A trawl is a large, bag-shaped net that is towed by a fishing vessel. Trawlers are generally large boats ranging from 70 feet to over 200 feet in length. The doors, because of the way they are built and rigged to the trawl, keep the mouth of the trawl open as it moves through the water. The headrope is equipped with floats forming the upper opening. The footrope is rigged with weights forming the lower opening. Trawlers use sophisticated ultrasonic devices both for location of fish underwater and for species identification.
Pelagic trawls sometimes contact the ocean bottom, but their heavy doors and wires are kept off the bottom to avoid damage to benthic habitat. Fishing with pelagic trawls is a selective method of fishing, because the nets can be operated in ways to minimize the incidental catch of non-target species. Pollock are usually harvested with few other species.
Sole are generally captured in bottom trawls in mixtures of the various species which are sorted onboard the fishing vessel. Trawling is allowed only in certain areas and strict limits are enforced upon the amount of non-target species (such as crab or halibut) that may be caught. In fact, it frequently happens that a trawl fishery is closed because it reached the pre-set "bycatch" limit, and does not achieve full harvest of its target species.

Upon locating a school of the desired species, the vessel trawls through the school and captures the fish. The fish accumulate in the end of the trawl, which is called the "cod end", regardless of the species of fish being harvested. Electronic sensors tell the harvester exactly where the trawl is in relation to the fish and the ocean floor, while other sensors report how full the trawl becomes. When capture is complete, the trawl is brought to the surface.
Once the trawl full of fish reaches the surface of the water, one of two things happens. If the vessel has the ability to process the fish onboard, it is called a factory-trawler or a freezer-trawler or catcher-processor. These vessels simply pull the net aboard, empty the net, sort the species, and process the catch. If the vessel is only capable of catching fish, then it must deliver the catch to a processing plant. These processing plants might be in other vessels, called floating processors, or they might be on shore. The catcher-vessel (trawler) usually takes the fish onboard and stores the fish in refrigerated tanks below decks. This keeps the fish in top quality until they are delivered and processed. In either case, the fish are kept well-chilled, and they are processed within a few hours of harvest. Trawls are the only fishing method used to harvest pollock and sole. They are sometimes used to catch cod and sablefish but never halibut.