Trawling is a method of fishing where a large net is pulled through the water behind one or more boats. The net funnels fish, shrimp, or other marine life into a closed end as the boat moves forward, scooping up everything in its path. It is one of the most widely used commercial fishing techniques in the world, responsible for catching enormous volumes of seafood, but also one of the most environmentally controversial.
How Trawling Works
A trawl is not just a net. It is an entire rigging system that includes the net itself, heavy metal plates called trawl doors (also known as otter boards), cables called bridles, and sometimes weighted chains. The trawl doors are the key engineering component. As the boat drags them through the water, they act like underwater kites, using water pressure to spread apart and hold the mouth of the net wide open. The bridles connect the doors to the net’s wings, creating a funnel shape that guides marine life inward.
The boat tows this entire assembly at a controlled speed. Fish and other creatures entering the net’s path are pushed toward the narrow, closed tail end called the cod end. When the net is hauled back aboard, the cod end is opened and the catch is dumped onto the deck for sorting.
Bottom Trawling vs. Midwater Trawling
There are two main types, and they differ in where the net travels and what it catches.
Bottom trawling drags the net along the ocean floor. The trawl doors scrape across the seabed, and a weighted chain (called a tickler chain) often runs along the bottom edge of the net to disturb creatures hiding in the sediment. This method targets species that live on or near the seafloor: cod, flounder, sole, halibut, and shrimp. It is also the more destructive of the two, because it physically disturbs the seabed with every pass.
Midwater trawling tows the net through the open water column, never touching the bottom. It targets schooling species like sardines, mackerel, shrimp, and squid. Because the net doesn’t contact the seafloor, midwater trawling causes far less habitat damage, though it still raises concerns about bycatch and overfishing of targeted species.
Environmental Damage From Bottom Trawling
Bottom trawling is often compared to clear-cutting a forest. The U.S. Geological Survey describes it as “essentially rototilling the seabed.” Heavy gear tears up root systems, collapses animal burrows, and flattens complex structures like coral and sponge beds that took decades or centuries to grow. Areas of soft mud can be scraped down to bare rock, eliminating the habitat that sediment-dwelling creatures depend on. Species diversity drops, and the physical complexity of the seafloor is simplified in ways that ripple through the food web.
Bycatch is the other major problem. Trawl nets are not selective. They capture whatever enters the net’s path, including juvenile fish, sea turtles, sharks, rays, and species with no commercial value. In shrimp trawl fisheries, bycatch can make up 60 to 80 percent of the total catch by weight. That means for every pound of shrimp pulled from the water, three to four pounds of other marine life may be caught and discarded, often dead or dying.
There is also a climate dimension. Ocean sediments store vast amounts of organic carbon, built up over thousands of years. When bottom trawling churns up those sediments, some of that carbon gets broken down by microbes and released as carbon dioxide. One estimate published in Nature Geoscience suggests trawling remineralizes 0.16 to 0.4 billion metric tons of sedimentary organic carbon globally each year, equivalent to 0.58 to 1.47 billion metric tons of CO₂ escaping from the seabed into the water. Some of that CO₂ eventually reaches the atmosphere.
Efforts to Reduce the Harm
Regulators have developed several tools to make trawling less destructive. Turtle excluder devices (TEDs) are metal grids installed inside shrimp trawl nets that allow sea turtles to escape through an opening while shrimp pass through to the cod end. Bycatch reduction devices (BRDs) work on a similar principle, using escape panels, chutes, or other net modifications that give finfish an exit route while retaining shrimp. In the southeastern United States, both TEDs and certified BRDs are required in commercial shrimp trawl fisheries.
Outright bans are gaining momentum in some parts of the world. In 2024, Greece became the first European country to ban bottom trawling in all of its marine protected areas, with the practice ending in its three national marine parks by 2026 and across all protected ocean areas by 2030. The European Union has pledged to phase out bottom trawling in all member states’ marine protected areas by 2030, though this regulation is not yet binding and enforcement varies by country.
Trawling Beyond Fishing
The word “trawling” has taken on a second life as a metaphor. In everyday language, trawling through something means searching through a large amount of material to find what you want, the way you might trawl through job listings or old emails. The image is the same: casting a wide net and seeing what you pull in.
In technology and law, “data trawling” refers to scraping massive amounts of publicly available information from the internet, often to train artificial intelligence models. The parallel to fishing is deliberate. Just as a bottom trawl scoops up everything on the seafloor, data trawling collects personal information (names, addresses, photos) along with the data actually being sought. The practice is drawing increasing legal scrutiny, particularly around privacy regulations in Europe.
It is worth noting that “trawling” and “trolling” are different words with different meanings, though they are frequently confused. Trolling in fishing means dragging a baited line behind a slow-moving boat to attract individual fish. Trawling means dragging a net. Online, “trolling” refers to deliberately provoking people, while “trawling” refers to searching broadly through information.