Swordfish (Xiphias gladius): Facts, Speed, Sword and Diet

What does a swordfish look like?

A swordfish has a long, rounded body that tapers to a large, crescent-shaped tail. On each side of the tail base is a single strong keel, a ridge that helps the tail cut through the water. The first dorsal fin is tall, stiff and curved like a shark’s fin, and it cannot fold down into a groove the way a sailfish’s sail can. A tiny second dorsal fin sits far back near the tail.

The famous sword is the upper jaw, extended into a flat blade made mainly of the premaxillary bones. In cross-section it is a flattened oval with sharp edges, unlike the round, spear-like bills of marlins and sailfish, and it is longer relative to the body than in any other billfish. The lower jaw is much shorter and pointed.

Adult swordfish have no scales, no teeth and no pelvic fins. Young fish have rough, prickly scales and teeth, but they lose both, along with the visible lateral line, by the time they reach about 1 m (3 ft) in length. The back is blackish-brown, fading to a lighter brown or silvery color below. The eyes are very large, an adaptation to hunting in dim, deep water.

How big do swordfish get?

Swordfish are among the largest bony fish in the sea. They commonly reach about 3 m (10 ft) long including the sword, and females grow much bigger than males: almost all swordfish heavier than 140 kg (300 lb) are female. Pacific swordfish reach larger sizes than those of the northwest Atlantic and the Mediterranean.

Swordfish grow quickly in their first years. Their age is hard to determine, because the ear bones that scientists normally use to age fish are tiny in this species, so researchers count growth rings in fin rays instead. Most estimates suggest a lifespan of at least 9 years.

Where do swordfish live?

Swordfish live in all the world’s tropical and temperate oceans, from about 60°N to 45°S, including the Mediterranean Sea. They are highly migratory, generally moving toward cooler, food-rich waters in summer and returning to warmer waters to spawn. They often gather along ocean fronts, where currents meet and prey is concentrated.

They prefer surface water of about 18–22 °C (64–72 °F) but tolerate a wider range than any other billfish, from about 5 to 27 °C (41 to 81 °F). Most swordfish follow a daily rhythm called diel vertical migration. At dawn they descend into the twilight zone, usually 200–600 m (650–1,970 ft) deep, and at night they rise to feed near the surface. Satellite tags on swordfish in the Tasman Sea recorded a median daytime depth of about 495 m (1,620 ft), and dives to nearly 2,900 m (9,500 ft) have been documented.

Swordfish are also seen basking at the surface by day, with the tall first dorsal fin sticking out of the water. They are solitary or travel in very loose groups, and they sometimes leap clear of the water.

What do swordfish eat?

Swordfish are opportunistic carnivores. Adults eat squid and a wide range of fish, including mackerels, herring, hakes, rockfish, barracudinas and lanternfishes, as well as some crustaceans. In a study of 168 swordfish from the northwest Atlantic, 82 percent had eaten squid and 53 percent had eaten fish.

The popular image of a swordfish spearing its prey is not supported by evidence. Instead, prey taken from swordfish stomachs often bears clean cuts, which shows that swordfish slash at larger animals with sideways swipes of the sword to injure them before swallowing them. Small prey is swallowed whole. How the sword works, and what it is not used for, is covered in What is a swordfish’s sword for?

Young swordfish start life eating zooplankton, including the larvae of other fish, and move on to small squid, fish and crustaceans as they grow.

How fast can a swordfish swim?

The swordfish is often described as one of the fastest fish in the ocean, with figures of 80–100 km/h (50–60 mph) repeated in books and websites. No one has actually measured a swordfish at top speed, and studies of related billfish suggest the real maximum is far lower.

The swordfish is still built for speed. It has a streamlined body, a stiff crescent tail and smooth, scaleless skin. In 2016 scientists also discovered an oil-producing gland at the base of the sword that releases oil through tiny pores onto the head, which may reduce drag. The evidence behind the speed claims is examined in How fast can a swordfish swim?

Heated eyes and brain

Most fish have the same body temperature as the water around them. The swordfish is an exception in one key area: next to its eyes sits a heater organ made from a modified eye muscle. Its cells are packed with mitochondria and produce heat instead of movement, and a network of blood vessels acts as a heat exchanger to keep that warmth in place.

This heater keeps the swordfish’s eyes and brain about 10–15 °C (18–27 °F) warmer than the surrounding water, even during deep dives into water as cold as about 5 °C (41 °F). Experiments have shown that warm eyes detect fast movement far better than cold ones, which helps the swordfish track darting squid in the dark. Swordfish also keep their main red swimming muscles warmer than the water. The details are explained in How do swordfish heat their eyes?

Reproduction and life cycle

Swordfish spawn in warm water, usually above about 23–24 °C (73–75 °F). In the Caribbean, the Gulf of Mexico and other warm parts of the western Atlantic they spawn all year, while in cooler regions spawning happens in spring and summer. Spawning grounds in the North Atlantic include the waters south of the Sargasso Sea and off the southeastern United States, and in the Mediterranean a well-known spawning area lies off Italy, where activity peaks in July and August.

Females release their eggs in several batches, and a large female can produce between 1 million and 29 million eggs. The floating eggs measure 1.6–1.8 mm across and hatch after about two and a half days. The larvae are about 4 mm (0.16 in) long, with short jaws and prickly scales. By about 12 mm (0.5 in), both jaws have grown long and equal in length, and as the larva grows, the upper jaw outpaces the lower one to form the sword. Swordfish become able to breed at about 4–6 years old.

Predators and threats

Adult swordfish have few natural enemies. Orcas sometimes prey on them, and shortfin mako sharks attack them too, although this can be dangerous for the shark: dead or dying shortfin makos have been found with broken swords embedded in their heads. Young swordfish are eaten by many ocean predators, including sharks, blue and black marlins, sailfish, yellowfin tuna and dolphinfish.

Swordfish also host almost 50 kinds of parasites, from tapeworms and roundworms to copepods. Lampreys, remoras and cookiecutter sharks attach to or bite their skin, and the leaps of swordfish may help shake some of these hitchhikers off.

The main threat to swordfish is fishing. Most are caught on longlines, fishing lines many kilometers long carrying hundreds or thousands of baited hooks, and many young fish are taken before they can breed.

Conservation status

The IUCN lists the swordfish as Near Threatened in an assessment published in 2022, meaning it is close to qualifying for a threatened category. The picture varies greatly between regions, because swordfish are managed as separate stocks.

The North Atlantic stock fell sharply in the 1990s. In 1998, a “Give Swordfish a Break” campaign persuaded hundreds of US chefs to take North Atlantic swordfish off their menus. An international rebuilding plan followed in 1999, and the United States closed nursery areas off Florida and in the Gulf of Mexico to longline fishing. The stock was declared rebuilt in 2009, and the 2022 assessment found it neither overfished nor subject to overfishing. The Mediterranean stock, by contrast, has been heavily overfished and has been under an international recovery plan since 2016.

Swordfish and people

Swordfish have been fished since ancient times. In the Strait of Messina between Sicily and mainland Italy, a traditional harpoon fishery is still carried out from boats called feluccas. Today swordfish is a popular food in Europe and North America, usually sold as firm steaks. Because swordfish are large, long-lived predators, mercury builds up in their flesh, and food safety agencies publish consumption advice for it.

Swordfish do not attack people, but a struck, hooked or harpooned swordfish can be dangerous. Museums hold pieces of wooden ships pierced by swordfish swords, and in 1967 a swordfish rammed the research submarine Alvin at a depth of about 610 m (2,000 ft) and became stuck in its outer skin. The word “swordfish” also found a place in popular culture as a secret password, after it was used in the 1932 Marx Brothers film Horse Feathers.