Sword Fish Facts: How Do Swordfish Reproduce?

Swordfish (Xiphias Gladius) is a species of billfish. They are known for their easily identifiable sword like protrusion from their faces, which is used to slice at prey and make them easier to catch. They are one of the fastest predators of the ocean, swimming at speeds of up to 50 miles per hour.

They are the only member of the Xiphiidae family – although they do seem superficially similar to fish such as Marlin, sailfish they are in fact a different species. The difference between marlin, other billfish and Swordfish is the shape and texture of their bill. Marlins have a rougher rounded, shorter bill, and they also come in a variety of species. The closest relative to swordfish is Chinese sword fish (Psephurus Gladius) which is a sturgeon and lives in fresh water and not salt water. Unlike the swordfish, the Chinese sword fish have a much longer life-span.

Swordfish (Xiphias gladius)

The sword-like protrusion of this species allows them hunt many of their favorite prey in large numbers. This includes squid, octopus, various pelagic fish. This includes fish such as mackerel and bluefish. Swordfish prefer to hunt in the open sea, which is why these pelagic fish are often what they hunt for. Areas close to reef and the ocean floor is not exactly favorite hunting grounds for swordfish. However they do tend to swim in water depths of up to 2,100 feet, which is much deeper than other billfish such as Marlin.

The sword fish’s unique bill also puts it at a very high position on the food chain, with its only predators being killer whales, tuna, sharks and man.

Swordfish Reproduction

At birth, the ‘sword’ is not present on the swordfish, but rather a set of teeth are. Over time the teeth are lost and the ‘sword’ grows as the fish matures. They are also born with scales, which disappear by the time full maturity is reached. Swordfish are considered fully mature at 4-5 years old. This is when they can begin reproduction. Mature females are considerably larger than males.

Swordfish reproduce by laying eggs, that is to say that they are oviparous. Female swordfish can lay anywhere between 1,000,000 and 30,000,000 eggs at one time and fertilization is external. The mother lays eggs and allows them to develop externally from her body.  It is a common method of reproduction for many fish.
Swordfish usually grow to be up to 10 ft long and weigh 150-200 lbs on average, although they have been noted to have grown to be up to 16 ft long and weigh an incredible 1000+ lbs in rare cases.

However, before they reach such significant size, their lifecycle starts as a small egg measuring only 1.6 – 1.8 mm in diameter. Swordfish eggs develop into embryos in approximately 2.5 days after successful fertilization. Soon, it develops into a pelagic larvae measures only 4 mm in length. The signature ‘sword’ of the swordfish starts to develop when the larvae reaches 12 mm in length. The body of the fish starts to narrow around this stage as well, gradually resembling the shape of an adult swordfish. At around 50 cm in length, the development of the dorsal fin will also start to resemble the shape of an adult swordfish.

Reproductive behavior and mating will be mostly limited from March to July in most waters. This is because these months provide the warmer temperatures that they prefer. In some waters, due to the warmer temperatures for an extended period of time, they will be able to extend their mating season. Batch spawning occurs in warm waters above 75 °F.

The reason that the sword fish tend to stick to the warmer regions of water during reproduction and throughout most of their life is a nearly negligible source of inner body heat regulation. As ectothermic animals, which rely almost entirely on the environmental heat to stay alive. Even when they are not mating, they prefer to be in water temperatures of approximately 65 – 75 °F. Although they can venture into colder waters, they will not stay in colder waters for too long. As highly migratory species, they will most likely move to waters with favorable temperatures as needed.

Swordfish can be found slicing through the warmer waters of the Pacific, Atlantic and Indian oceans at high speeds. It’s possible for them to reach high speeds when swimming as their ‘sword’ can both help in swimming function and it breaks up the water around the fish allowing it to propel itself forward faster. Surprisingly, this mostly nocturnal fish do not tend to live for much longer than 10 years.

Swordfish Population

In the 1980’s, swordfish population was dwindling. Fortunately, due to efforts of many, the population of swordfish have recovered from its lowest point. These efforts include the U.S. Atlantic Swordfish Fishery Management Plan that was implemented by the NOAA Fisheries as early as 1985. The establishment of minimum size limit and area closures that occurred in the years that followed must have helped the swordfish get a chance to reproduce and rebuild their population. Fortunately, as of 2009 the ICCAT have declared that the North Atlantic swordfish stock was fully rebuilt.

Fishing vessel on its way to catch swordfish.
Fishing Vessel

Swordfish are a commercially sought ‘food fish’ all over the world including the USA, Canada, Portugal, Brazil, Japan, Spain, Taiwan, and Uruguay. Harpoons or fishing rods are prevalent tools used to catch sword fish as they are adept at using their ‘swords’ to cut through nets when trapped. Even when the harpoon method is used, swordfish are still extremely dangerous and they put up a fierce resistance. They often end up impaling their swords in the ocean floor or cause damage to the boats with their swords. Even with these challenges and potential dangers of catching swordfish, they are sought after by many fishermen due to high consumer demand. They are known as a good source of potassium, protein, omega 3 fatty acids, Vitamin D and many minerals. Its is commonly grilled and served with lemon as a dish on the dinner table.

The Encyclopedia of Animals; A Complete Visual Guide by Drs. Fred Cooke, Hugh Dingle, Stephen Hutchinson, George McKay, Richard Schodde, Noel Tait and Richard Vogt.