Trawling is a method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different species of fishes or sometimes targeted species. Trawls are often called towed gear or dragged gear. The boats that are used for trawling are called trawlers or draggers. Trawlers vary in size from small open boats with as little as 30 hp (22 kW) engines to large factory trawlers with over 10,000 hp (7.5 MW). Trawling can be carried out by one trawler or by two trawlers fishing cooperatively (pair trawling). Trawling can be contrasted with trolling. While trawling involves a net and is typically done for commercial usage, trolling instead involves a reed, rod and a bait or a lure and is typically done for recreational purposes. Trawling is also commonly used as a scientific sampling, or survey, method. Bottom vs. midwater trawling Trawling can be divided into bottom trawling and midwater trawling, depending on how high the trawl (net) is in the water column. Bottom trawling is towing the trawl along (benthic trawling) or close to (demersal trawling) the sea floor. Bottom trawling is an industrial fishing method in which a large net with heavy weights is dragged across the seafloor, scooping up everything in its path. Bottom trawling can be disadvantageous because it stirs the sediments that lie on the sea bed and can harm some marine species. It also causes water pollutants to mix with some plankton, which in turn will move into the food chain, which will then create harmful algae blooms leading to insufficient oxygen. A 2021 study estimated that greenhouse gas emissions from bottom trawling were as much as aviation. Midway trawling or pelagic trawling target fishes that are living in the upper water column of the ocean. The funnel shaped trawl nets are hauled by one or two boats. This method is generally used to catch fishes of a single species. Unlike bottom trawling, this type of trawl does not come into contact with the sea bed and hence is not involved in damage of marine habitat. Some species caught with this trawling method are mackerel, herring, and hoki. However there may be some disadvantages in using this method as in the process of catching the targeted species of fish,