Pelagic trawling: This type of trawling involves dragging a net through the middle of the water column to catch fish that feed there. In waters around Britain and Ireland, this is most likely to be mackerel and herring. As pelagic trawls do not make contact with the seabed this type of commercial fishing does not cause as much damage to the marine environment as seabed trawling. However, there can be issues with high levels of bycatch of non-target species. Some of the largest fishing vessels in the world, such as Atlantic Dawn and FV Margiris are pelagic trawlers and have proved extremely controversial due to the huge quantities of fish they are capable of catching.
Seabed trawling: This is a type of trawling (also called bottom trawling) in which nets are dragged across the seabed to catch the species of fish which live and feed there. In the UK and Ireland this is likely to be cod, haddock, whiting and similar species. In some types of seabed trawling the net is held open by large steel boards (called otter boards) which slide apart once the boat begins to move and hold the net open wide. A system of wheels and floats can also be used to both keep the net open and allow it to pass over rocks and other seabed obstuctions. The net moves through the sea fast enough to catch fish in the mouth of the net or force them to swim in front of the net until they become exhausted and end up falling back into the net where the force of water flowing into the net prevents them from escaping. The size of the mesh dictates which fish can pass through the net and avoid capture, with a larger mesh size allowing juvenile fish and small fish with no commercial value the chance of avoiding capture. As these type of trawl nets are in constant contact with the seabed they cause damage to the marine environment with seaweed beds, corals and other features which provide a habitat for marine creatures likely to be destoryed by seabed trawls. It is also calculated that seabed trawling releases large amounts of carbon which has been stored in the seabed.
Beam trawling: This is a type of trawling typically used to catch flatfish such as lemon sole, Dover sole and plaice over soft sand and mud seabeds. Beam trawlers use two separate nets which are lowered to the seabed from derricks on the side of the boat. Each net is held open by a solid metal beam. ‘Tickler chains’ are used to lash the seabed and get flatfish out of the sediment so they can be scooped up by the net. Beam trawling is highly damaging to the marine environment as the trawls can cause permanent alterations to the seabed and can kill and damage non-target species which live and burrow into the sediment. There can also be problems with high levels of bycatch.
Pulse trawling: Pulse trawling is an adaptation of beam trawling which fires an electrical pulse through the nets which shocks and stuns fish out of the seabed and allows them to be scooped into the nets. Dutch trawlers have experimented with pulse trawling but it has yet to gain major acceptance in commercial fishing. Fishermen claim it is less damaging than a traditional beam trawl, but there is concern that using pulses of electricity to shock fish out of the seabed may have significant and as yet poorly understood long-term impacts on the marine environment. There is a longer article going into greater depth about the details of pulse trawling and the environmental damage some claim this method of fishing causes available here.
Pair trawling: This is a type of pelagic trawling which sees two vessels dragging one bet between them. The nets used in pair trawling can be massive. Pair trawlers usually use nets which are very large with the mouth of the net being over 200 metres (656 ft) wide and 100 metres (328 ft) deep. Pair trawling has many advantages. Fuel costs are lower, the net can be towed faster allowing more ground to be covered and the two vessels can catch and store much more fish working together than they would individually. Pair trawling for bass is banned in UK territorial waters due to the large cetacean (marine mammal) bycatch which is caused by this type of fishing.
Purse Seining: Purse seining is a method of fishing which is used to catch pelagic fish that are found in a shoal or school such as herring, mackerel, sardines and many species of tuna. Purse seining works by drawing a vast net around a school of fish. The net is then pursed (drawn closed at the bottom) trapping the fish inside and the net can then be pulled onto the vessel. Small-scale purse seining can be relatively low-impact as the seabed is not damaged, however, large-scale purse seining can be highly damaging to fish stocks due to the large numbers of fish which can be caught. There is also significant bycatch associated with purse seining for tuna in many parts of the world as dolphins, whales, sharks and turtles inadvertently caught in the net.
Dredging: Dredging is used to take shellfish such as oysters, mussels and scallops from the sea. Dredgers tow metal cages across shellfish beds. Metal beams or teeth are used to scrape the shellfish free from the seabed when they then fall back into the cage or into a net or bag attached to the cage. Hydraulic dredgers also exist which spray jets of water onto the shellfish to dislodge them from their location. Large vessels can drag as many as twenty cages behind them. Dredging is one of the most destructive methods of commercial fishing as it tears through shellfish beds and causes severe damage to the seabed and non-target species. Shellfish beds that have been intensively dredged are unlikely to recover from the damage that has been caused to them.
Other Types of Commercial Fishing
Long-lining: Long-lining is a method of commercial fishing used to catch a range of species. Small-scale inshore long-lines can be used to catch mackerel in a way that causes very little damage to the marine environment. However, large-scale long-lining is used to catch species such as marlin, tuna and swordfish. These vessels can unreel up to thirty miles of heavy line from the back of a ship. Thousands of hook snoods – baited by hand with small live fish – are attached to this line which is left at sea for a day and then retrieved, along with any fish that have taken the baits. Although long-lining does not damage the seabed in the way that trawling does it is still harmful to the environment as turtles, sharks and rare marine birds are all inadvertently caught by long-lines across the world, while long-lines that float on or near the surface also catch rare and endangered sea birds such as albatross.
Gill nets and drift nets: These types of nets hang in the sea like a wall and catch fish that swim into them. Gill nets are so-called because fish are caught by the gills. Drift nets are similar to gill nets but are suspended from a boat and allowed to move and drift in the tide. While small-scale gill nets can be used responsibly by inshore fishermen (especially when the mesh size is large enough to allow immature fish to swim through) large-scale drift nets can have significant problems with bycatch. Gill nets longer than 2.5 kilometres (1.5 miles) across have been banned the EU since 1992. These type of nets which have been lost or abandoned by commercial fishing vessels continue to catch fish – potentially for years – and are known as ghost nets.
Lobster and Crab Pots: Lobster and crab pots (also known as traps or creels) are cages made out of metal, wood, rope or a combination of different materials. They are baited with dead fish and lowered to the seabed on ropes and a buoy is used to mark the location. Their design allows crabs and lobsters to enter the pot and become trapped inside, with the fishermen returning to empty the pot after a day or two. This is a low-impact form of commercial fishing as there is little to no bycatch and immature and berried (egg carrying) crabs and lobsters can be returned to the sea unharmed.