Bottom trawling is a fishing practice that herds and captures the target species, like ground fish or crabs, by towing a net along the ocean floor.
Floats are attached to the headrope, top of trawl opening, while weights and special gear are attached to the footrope, bottom of trawl opening, to keep the net open as it moves through the water across the ocean floor. The mesh is designed to confine fish inside the net, trapping them in the codend as the trawl is hauled to the surface. A sweep attached to the net's footrope collects marine animals as they lay on the bottom or gather before the trawl opening. The trawl gear may be constructed and rigged for various target species over different types of bottom surfaces. Sweep types include:
- Chain sweep for smooth surfaces.
- Cookies (small steel or rubber rollers) for soft and irregular ocean floors.
- Rockhoppers (large molded rubber rollers) for rocky bottoms.
- Raised footropes that reduce groundfish bycatch.
Target Species
- Whiting
- Red hake
- Dogfish
- Crab
- Shrimp
- Flounder
Fisheries Regions
- Alaska
- New England/Mid-Atlantic
- Pacific Islands
- Southeast
- West Coast
- International
Risks to Sea Turtles
Many sea turtle species rest and forage on the bottom and are at risk of being captured in bottom trawls. Capture in a bottom trawl could result in:
- Drowning from being trapped in the net and held underwater for the duration of the trawl.
- Broken appendages or shell from the weight of the catch on top of them.
- Injury from the drop to the deck when the net is emptied aboard the fishing vessel.
- Stress and exhaustion from capture and release.
Risks to Marine Mammals
Marine mammals can become entangled by trawl gear when swimming to forage or migrate, with risks differing widely between species. Species that forage on or near the sea floor are at risk of being captured or entangled in netting or tow lines (also called lazy lines).
Pilot whales and common dolphins in the Atlantic are particularly susceptible to being caught in bottom trawls.
Bycatch Reduction
Sea turtle mortality in trawl gear was once very high. Turtle excluder devices have greatly reduced these risks in some trawl fisheries and might even allow small cetaceans to escape as well. TEDs are currently only required in trawl fisheries targeting shrimp and summer flounder.
Some recommended voluntary mitigation measures for reducing marine mammal bycatch include:
- Reducing the number of turns per tow at night.
- Reducing the duration of each tow.
- Encouraging frequent radio communications between captains to raise awareness of nearby animals.
More Information
Humpback whales swimming underwater. Credit: Kogia.org
An endangered leatherback sea turtle swimming at the ocean surface. Once prevalent in almost every ocean, the leatherback population is rapidly declining in many parts of the world. As one of NOAA Fisheries’ Species in the Spotlight, we have made it a priority to focus recovery efforts on stabilizing and recovering the leatherback sea turtle to prevent its extinction. Photo credit: NOAA Fisheries
Gray whale breaching. Credit: NOAA Fisheries/Merrill Gosho.
A Rice’s whale just under the surface of the water in the Gulf of Mexico. This endangered whale was recognized as a separate species from the Bryde’s whale in 2021. Credit: NOAA Fisheries under NOAA Permit No. 21938.