Bottom trawling

Bottom trawling is a way of fishing that involves towing a large net along the ocean floor. This method is often called dragging. Scientists divide this practice into two main types. Benthic trawling means the net is pulled at the very bottom of the ocean. Demersal trawling means the net is towed just above that bottom zone. This method is different from midwater trawling, which catches fish higher up in the water. Bottom trawling targets groundfish and species like cod, squid, shrimp, and rockfish.

How the net works depends on the gear used. In beam trawling, a solid metal beam holds the mouth of the net open. This is often used by smaller boats near the shore. Another way is otter trawling, which uses large boards made of wood or steel. These boards use water force to push outward and keep the net open. The boards act like a plough to stir up the seabed. The net itself is shaped like a funnel. It is wide at the mouth and gets narrow at the end, called the cod end.

People have used forms of trawling for a very long time. In 1376, during the reign of Edward III, people even complained about a tool called the wondyrchoum. This was an early beam trawl with a wooden beam. In Japan, a similar method called Utase Ami was used during the Edo era in 1704. Later, fishermen in Brixham, Devon, developed sailing trawlers. These boats could tow much larger nets in deeper waters. By the late 1800s, there were over 3,000 sailing trawlers in UK waters.

There are many important facts about the scale of this fishing. Global catch from bottom trawling is estimated at over 30 million tonnes every year. This is a larger amount than any other fishing method. Trawling can also affect the environment in big ways. A 2021 paper estimated that it might release 600 to 1500 million tons of carbon dioxide a year. This happens because the nets disturb carbon in the seafloor. Trawling also stirs up massive amounts of sediment. Scientists estimate this mass is about 22 gigatonnes per year.

This way of fishing changes the world beneath the waves. It can move boulders and leave deep scars in the mud. It can also hurt living things like the slow-growing Lophelia pertusa coral. Some areas, like the Grand Banks, have seen fish populations drop because of it. To help, some nets now use turtle excluder devices. These are special parts that let animals like turtles escape. Some areas are also set aside as marine protected areas to keep the seafloor safe.