Commercial Fishing Methods
There are a wide range of methods used to catch fish commercially – some of the small scale methods of catching fish are relatively low-impact on the environment and do not catch a large amount of fish, whereas large scale commercial fishing can catch massive amounts of fish while also wreaking havoc on the marine environment. Below the main methods of both large and small scale commercial fishing are described.
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. Pelagic trawling does not actually cause much damage to the environment as the nets are not dragging along the seabed. However, this type of trawling can catch huge amounts of fish. Atlantic Dawn, the biggest trawler in the world, can catch 400 tons of pelagic fish every 24-hours, and species such as jack mackerel have been fished down to a fraction of their former abundance in South American and Australian waters due to intensive pelagic trawling.
Bottom trawling is a method of trawling which drags a net along the seabed in order to catch fish that live and feed there. Bottom trawling is sometimes referred to as demersal trawling as the nets are dragged through the demersal zone of the sea which is the area on and just above the seabed. The majority of commercially important species such as cod, haddock, plaice, sole and whiting are all caught by bottom trawling. There are several types of bottom trawling.
Otter Trawling: This is the most common type of trawling. A boat drags a net along the seabed. The net is held open by large steel boards (called otter boards – hence the name) 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 hold the mouth of the net open. The net moves through the sea fast enough to force fish that are in its path into the back section of the net (known as the cod end) and the force of water flowing into the net prevents them from swimming out of the net and escaping. The size of the mesh dictates which fish can pass through the net and avoid capture, with a larger mesh size to allow small species with no commercial value and immature fish a chance of avoiding capture. An otter trawl does not scoop up fish as such. Instead, fish are either attracted or disoriented by the net dragging along the seabed and then find themselves caught in the mouth of the net. The fish will try to swim away but eventually find itself exhausted and end up falling back into the cod end of the net.
Beam Trawling: A beam trawler is a type of trawler which lowers two separate nets to the seabed from derricks on the side of the boat. Each net is held open by a solid metal beam which can be up 16 metres across. ‘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 trawls are typically used to catch flatfish commercially, particularly lemon sole, Dover sole and plaice as these species are most likely to bury themselves underneath sand and sediment. However, beam trawls with heavier gear and chains can be used to fish rockier and rougher ground. Beam trawls can leave tracks up to 10cm deep in the seabed and kill a huge range of species such as starfish, crabs, and brittle stars. While these species may be commercially unimportant the knock on effect of removing them from the food chain can be devastating. It is estimated that for every 1lb of marketable fish caught by a beam trawler, 16lb of marine life has been killed.
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 have been horror stories about masses of dead fish and devastation to the marine environment caused by pulse trawling. It also offers significant fuel savings to trawler owners which may be the real reason they are so keen to push forward with this technology. 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: As the name implies pair trawling involved two vessels dragging one bet between them. The nets used in pair trawling can be massive. Pair trawlers fishing for mackerel in the open sea can drag a net which is 1.6 kilometres long with a mouth opening measuring 60 by 120 metres. To put this into perspective a net this big would be able to fit ten 747 Boeing jumbo jets inside of it. 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 (dolphin, whale and 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 devastating. The biggest purse seines can use a net thousands of metres long which goes to a depth of 250 metres and use a speedboat to quickly draw the net around the fish. These kind of nets can catch up to 400 tons of small species such as jack mackerel, or a whole school of tuna. There is also massive bycatch associated with purse seining for tuna, with dolphins, whales, sharks and turtles inadvertently caught in the net and then thrown back into the sea dead once they have been brought onboard.
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 causing major damage. Shellfish beds that have been intensively dredged are unlikely to ever 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, Drift Nets and Tangle Nets: These type of nets hang in the sea like a wall and catch fish that swim into them. Gill nets are so called because fish get caught by the gills, while tangle nets unsurprisingly see the fish become tangled in the netting. 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 be massively damaging due to huge bycatch. Large scale gill nets (longer than 2.5 kilometres across) have been banned by EU fishing vessels since 1992. Gill, tangle and drift 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) are cages made out of metal or wood and rope. Their design lets crabs and lobsters enter the pot to take the bait but they cannot then get out. They are baited with dead fish and lowered to the seabed on ropes, usually about a dozen at a time. A buoy is used to mark the location and fishermen will return to retrieve their catch after a day or two. This is a responsible form of commercial fishing as there is next to no bycatch – only the target species are taken, and even if fish enter the pot they usually survive and can be returned to the sea. Small, immature and berried (egg carrying) crabs and lobsters can also be returned to the sea.