Bottom trawling facts for kids
Bottom trawling is a way of fishing where a large fishing net (called a trawl) is pulled along the bottom of the ocean. It's sometimes called "dragging" because the net drags across the seabed. There are two main types of bottom trawling. One is called benthic trawling, where the net is pulled right on the very bottom. The other is demersal trawling, where the net is pulled just a little bit above the seabed.
Another type of fishing is midwater trawling. This is when a net is pulled higher up in the water, not near the bottom. Midwater trawling catches fish that swim in the open water, like anchovies, tuna, and mackerel. Bottom trawling, however, catches fish and other creatures that live on or near the ocean floor. This includes fish like cod, squid, shrimp, and rockfish. A special kind of boat called a trawler is used for this type of fishing. Trawlers can be small boats or very large factory ships.
How Trawl Nets Work
When a trawl net is pulled, special parts called "trawl doors" drag on the seabed. These doors stir up a cloud of mud and sand. This muddy cloud helps hide the net as it comes along. The noise from the trawl doors can also attract fish. Fish start swimming in front of the net's opening. As the trawl keeps moving, the fish get tired and slowly fall back into the net. Eventually, they are too tired to swim away and get caught.
The speed at which the trawl is pulled depends on how fast the target fish can swim. It also depends on the fishing gear being used. For most fish, a speed of about 4 knots (which is about 7 kilometers per hour) works well. Trawling is a very effective way to catch a lot of fish. However, some people compare it to "fishing with bulldozers" because it can scrape up almost everything on the ocean floor.
Impact on the Ocean
Bottom trawling has been used for over 100 years in places with a lot of fishing, like the North Sea and the Grand Banks. For a long time, overfishing was seen as the main reason for big changes in the fish populations in these areas. More recently, people have become worried about the damage that bottom trawling does directly to the seabed.
One special concern is a type of deep-water coral called Lophelia pertusa. This coral grows very slowly and provides a home for many different deep-sea creatures. Fishing gear can easily damage these corals. Because of these concerns, on November 17, 2004, the United Nations General Assembly asked countries to think about temporarily banning bottom trawling in the open ocean. Bottom trawling has been blamed for reducing the numbers of many fish species, including orange roughy, barndoor skate, and sharks.
Images for kids
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The Celtic Explorer, a research vessel used for bottom trawling.
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An otter trawl.
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A pelagic (midwater) otter trawl. 1: trawl warp, 2: otter boards, 3: longline chains, 4 hunter, 5: weights 6: headline with floats, 7: pre-net, 8: tunnel and belly, 9: codend.
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A satellite image showing mud trails left by trawlers off the Louisiana coast.
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(A) A coral community and seabed on an untrawled seamount. (B) The exposed bedrock of a trawled seamount. Both are 1,000–2,000 meters (1094–2188 yards) below the surface.
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A beam trawl.
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A detail.
