The use of certain kinds of commercial fishing technologies can result in large bycatches—incidental catches of unwanted fish, sea turtles, seabirds, and marine mammals. Because the bycatch animals have little or no economic value, they are usually jettisoned, generally dead, back into the ocean. This non-selectivity of commercial fishing is an especially important problem when trawls, seines, and drift nets are used. The bycatch consists of unwanted species of fish and other animals, but it can also include large amounts of undersized, immature individuals of commercially important species of fish.

The global amount of bycatch has been estimated in recent years at about 30 billion tons (27 million tonnes), or more than 1/4 of the overall catch of the world's fisheries. In waters of the United States, the amount of unintentional bycatch of marine life is about 2.2 billion lb per year (1 billion kg per year). However, the bycatch rates vary...