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Fishing
Fish of the Week: Bluefin Tuna
Bluefin tuna are prepared for consumption in many ways, but are most commonly associated with sushi and sashimi.
by Ken Schultz
TUNA, BLUEFIN Thunnus thynnus. Other names: Atlantic bluefin tuna, northern bluefin tuna, tunny fish, horse-mackerel; Arabic: tunna; Chinese: cá chan, thu; French: thon rouge; Italian: tonno; Japanese: kuromaguro; Norwegian: sjorjf, thunfisk; Portuguese: atum, rabilha; Spanish: atún aleta azul, atun rojo; Turkish: orkinos.
The bluefin tuna is the largest member of the Scombridae family and one of the largest true bony fish. It is a pelagic, schooling, highly migratory species with enormous commercial value, especially in large sizes, and of great recreational interest, albeit only to the relative few who have the means and equipment to venture to appropriate offshore environs.
The red flesh of the bluefin has made the species coveted for food, especially in Japan, where giant specimens are sold at daily auction for prices that are far greater than other species especially late in the season when the meat contains the most fat. To date the largest price paid for a single Atlantic bluefin was $90,000 U.S. at the Tokyo market, making this species the most economically valuable wild animal on the planet. Bluefin tuna are prepared for consumption in many ways, but are most commonly associated with sushi and sashimi.
High demand for its dark red flesh has made the bluefin tuna the object of intense commercial and recreational fishing efforts and resulted in a dwindling population of adult fish. This species, as well as its cousin the southern bluefin tuna, is gravely overfished, and its numbers have declined dramatically in the western Atlantic, the eastern Atlantic, and the Pacific. According to some estimates, the population of the species in the western Atlantic has declined by about 87 percent since 1970. The southern bluefin tuna population may have declined by as much as 90 percent. Commercial fisheries continue to be very active in the Bay of Biscay, off the Iberian Peninsula, in the Mediterranean, and off North America. Fisheries in the North Sea and off South America have collapsed.
Since both species are slow to mature, they are very vulnerable to overexploitation. Although some catch quota restrictions have been established, the continued catch of small bluefins as well as large ones (called giants) in some regions, the failure to restrict harvest in others, the ignorance of restrictions by commercial fishermen of some countries, the lack of punishment or enforcement, and the managerial treatment of bluefins on a separate two-stock basis instead of one interpolar migratory one, are leading reasons for both species of bluefin tuna to be further troubled if not endangered. In 1996, scientists warned that existing worldwide catch quotas would have to be cut by 80 percent for populations to recover in 20 years, but they were raised instead.
Identification. The bluefin tuna has a fusiform body, compressed and stocky in front. It can be distinguished from almost all other tunas by its rather short pectoral fins, which extend only as far back as the eleventh or twelfth spine in the first dorsal fin. There are 12 to 14 spines in the first dorsal fin and 13 to 15 rays in the second. The anal fin has 11 to 15 rays. It has the highest gill raker count of any species of Thunnus, with 34 to 43 on the first arch. The ventral surface of the liver is striated and the middle lobe is usually the largest.
The back and upper sides are dark blue to black with a gray or green iridescence. The lower sides are silvery, marked with gray spots and bands. The anal fin is dusky with some yellow. The finlets are yellow, edged with black. The caudal keel is black at the adult stage, but is semi_transparent when immature.
Size/Age. Bluefin tuna can grow to over 10 feet in length, and are commonly found at lengths from 16 to 79 inches. Adults weigh from 300 to 1,500 pounds, although fish over 1,000 pounds are rare. The all-tackle world record is a fish from Nova Scotia that weighed 1,496 pounds when caught in 1979. The species reportedly can live up to 40 years.
Distribution/Habitat. Bluefin tuna occur in subtropical and temperate waters of the north Pacific Ocean, the North Atlantic Ocean, and in the Mediterranean and Black Seas. They are widely distributed throughout the Atlantic. Distribution in the western Atlantic occurs along Labrador and Newfoundland southward to Tobago, Trinidad, Venezuela, and the Brazilian Coast; they are especially encountered by anglers off Nova Scotia and Prince Edward Island, Cape Cod, Montauk, New York, the canyons offshore of New York and New Jersey, the North Carolina region, and the Bahamas. Distribution in the eastern Atlantic extends as far north as Norway and Iceland, and as far south as northern West Africa. Atlantic bluefin tuna spawn in the Gulf of Mexico between April and June and in the Mediterranean Sea in June and July.
Life History/Behavior. Bluefin tuna are warm-blooded and able to maintain their body temperatures up to 18 degrees Fahrenheit above the surrounding water, which makes them superbly adapted to temperate and cold waters. They retain 98 percent of muscular heat, may have the highest metabolism of any known fish, and are among the fastest and most wide_ranging animals on Earth. When hunted or hunting, they can accelerate to 35 miles per hour.
Bluefins are schooling fish and do congregate by size, although the largest schools are formed by the smallest individuals, and the smallest schools are comprised of the largest fish. They swim in a single file, side-by-side (soldier formation), or in an arc (hunter formation). Sometimes bluefins swim below a school of yellowfin tuna, relying on the skittish yellowfin to alert them to predators.
Extensive migrations appear to be tied to water temperature, spawning habits, and the seasonal movements of forage species. Specimens tagged in the Bahamas have been recaptured as far north as Newfoundland and Norway and as far south as Uruguay. In some cases the recaptured fish had traveled 5,000 miles in fifty days. The giants of the species make the longest migrations.
During spawning, a giant female may shed 25 million or more eggs. Larvae have once chance in 40 million of reaching adulthood eight years later, but the survivors grow rapidly and may be 2 feet long and weigh 9 pounds by the end of their first year. By age 14 they may be over 8 feet long and weigh 700 pounds. Bluefins in the western Atlantic are sexually mature at approximately age 8 (80 inches curved fork length), and in the eastern Atlantic at about age 5 (60 inches).
Food. The diet of bluefin tuna consists of squid, eels, and crustaceans as well as pelagic schooling fish such as mackerel, flying fish, herring, whiting, and mullet.
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