A fishing trawler is a commercial vessel designed to catch fish by pulling large nets through the water. It’s one of the most common types of fishing boat in the world, used to harvest everything from shrimp and cod to herring and krill. What sets a trawler apart from other fishing boats is its method: rather than using hooks, traps, or stationary nets, it drags (or “trawls”) a funnel-shaped net behind it, scooping up fish as it moves.
How Trawling Works
The basic concept is simple. A trawler tows a large cone-shaped net with an open mouth at the front and a closed end, called the cod-end, at the back. As the boat moves forward, water flows through the net while fish are funneled toward the cod-end and trapped. The net is held open by a combination of weighted cables, floats, and specialized metal plates called “doors” or “otter boards” that spread outward under water pressure, keeping the mouth of the net wide.
Modern trawlers rely on acoustic monitoring systems to make this process far more precise than it was decades ago. Sensors mounted on the net measure its shape, depth, water flow, and even catch rate in real time. Echo-sounders detect schools of fish ahead of the vessel, measuring their size and density based on the strength of reflected sound waves. A headline sensor tracks the distance between the top of the net and the seafloor. Systems like Simrad’s PX TrawlEye and NOTUS’s Trawl Master give the crew a live picture of what’s happening hundreds of meters behind and below the boat, letting them adjust speed and course to optimize the catch.
Bottom Trawling vs. Midwater Trawling
The two main types of trawling differ by where in the ocean they operate, and this distinction shapes the vessel, the gear, and the environmental consequences.
Bottom trawlers drag their nets along the seafloor to catch species that live near the seabed: cod, flounder, shrimp, sole, and other groundfish. The net’s lower edge is weighted with heavy chains or rollers that maintain contact with the bottom. This is effective but destructive, a point covered in more detail below.
Midwater (pelagic) trawlers tow their nets through the open water column, typically in the upper 200 to 1,000 meters. These nets are generally much larger than bottom trawl nets and target schooling species that swim in dense groups: herring, mackerel, anchovy, sardines, pollock, blue whiting, sprat, capelin, and jack mackerel. Pelagic trawling is also the primary method for harvesting Antarctic krill, and it can occasionally target faster species like tuna. A pelagic trawl can be towed by a single vessel or by two boats working in tandem, known as pair trawling.
There’s also a hybrid approach called semi-pelagic trawling, where the net can operate near the seabed or higher in the water column depending on the target species.
Types of Trawler Vessels
Trawlers come in a wide range of sizes, from small inshore boats under 15 meters to massive factory ships over 100 meters long. The key design differences come down to where on the boat the net is deployed.
Stern trawlers are the most common modern design. They launch and retrieve the net over the back (stern) of the vessel through an open stern door or ramp. A typical stern trawler has the wheelhouse positioned forward, giving the crew an unobstructed working deck at the rear. A hydraulic winch on the main deck controls the trawl cables, and a gantry overhead swings between positions to guide the net. This layout is safer and more efficient than older designs because the crew works on a wide, open deck rather than along the narrow sides of the boat.
Side trawlers, the older design, deploy and haul nets over the side. They’ve largely been replaced by stern trawlers on larger vessels but still exist in some smaller fleets. Beam trawlers use a rigid metal beam to hold the net mouth open instead of otter boards, and they’re especially common in European flatfish fisheries. Outrigger trawlers use long poles extending from the sides to tow multiple smaller nets simultaneously, a setup widely used in tropical shrimp fisheries.
Factory Trawlers and On-Board Processing
The largest trawlers are essentially floating fish-processing plants. Known as factory trawlers or freezer trawlers, these vessels can catch, process, freeze, and store fish without ever returning to port for weeks or even months at a time.
On the processing deck, fish pass through graders that sort them by size, heading machines that remove heads, and automated conveyor systems. The cleaned fish are then loaded into plate freezers, which use pressurized ammonia cooling systems to freeze the catch rapidly. Below the processing deck, the freezer hold uses specialized tube air-coolers mounted along the ceiling to maintain even temperatures throughout the storage area without requiring ductwork or fans. The goal is to freeze fish at sea as quickly as possible, preserving quality that would otherwise degrade during a long trip back to shore.
A vessel like the America’s Finest, a purpose-built factory trawler operating in Alaska, was designed around this concept of maximum automation, with the processing layout specifically engineered to reduce the manual labor required at each step.
Environmental Impact of Trawling
Trawling’s environmental footprint varies dramatically depending on the type. Bottom trawling is one of the most ecologically damaging fishing methods still in widespread use. Midwater trawling is generally far less harmful to habitats, though it raises its own concerns about bycatch.
Bottom trawling essentially plows the seafloor. A U.S. Geological Survey analysis found that the amount of sediment stirred up by bottom trawling globally is roughly equal to all the sediment deposited on the world’s continental shelves by rivers each year: nearly 22 gigatons. That resuspended sediment changes water chemistry, lowers light levels, and reduces photosynthesis in ocean plants that form the base of marine food webs. The physical damage is equally severe. Trawl gear tears up root systems of seafloor plants, destroys animal burrows, and can convert soft mud habitats to bare rock, eliminating the creatures that depend on sediment to survive. Species diversity drops, and complex habitats that took decades or centuries to develop can be flattened in a single pass.
Bycatch, the unintended capture of non-target species, is a persistent problem across all types of trawling. Trawl nets are not perfectly selective, and they can catch juvenile fish, sea turtles, sharks, marine mammals, and seabirds depending on the fishery and the gear used. Modern modifications like escape panels, turtle excluder devices, and mesh size regulations have reduced bycatch in many fisheries, but it remains a significant concern.
How Trawling Is Regulated
International and regional bodies now restrict where and how trawling can occur, particularly in deep-sea areas with fragile ecosystems. The South Pacific Regional Fisheries Management Organisation (SPRFMO), for example, requires that bottom trawling occur only within designated bottom trawl management areas, while midwater trawling is confined to either midwater or bottom trawl management areas. Fishing outside these zones is prohibited unless a vessel is operating under a strict exploratory fishing protocol.
These regulations also include encounter rules. If a trawler catches certain organisms that indicate a vulnerable marine ecosystem, such as deep-sea corals or sponges, above specific weight thresholds, the vessel must immediately stop fishing within a buffer zone around that location and report the encounter. Large-scale pelagic driftnets and all deepwater gillnets are banned entirely in some convention areas.
Many coastal nations have their own additional restrictions. Some have banned bottom trawling in certain zones outright, particularly near coastlines and over sensitive reef systems. Gear modifications like larger mesh sizes, which allow juvenile fish to escape, are mandatory in many regulated fisheries.
Life and Risk on a Trawler
Working on a commercial trawler is one of the most dangerous jobs in the world. Between 2000 and 2016, the U.S. commercial fishing industry had a fatality rate of 115 deaths per 100,000 workers, compared to an average of 4 deaths per 100,000 across all U.S. occupations. During that period, 755 commercial fishermen died on the job. Nearly half of those deaths, 48%, happened after a vessel disaster such as capsizing or sinking. Another 30% resulted from falling overboard, and 13% were caused by traumatic injuries sustained while working on deck.
A typical trawler crew includes a skipper (captain), a mate (second in command), an engineer, and several deckhands who handle the nets, sort the catch, and maintain equipment. On factory trawlers, additional crew work the processing lines below deck. Hours are long and fatigue is a constant factor. There’s a deep cultural acceptance of exhausting work schedules in commercial fishing, and researchers have identified this normalization of fatigue as a structural safety problem rather than simply an individual one. Noise exposure, repetitive upper-body strain, and the physical demands of working on a moving deck in rough seas contribute to chronic health problems that compound over a career.