The billfish are a group (Xiphioidea) of saltwater predatory fish characterised by prominent pointed bills (rostra), and by their large size; some are longer than 4 m (13 ft).

[4] However, the 5th edition of the Fishes of the World does recognise the Istiophoriformes as a valid order, albeit including the Sphyraenidae, the barracudas.

[11] Similarly, the pachycormid fish Protosphyraena and the plethodid fish Rhamphoichthys from the Late Cretaceous had both convergently evolved a highly billfish-like body plan, but are known to be very distantly related to actual billfish; these genera may have instead served as a Cretaceous ecological analogue to billfish.

FishBase follows Nakamura (1985)[13] in recognizing M. mazara as a distinct species, "chiefly because of differences in the pattern of the lateral line system".

[44] Billfish can accidentally impale boats and other floating objects when they pursue the small fish that aggregate around them.

The problem he posed was how dolphins can swim and accelerate so fast when it seemed their muscles lacked the needed power.

In 2009, Taiwanese researchers from the National Chung Hsing University introduced new concepts of "kidnapped airfoils and circulating horsepower" to explain the swimming capabilities of swordfish.

Like tuna, mackerel and other scombroids, billfish streamline themselves by retracting their dorsal fins into a groove in their body when they swim.

Sailfish raise them if they want to herd a school of small fish, and also after periods of high activity, presumably to cool down.

[50] They are sometimes referred to as "rare event species" because the areas they roam over in the open seas are so large that researchers have difficulty locating them.

[51][52] Unlike coastal fish, billfish usually avoid inshore waters unless there is a deep dropoff close to the land.

[42] Instead, they swim along the edge of the continental shelf where cold nutrient rich upwellings can fuel large schools of forage fish.

Billfish can be found here, cruising and feeding "above the craggy bottom like hawks soaring along a ridge line".

– IGFA Conservation Director, Jason Schratwieser[55] Billfish are among the most coveted of big gamefish, and major recreational fisheries cater to the demand.

They are often built to luxury standards and equipped with many technologies to ease the life of the deep sea recreational fisherman, including outriggers, flying bridges and fighting chairs, and state of the art fishfinders and navigation electronics.

[53] The boats cruise along the edge of the continental shelf where billfish can be found down to 200 metres (600 ft), sometimes near weed lines at the surface and submarine canyons and ridges deeper down.

Billfish are caught deeper down the water column by drifting with live bait fish such as ballyhoo, striped mullet or bonito.

The study concluded that, while tag and release programs have limitations, they provided important information about billfish that cannot currently be obtained by other methods.

According to the United States Food and Drug Administration, swordfish is one of four fishes, along with tilefish, shark, and king mackerel, that children and pregnant women should avoid due to high levels of methylmercury found in these fish and the consequent risk of mercury poisoning[broken anchor].

Swordfish are subject to particularly intense fisheries pressures, and although their survival is not threatened worldwide, they are now comparatively rare in many places where once they were abundant.

[63] Because of these concerns about declining populations, sport fishermen and conservationists now work together to gather information on billfish stocks and implement programs such as catch and release, where fish are returned to the sea after they have been caught.

[64][65] The stocks for individual species in billfish longline fisheries can "boom and bust" in linked and compensatory ways.

[66] "Many of the world's fisheries operate in a data poor environment that precludes predictions about how different management actions will affect individual species and the ecosystem as a whole.

[68] In 2011, a group of researchers claimed they have, for the first time, standardized all available data about scombrids and billfishes so it is in a form suitable for assessing threats to these species.