Bottom trawling

Big boats use nets to catch fish.

These nets drag along the sea floor. This is called bottom trawling. The nets can catch shrimp and fish.

This can stir up the mud at the bottom. The mud can drift far away in the water. This can change the home for sea life. Some nets have tools to help turtles escape.

People use these nets to get food for us.

Bottom trawling is a way to catch fish.

Fishermen use a boat called a trawler. They pull a large net along the seafloor. This is also called dragging. Some nets stay at the very bottom. This is called benthic trawling. Other nets stay just above the bottom. This is called demersal trawling. These nets catch many things like shrimp, squid, and cod.

One way to use a net is with a beam. A solid metal beam keeps the net mouth open. Another way uses otter boards. These are large boards made of wood or steel. They use water power to push the net open. This helps the net act like a plough. It can stir up mud on the seabed. This mud can drift for many kilometres.

This way of fishing can change the ocean floor. It can move rocks and stir up sediment. It can also hurt sea life like deep water coral. To help, some nets have special parts. These parts let animals like turtles escape.

Bottom trawling is a way of fishing that involves towing a large net along the ocean floor. This method is often called dragging. Scientists divide this practice into two main types. Benthic trawling means the net is pulled at the very bottom of the ocean. Demersal trawling means the net is towed just above that bottom zone. This method is different from midwater trawling, which catches fish higher up in the water. Bottom trawling targets groundfish and species like cod, squid, shrimp, and rockfish.

How the net works depends on the gear used. In beam trawling, a solid metal beam holds the mouth of the net open. This is often used by smaller boats near the shore. Another way is otter trawling, which uses large boards made of wood or steel. These boards use water force to push outward and keep the net open. The boards act like a plough to stir up the seabed. The net itself is shaped like a funnel. It is wide at the mouth and gets narrow at the end, called the cod end.

People have used forms of trawling for a very long time. In 1376, during the reign of Edward III, people even complained about a tool called the wondyrchoum. This was an early beam trawl with a wooden beam. In Japan, a similar method called Utase Ami was used during the Edo era in 1704. Later, fishermen in Brixham, Devon, developed sailing trawlers. These boats could tow much larger nets in deeper waters. By the late 1800s, there were over 3,000 sailing trawlers in UK waters.

There are many important facts about the scale of this fishing. Global catch from bottom trawling is estimated at over 30 million tonnes every year. This is a larger amount than any other fishing method. Trawling can also affect the environment in big ways. A 2021 paper estimated that it might release 600 to 1500 million tons of carbon dioxide a year. This happens because the nets disturb carbon in the seafloor. Trawling also stirs up massive amounts of sediment. Scientists estimate this mass is about 22 gigatonnes per year.

This way of fishing changes the world beneath the waves. It can move boulders and leave deep scars in the mud. It can also hurt living things like the slow-growing Lophelia pertusa coral. Some areas, like the Grand Banks, have seen fish populations drop because of it. To help, some nets now use turtle excluder devices. These are special parts that let animals like turtles escape. Some areas are also set aside as marine protected areas to keep the seafloor safe.

Bottom trawling is a commercial fishing method involving the towing of a large net, known as a trawl, along the seafloor. This practice is also frequently called "dragging." Scientists categorize this method into two specific types based on depth. Benthic trawling occurs when the net is towed at the very bottom of the ocean. Demersal trawling occurs when the net is towed just above the benthic zone. This differs from midwater trawling, or pelagic trawling, which targets fish higher in the water column. While midwater trawling catches species like mackerel, bottom trawling targets groundfish and semi-pelagic species. These include cod, squid, shrimp, and rockfish.

The mechanism of a bottom trawl requires specific components to function. A net must have a way to keep its mouth open horizontally and vertically. It also needs a "body" to guide fish inward and a "cod-end" to collect them. In beam trawling, a solid metal beam holds the mouth open. This beam is attached to metal plates called "shoes" that slide over the seabed. Otter trawling uses a different mechanism involving large rectangular boards called otter boards. These boards are made of timber or steel. As the vessel tows the net, hydrodynamic forces push the boards outward. This prevents the mouth from closing and allows the boards to act like a plough. To keep the mouth open vertically, the net uses floats on the "headline" and weighted "bobbins" on the "foot rope."

Different gear designs serve specific fishing needs and environments. Beam trawling is a simple method often used by smaller vessels near the shore to catch prawns or flatfish. Otter trawling is more complex and can be adapted for different seafloor textures. For smooth, sandy ground, fishermen use small rubber discs as bobbins. For rougher ground, they use large metal balls. Some gear includes "rock-hopper" bobbins designed to lift the net over obstacles. The body of the trawl is shaped like a funnel. It is wide at the mouth and narrows toward the cod-end. The mesh size in the cod-end is a critical factor in fishery management. By regulating mesh size, authorities can prevent the capture of juvenile fish.

History shows that humans have used forms of trawling for centuries. An early record of conservation concerns dates to 1376 during the reign of Edward III. A petition to Parliament protested a device called the "wondyrchoum." This was an early beam trawl using a wooden beam and a net with very small mesh. The petitioners argued it destroyed fisheries by catching all fish, regardless of size. In Japan, a similar method called "Utase Ami" was recognized in 1704 during the Edo era. Later, the fishermen of Brixham in Devon developed sailing trawlers. This innovation allowed for much larger nets to be towed in deeper waters. By the end of the 19th century, over 3,000 sailing trawlers operated in UK waters.

The scale of global bottom trawling is immense. It is estimated that the global catch from this method exceeds 30 million tonnes per year. This volume is larger than any other fishing method currently in use. However, this high productivity comes with significant environmental costs. Trawling can cause the population collapse of species like orange roughy, shark, and barndoor skate. It also impacts the physical structure of the seafloor. The gear can overturn boulders and leave deep scars in muddy bottoms. In some regions, such as the Grand Banks, overfishing has caused major ecological changes. Research shows that trawled canyon sediments may contain 52 percent less organic matter than undisturbed areas.

Bottom trawling also has significant biogeochemical effects. As the nets drag, they stir up massive amounts of sediment. This creates suspended solid plumes that can drift for tens of kilometres. The estimated mass of resuspended sediment on the world's continental shelves is 22 gigatonnes per year. This process also affects carbon levels in the ocean. A 2021 paper estimated that bottom trawling contributes between 600 and 1500 million tons of carbon dioxide annually. This occurs because the activity disturbs carbon dioxide stored in the seafloor. While these specific carbon values are highly uncertain and criticized as overestimates, they highlight the connection between fishing and climate science.

Efforts to mitigate the impact of trawling focus on gear design and protected areas. To reduce bycatch, which is the accidental capture of non-target species, engineers have added turtle excluder devices. These can include simple square mesh panels or more complex bycatch grills. Many regions also establish marine protected areas where trawling is restricted. The impact on sea life depends heavily on the type of gear used. A global analysis found that otter trawls remove about 6% of biota per pass. In contrast, hydraulic dredges are much more destructive, removing 41% of biota per pass. Protecting sensitive species, such as the slow-growing deep-water coral Lophelia pertusa, remains a major challenge for international fisheries management.