Trawling is another fishing technique that results in high numbers of bycatch. The two trawling methods are pelagic and bottom (“dimersal”) trawling. Bottom trawling, the more environmentally destructive of the two, involves a fishing vessel which drags a funnel shaped weighted net, equipped with a mechanism to disturb the seabed, along the sea floor to sweep up everything in its path. This method is indiscriminate, uncontrollable in the numbers of organisms netted and is extremely destructive to ancient and fragile seabed communities.
Deepwater coral communities and sea mounts are being devastated by bottom trawls, an activity which has been compared to the clear-cutting of rainforests. Corals are among the oldest living animals on the planet and are slow growing organisms; some species only growing a millimeter every year. When coral communities are damaged they are lost for generations. In addition, bottom trawls may be extinguishing endemic communities of sponges, crustaceans, fish, and other species that we have yet to even discover.
Shrimp trawls in particular are notorious for their high level of bycatch, catching animals ranging from fish to endangered sea turtles. The unwanted dying or already dead bycatch is commonly discarded back into the water. Gear modifications such as Bycatch Reduction Devices (BRDs) and Turtle Excluder Devices (TEDs) greatly reduce unintended bycatch by allowing animals an escape. Unfortunately, laws and their enforcement mandating these devices vary greatly across nations.
Pelagic (or mid-water) trawls are designed to catch large schools of fish such as tuna, sea bass and anchovies by dragging a net higher in the water column. By doing this however, they are the cause of a high level of cetacean bycatch. Because the fish species targeted by these trawls are also important prey sources for dolphins, dolphins are the most common bycatch victims of this fishing method.