Marlin, Istiophoridae, is a member of a group of marine fish known as "billfish", and is closely linked to the freshwater trout. A marlin has an elongated body, a spear-like snout, and a long rigid dorsal fin, which extends forwards to form a crest. Its common name is thought to derive from its notional resemblance to a sailor's marlinspike. Marlin are known to be incredibly fast swimmers, reaching speeds of about 110 kilometres per hour (68 mph).
The larger species include the Atlantic blue marlin, Makaira nigricans, which have been reliably recorded in excess of 2 metres (6.6 ft) in length and 120 kilograms (260 lb) in weight, and the Black marlin, Makaira indica, which have been reliably recorded in excess of 5 metres (16 ft) in length and 670 kilograms (1,500 lb) in weight. They are popular sporting fish in certain tropical areas.
Marlin are rarely table fare, appearing mostly in fine dining restaurants. Most modern sport fishermen release marlin after unhooking. However, the old fisherman in Ernest Hemingway's novella The Old Man and the Sea was storied to have caught an 18-foot marlin in order to sell its meat at market.
Marlin makes an excellent substitute for swordfish.