Trawling is a globally common, industrial fishing method responsible for a significant portion of the world’s wild-caught seafood. This technique involves dragging a large, funnel-shaped net through the water behind one or more vessels. As an active fishing process, trawling is designed for efficiency and volume, making it a powerful tool in commercial fisheries worldwide.
What Defines Trawling
Trawling is classified as an active fishing method because the vessel must continuously move and pull the gear, contrasting with passive methods like stationary netting. The core component is the trawl, a large, cone-shaped net that tapers to a narrow closed end. This net is engineered to herd and capture fish as it is towed through the water column.
The primary goal of trawling is to achieve high-volume catches of commercially valuable species. Operations target various marine life, including demersal (bottom-dwelling) species like cod, flounder, and shrimp, as well as schooling pelagic species. The scale and active nature of the gear allow a single trawler to harvest enormous quantities of fish.
The Mechanics of Trawl Fishing
The successful deployment and operation of a trawl net rely on a sophisticated system of engineering components. The net itself is composed of three main sections: the wide-open mouth, the long tapering body, and the cod end, the closed terminal section where captured fish accumulate. This overall funnel shape concentrates the catch toward the retrieval point.
To keep the net open horizontally, heavy, wing-like metal plates called otter boards, or trawl doors, are attached to the towing lines. These doors are angled so that the hydrodynamic pressure of the water pushes them outward as the vessel moves forward, spreading the mouth of the net wide. Maintaining the vertical opening of the net requires a balance of buoyancy and weight.
The top edge of the net’s mouth, known as the headline, is equipped with floats to provide lift. The bottom edge, or footrope, is weighted with chains or rollers to keep it close to the desired depth. Winches on the vessel manage the heavy steel cables, or warps, that tow the gear, controlling its depth and retrieval speed. Modern trawlers utilize advanced sonar and echosounder technology to locate fish schools and monitor the net’s geometry underwater.
Bottom Trawling Versus Midwater Trawling
Trawling is separated into two categories based on where the net is towed in the water column: bottom trawling and midwater trawling. Bottom trawling, also known as demersal trawling, is designed to drag along or just above the seafloor to capture organisms that live on or near the seabed. This method uses heavily weighted footropes and reinforced gear, sometimes including heavy rubber wheels called “rock hoppers,” to withstand the friction and obstacles of the ocean bottom.
Bottom trawlers typically target groundfish and shellfish, such as sole, halibut, and shrimp. The operation is specifically tailored to scoop up these bottom-dwelling species. In contrast, midwater trawling, or pelagic trawling, is executed by towing the net in the middle of the water column, intentionally avoiding contact with the seafloor.
Midwater trawls are larger and are typically used to target fast-moving, schooling fish in open water. Species like tuna, mackerel, and pollock are the common targets for this method. Because these nets are towed off the bottom, the equipment is generally lighter and the operation focuses on precise depth control to intercept schools detected by sonar.
Ecological Impact of Trawling
The non-selective nature of trawl gear is a significant environmental concern, particularly regarding the issue of bycatch, the capture of non-target marine life. Trawl nets are often indiscriminate, collecting juvenile fish, protected species, and invertebrates alongside the intended catch. The discarded portion of the catch, which is often dead or dying, represents a substantial waste of marine resources.
In some fisheries, such as those targeting tropical shrimp, the bycatch rate has been reported to be extremely high, sometimes exceeding 90% of the total volume caught. This incidental capture includes vulnerable populations, such as sea turtles, marine mammals, and seabirds. The resulting mortality can severely impact the recovery rates and overall health of these non-target populations.
Bottom trawling also causes extensive physical damage to seafloor ecosystems. The trawl doors and heavy footropes scrape and plow the seabed, leaving behind visible “trawling scars” in the sediment. This action destroys complex, slow-growing habitats like deep-sea coral reefs and sponge gardens, which can take decades or even centuries to recover.
The physical disturbance of the seabed can also have chemical consequences for the water column. Scraping the bottom re-suspends large amounts of fine sediment, which reduces light penetration needed by photosynthetic organisms. This churning action can reintroduce buried pollutants, such as DDT and PCBs, which have settled on the seafloor, making them available to enter the marine food web once again.