Trawls have been used in oceanographic research since early studies of marine biodiversity. Trawls are specialized large nets, akin to those used by fishermen. Trawls can have a variety of forms depending on the organisms of most interest. For example, the type of trawl used to study large shoals of adult cod is different from the trawl used to collect small planktonic larval cod. There are both benthic (used along the surface of the ocean floor) as well as pelagic (used down to depths as great as 3000 meters) trawls. Some trawls sample at different water depths in a series to study the movement of organisms in the water column. Though trawls can be useful in the study of marine biodiversity, they have their limitations. Many animals have been shown to be good at avoiding capture in trawls and other species can be easily damaged or destroyed in the process (especially those from great depths and those that are soft-bodied, such as jellyfish). Therefore, scientists often combine trawls with other research tools such as Video Plankton Recorders, acoustic technologies, and larger imaging equipment. Trawls have proven themselves to be a mainstay of marine research and Census projects use them extensively.
Scientists and crew sort through large amounts of specimens brought up in trawls at different depths, providing important clues about movement of organisms. (Gulf of Maine Area Program - GoMA)
A diagram demonstrating how Sampling Nets and Multi-Frequency Echo Sounders can work together to study the movement of organisms. (Gulf of Maine Area Program - GoMA, W. Michaels)