Bottom trawling is a fishing method where a large, cone-shaped net is dragged along the ocean floor to catch fish and shellfish that live near the seabed. It’s one of the most productive commercial fishing techniques in the world, responsible for roughly 28% of all global fish catch over the past six decades. It’s also one of the most controversial, because the heavy gear scrapes and plows the seafloor, damaging habitats and scooping up enormous quantities of non-target marine life in the process.
How Bottom Trawling Works
A bottom trawl is essentially a massive net shaped like a funnel, towed behind one or two boats. The wide mouth of the net is held open by a headline across the top and a heavy ground rope along the bottom. Two lateral wings extend forward from the opening to funnel fish toward the narrowing body of the net, which ends in a “codend” where the catch accumulates. The ground rope is fitted with rubber discs, metal bobbins, and spacers that help the gear roll over rough terrain and protect the net from tearing.
There are three main categories. Beam trawls use a rigid metal beam to hold the net open horizontally. Bottom otter trawls use heavy, door-shaped boards (called otter boards) that angle outward through the water to spread the net apart. Bottom pair trawls skip the boards entirely and use two boats towing the same net from a distance. Some configurations also drag chains called tickler chains along the seabed to startle flatfish up off the bottom and into the net. Those chains penetrate the sediment to a depth of 4 to 8 centimeters.
What It Catches
Bottom trawling targets species that live on or near the seafloor. In the northeastern United States, the primary targets include Atlantic cod, haddock, pollock, several species of flounder (yellowtail, winter, witch, summer, and windowpane), monkfish, halibut, redfish, various hakes, skates, and spiny dogfish. Globally, bottom trawling also harvests shrimp, sole, scallops, and other groundfish species depending on the region.
The method is effective but indiscriminate. Bycatch rates average between 31% and 55%, meaning roughly a third to over half of what comes up in the net isn’t the intended species. That average masks enormous variation: some shrimp trawl fisheries produce bycatch rates above 80%, while certain scallop and clam fisheries stay under 5%. Over the past 65 years, industrial bottom trawlers have discarded an estimated 437 million tonnes of fish, worth roughly $560 billion had it been landed and sold. That accounts for about 60% of all fisheries discards worldwide, despite bottom trawling representing only 28% of total catch.
Damage to the Seafloor
The environmental impact starts at the seabed itself. Heavy trawl gear flattens the natural texture of the ocean floor, crushing structures that took decades or centuries to form. Coral, sponges, and other organisms that create habitat for smaller species are destroyed on contact. The gear homogenizes sediment that had been naturally sorted by currents and biological activity, stirring fine particles into the water column and making sediment systems unstable.
Beyond the physical restructuring, trawling kills bottom-dwelling animals directly and disrupts the chemical processes happening at the boundary between sediment and seawater. This interface is where nutrients cycle and organic material gets broken down. When trawl gear churns it up repeatedly, those processes are altered in ways that ripple through local food webs.
Carbon Stored in the Seafloor
Ocean sediments are one of the planet’s largest carbon stores, and bottom trawling disrupts that storage. When gear plows through the seabed, it resuspends buried organic carbon, exposing it to decomposition that converts it into CO2. Research published in Nature Geoscience found that trawling-induced resuspension and mixing account for roughly 10% of total carbon breakdown on the seafloor in heavily fished areas like the North Sea.
Globally, bottom trawling reduces the carbon stored in shelf sea sediments by an estimated 0.008 to 0.01 billion tonnes of carbon per year. Converted to CO2, that’s roughly 0.03 to 0.037 billion tonnes annually. For the North Sea specifically, the long-term reduction in carbon storage capacity averages about 3.67 tonnes of CO2 per square kilometer per year. These numbers are modest compared to fossil fuel emissions, but they represent a loss of natural carbon sequestration that would otherwise happen passively on the ocean floor.
Why Depth Matters
Bottom trawling happens across a wide range of depths, but the deeper it goes, the worse the tradeoffs become. Below about 600 meters, the ratio of discarded fish to commercially valuable catch rises sharply. So does the proportion of sharks and rays in the net, species that reproduce slowly and recover poorly from overfishing. At the same time, the commercial value of the catch drops.
Deep-sea species tend to grow slowly, mature late, and produce fewer offspring, making them especially vulnerable to heavy fishing pressure. Research has suggested that capping bottom trawling at a maximum depth of 600 meters could be an effective management strategy, protecting the most vulnerable species and habitats while preserving the fisheries that generate the most economic value.
Where Bottom Trawling Is Restricted
Regulations vary widely by country. More than half of U.S. federal waters are now off-limits to bottom trawling, with especially large closures along the West Coast where essential fish habitat designations prohibit the practice. In Alaska, coral gardens have visibly recovered in areas where trawling has been banned for over a decade.
The European Union’s 2023 action plan mandates ending bottom trawling in all marine protected areas by 2030. Norway has banned the practice near deep-sea coral reefs. Ghana announced at the 2025 UN Ocean Conference that it will ban bottom trawling and all industrial fishing from its waters entirely. Several West African nations, including Nigeria and Ivory Coast, already prevent foreign trawlers from obtaining licenses in their exclusive economic zones. China has maintained restrictions on bottom trawling since the 1950s, though enforcement remains inconsistent. New Zealand has closed one of the largest marine areas in the world to bottom trawling, though debate continues over whether its management goes far enough.
Alternatives and Gear Modifications
The simplest improvements involve redesigning existing trawl gear to reduce contact with the seafloor. Otter boards that hover above the bottom instead of dragging on it, elevated footropes, and lighter net constructions all decrease sediment disturbance while still catching the target species. Bottom seining, which uses a lighter net without heavy trawl doors or warps, can cover a similar or even larger area with substantially less pressure on the seabed.
More advanced approaches use electricity or light to lift fish off the bottom before they reach the net. Pulse trawling, for instance, replaces the heavy tickler chains of beam trawls with mild electrical pulses that cause flatfish to swim upward. Scientific trials have shown that pulse trawls cut sediment disturbance roughly in half (a median penetration of 1.8 cm versus 4.1 cm for tickler chains) while also reducing bycatch mortality among bottom-dwelling invertebrates and lowering fuel consumption.
For some species, the trawl itself can be replaced entirely. Pots and longlines are economically viable alternatives for species like Pacific cod and sablefish in Alaska, offering better size and species selectivity with far less impact on the seabed. The broader challenge is that no single alternative works for every fishery, and the economic incentives to keep trawling remain strong in many regions. Effective management typically combines gear modifications, spatial closures, and depth limits rather than relying on any single solution.