Animalia Life

Thunnus thynnus
FAMILY
Scombridae
TAXONOMY
Thunnus thynnus (Linnaeus, 1758). Pacific bluefin were considered a subspecies of T. thynnus for many years but have recently been raised to species level (Collette et al., 2001) as T. orientalis Temminck and Schlegel, 1844.
OTHER COMMON NAMES
English: Northern bluefin tuna; French: Thon rouge; Spanish: Atъn.
PHYSICAL CHARACTERISTICS
Maximum fork length more than 118 in (300 cm), commonly to 79 in (200 cm). A very large tuna, deepest near middle of first dorsal fin base. Two dorsal fins, separated by only a narrow interspace, the first with 11–14 spines, the second with 12–16 rays; anal fin with 11–16 rays, both second dorsal and anal fins followed by 7–10 finlets. Pectoral fins very short, less than 80% of head length, never reaching the interspace between the dorsal fins. Teeth small and conical in a single series. Gill rakers, 34–41. Caudal peduncle very slender with a strong lateral keel between two smaller keels. Corselet of large scales anteriorly; rest of body covered with small scales. Swim bladder large. Ventral surface of liver striated. Back metallic dark blue, lower sides and belly silvery white; first dorsal fin yellow or bluish, second dorsal reddish brown, anal fin silvery gray, anal finlets dusky yellow edged with black; no white margin on posterior margin of caudal fin.
DISTRIBUTION
North Atlantic Ocean from Labrador and Newfoundland south into Gulf of Mexico and Caribbean Sea. Replaced by the closely related Pacific bluefin tuna in the north Pacific.
HABITAT
Epipelagic, usually oceanic but seasonally coming very close to shore. Bluefin are found in moderately warm seas but are more tolerant of cold water than are most of their relatives. Offshore in the northwest Atlantic, large bluefin are taken at surface temperatures of 43.5–83.8°F (6.4–28.8°C). Tunas have evolved elaborate rete mirabilia, “wonder nets,” of capillaries that act as countercurrent heat exchangers. These heat exchangers form thermal barriers that prevent metabolic heat loss and enable bluefin to maintain a high internal temperature, as high as 83.8°F (28.8°C) for a bluefin taken in 45.1°F (7.3°C) water.
BEHAVIOR
Atlantic bluefin migrate long distances from their spawning grounds off Florida in the western Atlantic and in the Mediterranean Sea in the eastern Atlantic. Tag returns show there is some mixing between eastern and western Atlantic but there is ongoing debate about the proportion of individuals that cross the ocean.
FEEDING ECOLOGY AND DIET
Feed on a variety of fishes, crustaceans, and squids.
REPRODUCTIVE BIOLOGY
Onset of maturity is at approximately 4–5 years. Large adults (10 years and older) spawn in the Gulf of Mexico and in the Mediterranean Sea. Females weighing 592–661 lb (270–300 kg) may produce as many as 10 million eggs per spawning season.
CONSERVATION STATUS
Listed by IUCN as Data Deficient. Western Atlantic bluefin were fished intensively in the 1960s by purse seiners targeting small fish for canneries (Safina, 2001). Obvious depletion led to reduction in the east coast purse seine fleet. Western Atlantic bluefin catches averaged approximately 8,818 tons (8,000 metric tons) during the 1960s and 6,062 tons (5,500 metric tons) during the 1970s. During the 1970s, commercial targeting switched to large fish for export to Japan for sashimi. The offer to buy giant bluefin at $1.45 per pound (0.5 kg) (instead of the previous $0.20 to $0.50 per pound in autumn 1972) greatly increased U.S. fishing pressure on giant bluefin.
SIGNIFICANCE TO HUMANS
Important as both a food fish and a sport fish. FAO catch statistics for 1991–2000 show catches of 29.7–60.2 thousand tons (26.9–54.6 thousand metric tons) per year by 44 countries. The belly meat of bluefin, when containing much fat, reaches astronomical prices in the Japanese market for sashimi. Individual bluefin in prime condition have sold for as much as $68,000, approximately $45 per pound (0.5 kg), but a new record price of $173,600 (Ґ20 million) was reached for a 444-lb (201-kg) bluefin sold in Tokyo’s Tsukiji Central Fish Market in January 2001. The all-tackle game fish record for a “giant” bluefin is a 1,497-lb (679-kg) fish taken off Nova Scotia.

Other popular Animals