Trawl fishing is a method of catching fish by towing a large, funnel-shaped net through the water behind one or more boats. It is one of the most productive fishing techniques in the world, landing roughly 19 million tons of fish per year, about a quarter of all wild-caught seafood. It is also one of the most debated, because of the damage certain forms of trawling can cause to ocean habitats and the large amounts of unwanted marine life caught alongside target species.
How a Trawl Net Works
A trawl net narrows from a wide opening down to a closed pouch at the back called the codend, where fish accumulate as the net moves. Floats are attached along the top edge of the opening (the headrope) to hold it up, while weights and specialized ground gear line the bottom edge (the footrope) to keep it down. This combination holds the mouth of the net open while the boat drags it forward.
Most trawl operations use a pair of heavy, angled panels called otter boards, one on each side of the net mouth. Water pressure pushing against these boards forces them apart, spreading the net opening horizontally. The result is a wide, gaping funnel that herds fish inward as the boat moves. Once enough time has passed, the crew winches the net back aboard and empties the codend onto the deck or into a hold.
Bottom Trawling vs. Midwater Trawling
The two main types of trawling differ primarily in where the net travels in the water column, and that distinction has major consequences for both the species caught and the environmental footprint.
Bottom Trawling
Bottom trawls (also called demersal trawls) are designed to roll along the ocean floor. Bobbins or heavy rollers on the footrope help the net glide over the seabed while a sweep along the bottom collects animals living on or near the substrate. Target species are typically bottom-dwelling fish and shellfish: flathead, pink ling, blue grenadier, silver warehou, prawns, crabs, and various groundfish. This is the form of trawling that attracts the most environmental criticism, because it physically contacts the seafloor.
Midwater Trawling
Midwater (pelagic) trawls operate anywhere from just above the bottom to near the surface, targeting schooling fish that swim in open water. Common catches include jack mackerel, blue mackerel, sardines, and herring. Because the net never touches the seabed, midwater trawling has minimal impact on bottom habitats. The tradeoff is that it can only catch species that form dense schools in the water column.
Environmental Impact on the Seafloor
Bottom trawling is often compared to plowing a field. The U.S. Geological Survey describes it as “essentially rototilling the seabed,” tearing up root systems, animal burrows, corals, and sponges in its path. The effects go beyond the organisms directly hit. A global calculation of sediment disturbed by bottom trawling found that the practice stirs up roughly 22 gigatons of seabed material per year, approximately the same mass of sediment that all the world’s rivers deposit onto continental shelves annually.
That resuspended sediment changes water chemistry. It raises nutrient levels, lowers the amount of light penetrating the water, and reduces photosynthesis in marine plants that form the base of the food web. Areas trawled repeatedly can shift from soft, living mud to bare rock, eliminating the burrowing creatures that once lived there. Species diversity and habitat complexity both decline when the physical structure of the seafloor is simplified this way.
The Bycatch Problem
Bycatch, the unintended capture of non-target species, is a persistent issue with trawling, and it is especially severe in shrimp fisheries. The average shrimp-to-bycatch ratio by weight is about 1:10, meaning that for every kilogram of shrimp hauled aboard, roughly 10 kilograms of other marine life come with it. That other catch can include juvenile fish, rays, small sharks, and sea turtles.
Technology has helped reduce some of the worst outcomes. Turtle excluder devices, metal grids sewn into the net that guide turtles out through an escape hatch, are now 97 percent effective at keeping sea turtles out of shrimp trawls. Similar bycatch reduction devices use differences in size or swimming behavior to sort target species from everything else before it reaches the codend. These tools don’t eliminate bycatch entirely, but they have dramatically reduced mortality for certain vulnerable species.
Fuel Use and Carbon Footprint
Dragging a heavy net through water or along the seabed takes a lot of energy. Trawling is consistently one of the most fuel-intensive fishing methods. In the North Sea, bottom trawling for demersal fish burns roughly 0.99 liters of fuel per kilogram of catch. By comparison, Danish seining for the same types of fish uses about 0.25 liters per kilogram, and gillnetting comes in around 0.41 liters. The gap widens further for crustacean trawling, which can exceed 2 liters per kilogram of catch.
Midwater trawling is far more efficient than bottom trawling, using around 0.09 to 0.12 liters per kilogram for pelagic species. That’s in the same range as purse seining, another common method for catching schooling fish. The high fuel cost of bottom trawling is driven largely by the resistance of dragging gear across the ocean floor, which is why alternative methods that avoid bottom contact tend to perform better on energy metrics.
Scale: Industrial to Artisanal
Trawling exists across a wide spectrum of scale. Industrial trawlers are large, heavily mechanized vessels equipped with advanced fish-finding sonar and navigation systems. Some are factory ships that process and freeze their catch onboard, staying at sea for weeks. These operations account for the bulk of trawl-caught seafood globally.
At the other end, small-scale and artisanal fishers in many countries also use trawl nets, sometimes from boats under 20 meters. These operations typically make shorter trips closer to shore and supply local markets rather than international trade. The line between “artisanal” and “industrial” varies by country, but the environmental pressures of bottom trawling apply at any scale when the gear contacts the seabed.
Where Bottom Trawling Is Restricted
Growing awareness of seafloor damage has led to trawling bans in certain waters. Sweden and Greece have both prohibited bottom trawling. The European Union has set a goal of banning the practice in all its marine protected areas by 2030, though implementation varies by member state. England, by contrast, decided in 2025 to allow bottom trawling to continue in its protected waters, drawing criticism from environmental groups.
Many countries manage trawling through gear restrictions rather than outright bans. These include requirements for bycatch reduction devices, minimum mesh sizes to let juvenile fish escape, seasonal closures during spawning periods, and designated zones where trawling is prohibited to protect sensitive habitats like deep-sea coral reefs. The regulatory picture is uneven globally, with some of the most heavily trawled regions in the developing world having limited enforcement capacity.