Many fishing gears and methods, particularly drift gillnets and drift longlines, do not discriminate between commercially valuable fish and those that are too small to eat or sell, of limited commercial value, or illegal to keep. Gillnets are vertical walls of netting designed to entangle fish by their gills. While driftnets longer than 2.5 kilometers are illegal, smaller nets are regularly used to catch swordfish, sharks and tunas. Gillnets entangle and kill anything larger than the net's mesh size, resulting in tremendous amounts of bycatch. Longlines are single-stranded fishing lines many miles long, with up to 3,000 baited hooks.
Drift longlines targeting tunas and swordfish catch every creature that bites one of the hooks.
Each year, tens of thousands of billfish, sharks, juvenile swordfish and tunas, sea turtles, seabirds, and many other wild marine animals die on longlines. Because of their unique characteristics, tunas, sharks, swordfish, and marlins are particularly hard hit by non-selective gears such as drift gillnets and drift longlines. An estimated 40,000 juvenile swordfish were discarded dead by the U.S. commercial fleet in 1996. Bykill is the number one killer of sharks worldwide. U.S. longliners in the Atlantic and Gulf of Mexico killed and discard and estimated 40,000 blue sharks in 1995.