Modern trawlers fish for pollock, cod, sole, rockfish, shrimp, and many other species by towing nets behind them in which the catch is trapped by the forward movement of the boat. The nets are towed through mid-water for some kinds of fish, and on the bottom for others. Trawlers range in size from about forty feet for shrimpers and smaller inshore boats to five hundred feet or more for large, offshore factory ships that can also process their catch on board. Some smaller trawlers make day trips from their home ports, while others stay out for weeks or even months. Trawlers fish in every ocean in the world.