fish towards the disturbance. The net is held open vertically on an otter trawl by floats attached to the "headline" (the rope which runs between the trawl doors along the upper mouth of the net), and weighted "bobbins" attached to the "foot rope" (the rope which runs along the lower mouth of the net). These bobbins vary in their design depending on the roughness of the sea bed which is being fished, varying from small rubber discs for very smooth, sandy ground, to large metal balls, up to 0.5 m in diameter for very rough ground. These bobbins can also be designed to lift the net off the seabed when they hit an obstacle. These are known as "rock-hopper" gears.

Body of the net

The majority of the structure of the net is wide diamond-meshed material, which holds the fish inside the net, but is not used for capturing fish. Into this area, fish escape devices can be fitted. These can be simple structures like "square mesh panels", which are easier for smaller fish to pass through, or more complicated devices, such as bycatch grills.

Cod end

The business end of the net, the cod end is where fish are finally "caught". The size of mesh in the cod end is a determinant of the size of fish which the net catches. Consequently, regulation of mesh size is a common way of managing mortality of juvenile fishes in trawl nets.

Process of benthic trawling

The idea that fish are passively "scooped up" is commonly held, and has been since trawling was first developed, but has been revealed to be erroneous. Since the development of scuba diving equipment and cheap video cameras it has been possible to directly observe the processes that occur when a trawl is towed along the seabed.

The trawl doors disturb the sea bed, create a cloud of muddy water which hides the oncoming trawl net and generates a noise which attracts fish. The fish begin to swim in front of the net mouth, but do not seam to be distressed by it. As the trawl continues along the seabed, fish begin to tire and slip backwards into the net. Finally, the fish become exhausted and drop back, into the "cod end" and are caught. The speed that the trawl is towed at depends on the swimming speed of the species which is being targeted, but for most demersal species, a speed of around 4 knots (7 km/h) is appropriate.

Ecological effect

Bottom trawling has operated for over a century on heavily fished grounds such as the North Sea and Grand Banks. Although overfishing has caused huge ecological changes to the fish community on the Grand Banks, concern has been raised recently about the damage which benthic trawling inflicts upon seabed communities. A species of particular concern is the slow growing, deep water coral Lophelia pertusa. This species is home to a diverse community of deep sea organisms, but is easily damaged by fishing gear.On November 18, 2004 the United Nations General Assembly urged nations to consider temporary bans on high seas bottom trawling.