A bottom trawl consists of a large net with a wide mouth and a small enclosed codend. They look like pelagic trawl nets, but are normally much smaller (maximum length ca. 200 meters). The opening of the net has two heavy doors that keep the net open and down on the ocean floor. These "otter doors" can weigh several tons. In addition to the heavy doors, the bottom of the net is a thick metal cable, which is equipped with heavy steel balls or rubber bobbins, so that the net can roll over the sea floor.
Bottom trawls are used to catch demersal fish, such as cod and flatfish and crustaceans, like shrimps and prawns. The mesh size of the net regulates the size and species to be captured.
As the bottom trawl net drags along the seafloor, everything in its path is crushed, ripped up, or smothered as the seabed is turned over. Thereby bottom trawls can destroy large areas of seafloor habitats that provide food and shelter for a wide variety of marine species and are important nursery areas for young generations of fish and invertebrates. But the damage is not limited to the destruction of the sea floor: all creatures in the path of the net, like fish, crustaceans and other invertebrates are caught. The fishing vessels only keep the commercial species and discard the remaining. Thus, many creatures end up thrown overboard dead or dying, including vulnerable deep-sea corals that can live for hundreds of years or more. Within a few weeks or months, bottom trawl fisheries can destroy what took nature many thousands of years to create.
Deep Sea Fishing
Because coastal and open-water fisheries have become more and more depleted, industrial fishing vessels have increasingly turned to exploiting deep-sea species. The primary method of deep-sea fishing is bottom trawling. Modern trawlers are capable of fishing seamounts, deep-sea canyons and rough seafloor in up to 2000 meters ocean depth. To catch a few target fish species of commercial value, important and diverse deep-sea ecosystems like deep sea corals and sponge gardens are destroyed and many other species are caught as bycatch and thrown back into the ocean, dead or wounded.
Learn more about the techniques of modern fisheries here.