High Quality Content by WIKIPEDIA articles! The term billfish refers to a group of predatory fish characterised by prominent bills, or rostrums, and by their large size (some are longer than four metres (13 feet)). Billfish include sailfish and marlin, which make up the family Istiophoridae, and swordfish, sole member of the family Xiphiidae. They are apex predators which feed on a wide variety of smaller fish, crustaceans, and cephalopods.