Bottom trawling

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.