Fishing trawlers are working vessels designed to catch fish or shellfish by towing a large net through the water or along the seafloor. They matter because much of the seafood sold around the world comes from trawl fisheries. A trawler combines ship design, fishing gear, navigation, and onboard processing into one mobile harvesting system.
Understanding how trawlers work helps explain both seafood production and the environmental choices involved in fishing.
Key Facts
- Trawl speed is often about 2 to 5 knots, depending on the target species and gear type.
- Distance towed can be estimated with d = vt, where d is distance, v is speed, and t is time.
- Otter boards spread the trawl mouth open by using water resistance as the vessel moves forward.
- A demersal trawl targets organisms near or on the seabed, while a pelagic trawl targets fish in midwater.
- Catch per unit effort can be written as CPUE = catch mass / fishing time.
- Onboard processing often includes sorting, washing, gutting, chilling, freezing, and storing the catch.
Vocabulary
- Trawler
- A trawler is a fishing vessel that catches marine life by towing a net through the water or across the seafloor.
- Trawl net
- A trawl net is a cone-shaped fishing net with a wide mouth and a narrow codend where the catch collects.
- Otter board
- An otter board is a heavy angled panel that pulls sideways through the water to hold the trawl net open.
- Codend
- The codend is the closed rear section of a trawl net where fish and other catch accumulate.
- Bycatch
- Bycatch is the unintended capture of non-target species during fishing operations.
Common Mistakes to Avoid
- Thinking all trawlers scrape the seafloor, which is wrong because pelagic trawlers tow nets through midwater and may never touch the bottom.
- Ignoring towing speed when estimating fishing effort, which is wrong because the area swept by a net depends on both time and vessel motion.
- Assuming the net catches every fish in its path, which is wrong because fish behavior, mesh size, escape gaps, and water flow all affect capture.
- Treating all catch as marketable fish, which is wrong because trawls can bring up undersized fish, damaged catch, non-target species, and debris that must be sorted.
Practice Questions
- 1 A trawler tows its net at 4 knots for 3 hours. If 1 knot is 1.852 km/h, how far does the vessel travel in kilometers during the tow?
- 2 A trawler catches 1800 kg of fish during a 6 hour tow. Calculate the catch per unit effort in kg/h.
- 3 A fishing crew switches from a bottom trawl to a pelagic trawl in the same region. Explain how the target species, net position, and possible seafloor impact would change.