PDA

View Full Version : New Week 26 - Swordfish



Catiana
May 25th, 2009, 04:12 PM
Swordfish
(Xiphias gladius)

The swordfish also called the broadbill, is the only member of the family Xiphiidae. As its name implies, this magnificent fish is characterized by an upper jaw that extends to form a flat, sharp-edged "sword." Swordfish are impressive jumpers and powerful fighters—thrilling for anglers and boaters alike.

Physical Characteristics
The swordfish has a stout, fairly rounded body and large eyes. The first dorsal fin (rising from the back of the fish) is tall and crescent-shaped. The second dorsal fin is quite separate from the first and very small. Both are soft-rayed—having thin, bony rods that extend from the base of the fin and support the fin membrane. The anal fins approximate the shape of the dorsal fins, but are noticeably smaller. Ventral fins, found on the underside of fish, are absent. There is a strong, longitudinal keel, or ridge, on either side of the caudal peduncle (the base of the tail where the tail fins project from), which leads to a broad, crescent-shaped tail. Adult swordfish have neither teeth nor scales.

The swordfish snout elongates into a true sword shape. Measuring at least one-third the length of the body, it is long, flat, pointed, and very sharp. The lower jaw is much smaller, though just as pointed, ending in a very wide mouth.

The bodies of swordfish fry (recently hatched fish larvae) are quite different form those
of the adults. Their upper and lower jaws are equally prolonged; bodies are long, thin, and snakelike; they are covered with rough, spiny scales and plates; tails are rounded; and they have just one long dorsal and anal fin.

Swordfish coloration varies greatly among individuals. The dorsal side can range from dark brown to grayish-blue. This dark shading can extend anywhere from halfway down the side to almost the full extent of the body. The remaining area of the skin is tinged silvery white.

In Northeast waters, only the spearfish bears any resemblance to the swordfish. It is distinguished from the swordfish by its rounded sword, small teeth, a long, continuous dorsal fin, and ventral fins.

Size
Swordfish are very large fish. Today, the average fish caught in the commercial fishery weighs between 90 and 150 kilograms (1 kilogram equals 2.2 pounds). While fish over 200 kg are unusual, the largest recorded in the North Atlantic ocean weighed 550 kg. The largest fish to be caught on a tackle weighed 274 kg. These larger fish measure approximately 4.5 meters in length (1 meter equals 3.3 feet)—with a 3 meter body and a 5 meter sword.

Female swordfish grow faster, live longer, and are proportionally heavier than their male counterparts. Research shows that by 1 year of age, the female is already almost 4 kg. During the next 2 years, she triples her weight of the previous year. By age 4, the female is likely to weigh 70 kg, and at age 5, 110 kg. Similar data for males and older swordfish are inconclusive.

Longevity
Swordfish reach sexual maturity at about 2 to 3 years of age, and live for at least nine years. While they may survive longer, no such documentation exists. The majority of swordfish caught in the North Atlantic sport fishery are thought to be 4 to 5 years old.

Distribution
Swordfish are pelagic fish—living within the water column rather than on the bottom or in coastal areas. They are typically found at depths of between 180 meters and 580 meters, and are found worldwide in temperate and tropical waters. They are believed to prefer waters where the surface temperature is above 15°C (58°F), although they can tolerate temperatures as low as 10°C (50°F). There seems to be some correlation between larger size and the ability to tolerate colder temperatures. Few fish under 90 kg are found in waters less than 18°C (64°F).

Swordfish are summer and fall visitors to New England waters, entering the warming Atlantic coastal waters from far offshore in the Gulf Stream around June and departing in late October. Evidence suggests that such onshore-offshore seasonal migrations are more prevalent than are migrations between the northern feeding areas off Cape Hatteras and the southern spawning grounds off Florid and the Caribbean.

Behavior
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 remoras 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 and sperm and killer whales. 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.