Swordfish (Xiphias gladius) are large, highly migratory, predatory fish characterized by a long, flat bill in contrast to the smooth, round bill of the marlins. Swordfish are elongate, round-bodied, and lack teeth and scales as adults. They reach a maximum size of 14 ft (4.3 m) and 1,190 lb (540 kg). The International Game Fish Association's all-tackle angling record is a 1,182 lb (536 kg) fish taken off Chile in 1953.
Swordfish are distributed throughout the world's marine ecosystem, in tropical, subtropical and temperate waters and tend to concentrate where major ocean currents meet, and along temperature fronts. They inhabit the mixed surface waters where temperatures are greater than 15 °C but also can move and hunt in water as cool as 5 °C for short periods aided by specially adapted heat exchange organs which are able to increase the temperature of their brain and eyes by 10-15 °C.
Areas of greater apparent abundance occur north of Hawaii along the North Pacific transition zone, along the west coasts of the U.S. and Mexico and in the western Pacific, east of Japan. Migration patterns have not been described although tag release and recapture data indicate an eastward movement from the central Pacific, north of Hawaii, toward the U.S. West Coast. Acoustic tracking indicates some diel movement from deeper depths during the daytime and moving into the mixed surface water at night. At times they appear to follow the deep scattering layer, and small prey, as they undertake these vertical movements.
Females grow larger than males, as males over 300 lb (135 kg) are 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, as well as benthic species of hake and rockfish. Squid are important when available. Swordfish likely 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, tuna and some sharks.
Swordfish in cooking
Swordfish is a particularly popular fish for cooking. Since swordfish are large animals, meat is usually sold as steaks, which are often grilled. The color of the flesh varies by diet, with fish caught on the east coast of North America often being rosier.
However, many sources including the United States Food and Drug Administration warn about potential toxicity from high levels of methyl mercury in swordfish. The FDA recommends that women who are pregnant or who may become pregnant should eat no more than one seven-ounce serving a month; others should eat no more than one serving a week.
References
- Michael Hopkin (2005): Swordfish heat their eyes for better vision (http://www.nature.com/news/2005/050110/full/050110-2.html). Nature, 10 January 2005
- Fritsches, Kerstin A., Brill, Richard W., and Warrant, Eric J. (2005): Warm Eyes Provide Superior Vision in Swordfishes (http://www.current-biology.com/content/article/fulltext?uid=PIIS0960982204009960). Current Biology 15, 55"58
- FDA Consumer:Mercury In Fish:Cause For Concern? (http://www.fda.gov/fdac/reprints/mercury.html)