Bottom trawling is a commercial fishing method that involves dragging a large, heavy net along the ocean floor to catch marine life living on or near the seabed. This technique is designed to capture demersal species, such as cod, hake, shrimp, and flatfish. The gear, which includes weighted nets and metal plates (trawl doors), is towed by one or two vessels across the seafloor. This approach is highly effective for harvesting large quantities of groundfish, accounting for approximately a quarter of the world’s wild-caught seafood annually.
Physical Destruction of Seafloor Habitats
The primary harm caused by bottom trawling is the physical destruction of seafloor habitats. The heavy trawl doors and ground gear act much like a bulldozer, scraping and plowing the seabed, leveling complex underwater topography. This mechanical disturbance removes or crushes structural components that provide shelter and feeding grounds for thousands of marine species. Studies show that a single pass of a trawl net can displace boulders weighing up to 25 tons and eliminate habitat complexity.
This destructive force is particularly devastating to fragile, slow-growing ecosystems, which are often biodiversity hotspots. Deep-sea corals and sponges create intricate, three-dimensional structures that function as nurseries and refuges for fish and invertebrates. Bottom trawling can reduce the cover of structure-forming corals on seamounts by as much as 95 to 98%. The removal of these structures converts complex, diverse habitats into homogenized, barren expanses, leading to a significant loss of species richness. The scale of this destruction is immense, with estimates suggesting that U.S. trawlers alone have altered a seafloor area greater than the state of California.
Mortality of Non-Target Marine Life
Bottom trawling is a non-selective fishing method, catching nearly everything in its path. This indiscriminate capture of unwanted organisms is known as bycatch, representing a significant waste of marine life. Globally, bottom trawling is responsible for a large portion of all fishing-related bycatch, with some estimates suggesting it accounts for nearly 50% of the total.
Bycatch rates are highly variable but can average between 31 and 55% of the total catch in bottom trawl fisheries, with some shrimp trawl fisheries reporting rates exceeding 80%. This includes non-commercial fish species, vulnerable and protected animals, juvenile target fish, sea turtles, marine mammals, sharks, and rays. Once caught, these non-target species are typically discarded back into the ocean, often dead or dying from the trauma of capture and decompression. The high rate of discarding disrupts the marine food web by removing a substantial amount of biomass from the ecosystem.
Disruption of Carbon Storage and Sediment Chemistry
The deep ocean floor plays a significant role in climate regulation as a “blue carbon” sink, where organic carbon is stored within the sediments over geological timescales. Bottom trawling severely disrupts this carbon storage by stirring up the undisturbed seafloor sediments. The heavy gear penetrates the seabed, mixing the layers and resuspending decades or centuries of buried organic matter into the water column.
Once this carbon-rich material is exposed to oxygenated water, it begins a process of rapid decomposition and mineralization. This process releases stored carbon back into the ocean as dissolved CO2. Scientific models estimate that global bottom trawling could release a substantial amount of carbon dioxide annually, comparable to the emissions of some major industrial sectors. Trawling also alters sediment chemistry by depleting macrofauna like worms and mollusks, whose burrowing activities (bioturbation) naturally help bury fresh carbon into deeper layers.
Impeding Ecosystem Recovery
The damage caused by bottom trawling is severe and long-lasting. The severity of the harm is compounded by the slow life history traits of many deep-sea organisms. Deep-sea corals and sponges, which are foundational species for many complex habitats, can grow at rates of only a few millimeters per year.
Because of these slow growth rates, the recovery of a deep-sea coral reef or sponge community from a single trawling event can take decades, or even centuries, to reach its original complexity. Trawling often occurs repeatedly in the same areas, which is common practice, preventing the ecosystem from progressing past the initial stages of recovery. In some areas, scientists have observed no clear signs of recovery in benthic communities even 14 years after trawling activities had stopped.