A trawl is a large, bag-shaped net that is towed by a fishing vessel. Trawlers (not to be confused with trollers) 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.
Two types of trawl gear are used in Alaska. Pollock are caught using mid-water, or pelagic, trawls, which are designed to fish off the ocean bottom to avoid damage to the benthic habitat. Fishing with mid-water 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 along with other bottom-dwelling species, which are sorted onboard the fishing vessel. Bottom 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.