Drift netting

Drift netting is a fishing technique where nets, called drift nets, hang vertically in the water column without being anchored to the bottom. The nets are kept vertical in the water by floats attached to a rope along the top of the net and weights attached to another rope along the bottom of the net. Drift nets generally rely on the entanglement properties of loosely affixed netting. Folds of loose netting, much like a window drapery, snag on a fish's tail and fins and wrap the fish up in loose netting as it struggles to escape. However the nets can also function as gill nets if fish are captured when their gills get stuck in the net. The size of the mesh varies depending on the fish being targeted. These nets usually target schools of pelagic fish. Drift netting is a fishing technique where nets, called drift nets, hang vertically in the water column without being anchored to the bottom. The nets are kept vertical in the water by floats attached to a rope along the top of the net and weights attached to another rope along the bottom of the net. Drift nets generally rely on the entanglement properties of loosely affixed netting. Folds of loose netting, much like a window drapery, snag on a fish's tail and fins and wrap the fish up in loose netting as it struggles to escape. However the nets can also function as gill nets if fish are captured when their gills get stuck in the net. The size of the mesh varies depending on the fish being targeted. These nets usually target schools of pelagic fish. Traditionally drift nets were made of organic materials, such as hemp, which were biodegradable. Prior to 1950, nets tended to have a larger mesh size. The larger mesh only caught the larger fish, allowing the smaller, younger ones to slip through. When drift net fishing grew in scale during the 1950s, the industry changed to synthetic materials with smaller mesh size. Synthetic nets last longer, are odourless and may be nearly invisible in the water, and do not biodegrade. Most countries regulate drift net fisheries within their territories. Such fisheries are also often regulated by international agreements. Drift net fishing became a commercial fishing practice because it is cost effective. Nets can be placed by low-powered vessels making it fuel efficient. Drift nets are also effective at bringing in large amounts of fish in one catch. Prior to the 1960s net size was not limited, and commercially produced nets were commonly as long as 50 kilometres (31 mi). In 1987 the U.S. enacted the Driftnet Impact, Monitoring, Assessment and Control Act limiting the length of nets used in American waters to 1.5 nautical miles (≈1.7 miles, ≈2.778 km). In 1989 the United Nations General Assembly (UNGA) placed a moratorium on the practice of drift net fishing. In 1992 the UN banned the use of drift nets longer than 2.5 km long in international waters. Any fish that crosses the path of a drift net in the ocean may be tangled or caught in the net. Non-target individuals caught in the net are called by-catch. In 1994 United Nations Food and Agriculture Organization (UNFAO) estimated global by-catch rates to be as high as 27 million tons of fish discarded by fisheries each year. Many individuals of non-target species perish as by-catch in the cast of each drift net. As a result, many such species are now endangered. Species caught as by-catch include sharks, dolphins, whales, turtles, sea birds, and other marine mammals. Since nets are placed and may not be retrieved for days, air-breathing mammals that become tangled in the nets drown if they are unable to free themselves. In certain areas, exemption from punitive measure due to the unintentional by-catch of marine mammals, as outlined by the Marine Mammal Protection Act, is extended to commercial drift fishermen. In the 1990s, drift net fisheries were responsible for 30,000 tons of sharks and skates in global by-catch annually. While filming National Geographic's Incidental Kill in the California Channel Islands where swordfish and various sharks swim north, the divers discovered that many drift net boats had placed nets that night. The nets were one mile long each and nearly 100 feet (30 m) high placed to target swordfish and thresher sharks. They swam half the length of one net and in that length discovered 32 dead blue sharks in the net as well as 2 hammer head sharks, a sea lion, and a manta ray all of which were thrown back into the ocean when the net was hauled in. Although long line fisheries are the main contributor to sea bird by-catch, sea birds are also caught in drift nets in significant numbers. Studies conducted on 30 small-scale drift net fisheries in the Baltic Sea estimate that 90,000 sea birds die annually in drift nets. Bycatch is thrown back to the ocean either dead or with injuries that may result in death. If not eaten, dead animals decompose, as bacteria use oxygen to break down the organic matter. Large amounts of dead matter decomposing in the ocean causes the surrounding levels of dissolved oxygen to decrease. Drift nets lost or abandoned at sea due to storms causing strong currents, accidental loss, or purposeful discard become ghost nets. Synthetic nets are resistant to rot or breakdown, therefore ghost nets fish indefinitely in the oceans. Marine animals are easily tangled in ghost nets. The float line on the net allows it to be pushed in the current which causes ecological damage to plant life and substrate habitats as the nets drag the sea floor.