<P> 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). </P> <P> Trawling can be contrasted with trolling, where baited fishing lines instead of trawls are drawn through the water . Trolling is used both for recreational and commercial fishing whereas trawling is used mainly for commercial fishing . Trawling is also commonly used as a scientific sampling, or survey, method . </P> <P> 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 . Midwater trawling is towing the trawl through free water above the bottom of the ocean or benthic zone . </P> <P> Midwater trawling is also known as pelagic trawling . Midwater trawling catches pelagic fish such as anchovies, shrimp, tuna and mackerel, whereas bottom trawling targets both bottom - living fish (groundfish) and semi-pelagic fish such as cod, squid, halibut and rockfish . </P>

Which fish species has been reduced by huge marine trawling operations