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 . 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. Controversy Bycatch Any fish that crosses the path of a drift net in the ocean may be tangled or caught in the net. This leads to fish species becoming endangered or even extinct. Non-target individuals caught in the net are called by-catch.