Bottom trawling is a fishing method that involves dragging large, weighted nets across the seafloor to catch species living on or near the ocean bottom, such as cod, hake, shrimp, and sole. The practice has generated considerable environmental concern due to its wide-ranging impacts on marine ecosystems.
Damage to Ocean Floor Habitats
Bottom trawling inflicts substantial physical damage on seafloor habitats. The heavy trawl gear, which includes large metal doors and thick cables or chains, scrapes and pulverizes the seabed. This action has been likened to an “underwater bulldozer” or “rototilling” the ocean floor, destroying marine environments.
The physical disturbance affects delicate ecosystems. Cold-water coral reefs, which can take centuries to form, are particularly vulnerable; studies show up to 90% of a coral colony perishing from a single trawl pass. Sponge gardens, seagrass beds, and soft sediment habitats also suffer extensive harm as the gear tears up root systems and animal burrows. Recovery of these damaged habitats is often slow, taking decades or even centuries for some deep-sea coral communities to show new growth.
Threat to Marine Biodiversity
Beyond habitat destruction, bottom trawling poses a significant threat to marine biodiversity through the indiscriminate capture of marine life. This issue, known as bycatch, involves the unintentional netting of non-target species. Bycatch can include endangered marine mammals, sea turtles, sharks, seabirds, and juvenile fish, many injured or killed before being discarded.
The discard rate from bottom trawling is high, averaging between 31% and 55% of the total catch, though some shrimp fisheries report rates exceeding 80%. This large-scale removal of non-target species depletes fish stocks and disrupts marine food webs. When species that serve as food for larger predators are displaced or eliminated, it creates gaps in the food chain that can take years to recover. Areas subjected to frequent trawling have shown reductions in species diversity across all trophic levels, with recovery times potentially extending beyond 15 years.
Release of Stored Carbon
Bottom trawling also contributes to climate change by disturbing marine sediments that store vast amounts of carbon. The seafloor holds approximately 50 times more carbon than the atmosphere, with the top meter of sediment storing as much carbon as three meters of soil. When heavy trawling gear is dragged across the seabed, it resuspends these carbon-rich sediments into the water column.
This disturbance releases stored organic carbon, and in some cases, intensifies the oxidation of pyrite, a mineral found in marine sediments, leading to additional carbon dioxide (CO2) emissions. The released carbon can then enter the atmosphere, contributing to ocean acidification and global warming. Estimates suggest bottom trawling globally releases tens of millions of tonnes of CO2 annually, with some studies indicating figures as high as 370 million metric tons per year, though the precise magnitude remains a subject of ongoing research.