Swordfish
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| Swordfish Fossil range: 33.9–0 Ma Early Oligocene to Present[1] | ||||||||||||||
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| Xiphias gladius Linnaeus, 1758 |
Swordfish (Xiphias gladius), also known as Broadbill in some countries, are large, highly migratory, predatory fish characterized by a long, flat bill. They are a popular sport fish, though elusive. Swordfish are elongated, round-bodied, and lose all teeth and scales by adulthood. They reach a maximum size of 177 in. (455 cm) and 1,400 lb (650 kg). The International Game Fish Association's all-tackle angling record for a swordfish was a 1,182 lb (536.15 kg) specimen taken off Chile in 1953.
They are the sole member of their family Xiphiidae.
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[edit] Physiology
The swordfish is named after its sharp beak resembling a sword (Latin gladius), which together with its streamlined physique allows it to cut through the water with great ease and agility. Contrary to belief the "sword" is not used to spear, but instead may be used to slash at its prey in order to injure the prey animal, to make for an easier catch. Mainly the swordfish relies on its great speed, capable of reaching speeds up to 50 mph (80 km/h), and agility in the water to catch its prey. One possible defensive use for the sword-like bill is for protection from its few natural predators. The shortfin mako shark is one of the rare sea creatures big enough and fast enough to chase down and kill an adult swordfish, but they don't always win. Sometimes in the struggle with a shark a swordfish can kill it by ramming it in the gills or belly.
Females grow larger than males, with males over 300 lb (135 kg) being rare. Females mature at 4–5 years of age in northwest Pacific while males mature first at about 3 to 4 years. In the North Pacific, batch spawning occurs in water warmer than 24 °C from March to July and year round in the equatorial Pacific. Adult swordfish forage includes pelagic fish including small tuna, dorado, barracuda, flying fish, mackerel, forage fish as well as benthic species of hake and rockfish. Squid are important when available. Swordfish are thought to have few predators as adults although juveniles are vulnerable to predation by large pelagic fish.
While swordfish are cold blooded animals, they have special organs next to their eyes to heat their eyes and also their brain. Temperatures of 10 to 15 °C above the surrounding water temperature have been measured. The heating of the eyes greatly improves the vision, and subsequently improves their ability to catch prey. Out of the 25 000+ species of bony fish, only about 22 are known to have the ability to heat selected body parts above the temperature of the surrounding water. These include the swordfish, marlin, and tuna.
Swordfish are not schooling fish. They swim alone or in very loose aggregations, separated by as much as 10 meters from a neighboring swordfish. They are frequently found basking at the surface, airing their first dorsal fin. Boaters report this to be a beautiful sight, as is the powerful jumping for which the species is known. This jumping, also called breaching, is thought by some researchers to be an effort to dislodge pests, such as remora or lampreys. It could also be a way of surface feeding by stunning small fish as they jump out of the water, making the fish more easily captured for food.
Swordfish feed daily, most often at night when they rise to surface and near-surface waters in search of smaller fish. They have been observed moving through schools of fish, thrashing their swords to kill or stun their prey and then quickly turning to consume their catch. In the western North Atlantic, squid is the most popular food item consumed. But fish, such as menhaden, mackerel, bluefish, silver hake, butterfish, and herring also contribute to the swordfish diet.
Swordfish are vigorous, powerful fighters. When hooked or harpooned, they have been known to dive so quickly that they have impaled their swords into the ocean bottom up to their eyes. Although there are no reports of unprovoked attacks on humans, swordfish can be very dangerous when harpooned. They have run their swords through the planking of small boats when hurt.
The adults have few natural enemies, with the exception of large sharks, sperm whales, and orcas. They are easily frightened by small boats, yet paradoxically, large craft are often able to draw very near without scaring them. This makes swordfish easy to harpoon.
The swordfish is often mistaken for other billfish (like marlin), but upon examination their physiology is quite different.
[edit] Reproduction
Swordfish have also been observed spawning in the Atlantic Ocean, in water less than 250 ft (75 m) deep. Estimates vary considerably, but females may carry from 1 million to 29 million eggs in their gonads. Solitary males and females appear to pair up during the spawning season. Spawning occurs year-round in the Caribbean Sea, Gulf of Mexico, the Florida coast and other warm equatorial waters, while it occurs in the spring and summer in cooler regions. The most recognized spawning site is in the Mediterranean, off the coast of Italy. The height of this well-known spawning season is in July and August, when males are often observed chasing females. The pelagic eggs are buoyant, measuring 1.6–1.8 mm in diameter. Embryonic development occurs during the 2 ½ days following fertilization. As the only member of its family, the swordfish has unique-looking larvae. The pelagic larvae are 4 mm long at hatching and live near the surface. At this stage, the body is only lightly pigmented. The snout is relatively short and the body has many distinct, prickly scales. With growth, the body narrows. By the time the larvae reach half an inch long (12 mm), the bill is notably elongated, but both the upper and lower portions are equal in length. The dorsal fin runs the length of the body. As growth continues, the upper portion of the bill grows proportionately faster than the lower bill, eventually producing the characteristic prolonged upper bill. Specimens up to approximately 9 inches (23 cm) in length have a dorsal fin that extends the entire length of the body. With further growth, the fin develops a single large lobe, followed by a short portion that still reaches to the caudal peduncle. By approximately 20 inches (52 cm), the second dorsal fin has developed, and at approximately 60 inches (150 cm), only the large lobe remains of the first dorsal fin.
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