Trawling is a global commercial fishing technique that involves pulling a large net through the water or along the seafloor to catch fish and other marine life. Specialized vessels, known as trawlers, deploy, tow, and retrieve these extensive nets. This active method efficiently harvests significant quantities of seafood.
Understanding Trawling
Trawling captures large volumes of fish and shellfish by drawing a net through the marine environment. This technique efficiently harvests species that aggregate in schools or inhabit specific seabed areas. The trawl net, often cone-shaped, filters marine organisms from the water column or ocean floor into a ‘cod-end’ where the catch accumulates. Its ability to cover expansive areas makes it a common choice for commercial fisheries.
Trawling Methods and Equipment
Trawling encompasses distinct methods, primarily categorized by where the net is towed in the water column.
Bottom Trawling
Bottom trawling, also known as demersal trawling, involves dragging heavy, weighted nets along or just above the seafloor. This method targets species like cod, haddock, flatfish, shrimp, and crabs. Bottom trawls use timber or steel otter boards, also called trawl doors, to keep the net’s mouth open horizontally and maintain contact with the seabed. Floats on the top rope (headrope) and weights on the bottom rope (footrope) ensure the net remains open vertically. Specialized rollers or bobbins on the ground gear allow the net to move over uneven terrain and minimize snagging.
Midwater Trawling
In contrast, midwater trawling, or pelagic trawling, involves towing a net higher in the water column, avoiding contact with the seafloor. This technique catches schooling fish species that inhabit open waters, such as mackerel, herring, hoki, and anchovies. Midwater nets have their horizontal opening maintained by otter boards or by being towed between two vessels in a practice known as pair trawling. Trawlers use advanced sonar systems to locate and precisely target these fish schools. The net is typically deployed from the stern of the vessel, with the trawl doors spreading the net as the boat moves forward.
Environmental Impact of Trawling
Trawling, particularly bottom trawling, can alter marine ecosystems. The heavy nets and associated gear act like underwater bulldozers, damaging fragile seafloor habitats such as coral reefs, sponge beds, and seagrass meadows. This physical disruption can tear up root systems and animal burrows, transforming vibrant underwater communities into barren landscapes. Recovery for slow-growing, long-lived species like deep-sea corals can take decades or even centuries.
Beyond physical damage, bottom trawling also resuspends large quantities of sediment from the seabed. This process can change the water’s chemistry, reduce light penetration, and potentially re-release centuries-old carbon stored in marine sediments, contributing to greenhouse gas emissions.
Bycatch, the unintentional capture of non-target species, is a primary concern across both bottom and midwater trawling. This includes juvenile commercial fish, non-commercial organisms, and vulnerable species such as sea turtles, marine mammals, seabirds, and sharks. Bycatch can comprise a substantial portion of the total catch, sometimes reaching up to 60%.
This disrupts population structures and undermines the reproductive capacity of fish stocks. Many bycaught animals suffer injury or mortality from rapid pressure changes, lack of oxygen, or physical trauma before being discarded. The cumulative effect of habitat destruction and bycatch reduces species richness and overall biodiversity, impacting marine food webs.
Managing Trawling Operations
Efforts to manage trawling operations aim to mitigate environmental consequences and promote sustainable fishing practices. Regulatory measures include establishing fishing quotas, designating restricted areas or Marine Protected Areas (MPAs) where trawling is prohibited, and implementing seasonal closures to protect spawning grounds or allow vulnerable populations to recover.
Technological advancements and gear modifications also reduce impacts. Innovations like Turtle Excluder Devices (TEDs) provide escape openings for sea turtles. Bycatch Reduction Devices (BRDs), such as grid-like exclusion systems or larger mesh sizes, allow non-target species or undersized fish to escape while retaining the desired catch. The use of seabed mapping and habitat surveys helps fishermen avoid sensitive ecosystems, minimizing trawling’s physical footprint. These strategies, combined with ecosystem-based fisheries management, seek to balance economic benefits with protecting marine biodiversity and habitats.