Northern bluefin tuna
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| Northern bluefin tuna | ||||||||||||||
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| Thunnus thynnus (Linnaeus, 1758) |
The northern bluefin tuna (Thunnus thynnus), or giant bluefin tuna, is a species of tuna native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea and the Black Sea. Although not native to the Pacific Ocean, the species is now commercially cultivated off the Japanese coast. Northern bluefin tuna can live 30 years. Due to overfishing of this species, few known specimens grow to a mature age and typical specimens average one meter (3.3 ft) long and about 35 kg (77 lb) in weight. The largest recorded specimen was caught off Nova Scotia and recorded at 680 kg (1,500 lb). The species can reach a maximum length of about 4.3 meters (14 feet). Bluefin tuna are caught by sports fishermen using heavy-duty rod and reels and by commercial fishermen using purse seine gear. The northern bluefin tuna is an important commercial species, especially for sushi. It is this commercial importance that has led to severe overfishing of bluefin tuna stocks. The species is currently classified as critically endangered, and consumers have been recommended to avoid consumption of bluefin tuna until stocks recover. Canned tuna and tuna steaks originate from other species.
The species was in the past called the common tunny. It is often referred to simply as the "bluefin" or "bluefin tuna", but this name is ambiguous as it is also sometimes used for the southern bluefin tuna, Thunnus maccoyii, and the Pacific bluefin tuna, T. orientalis. Even the preferred name, northern bluefin tuna, is not unambiguous, because this is sometimes used for the longtail tuna T. tonggol. In Australia, canned T. tonggol can and is legally sold under the name "Northern bluefin tuna". This is also true in New Zealand and Fiji, although canned tuna is less common there.
The body of the northern bluefin tuna is cigar-shaped and robust. The head is conical and the mouth rather large. They typically prey on small fish such as sardines, herring, mackerel, squid and crustaceans. The color is dark blue above and gray below. Northern bluefin tuna can easily be distinguished from other members of the tuna family by the relatively short length of their pectoral fins. Their livers have a unique and definitive characteristic in that they are covered with blood vessels (striated). In other tunas with short pectoral fins, such vessels are either not present or present in small numbers along the edges.
The northern bluefin tuna is an important source of seafood, providing most of the tuna used in sushi. It is a particular delicacy in Japan where the price of a single giant tuna can exceed $100,000 on the Tsukiji fish market in Tokyo[1]. It is also popular in Taiwan, particularly in the town of Tungkang. As a result, some fisheries of bluefin are considered overfished, and this problem is compounded by the bluefin's slow growth rate and late maturity. The Atlantic population of the species has declined by nearly 90 percent since the 1970s [2]. The bluefin species are consequently listed as ones to "Avoid" on the Monterey Bay Aquarium's Seafood Watch program.
The tetraphyllidean tapeworm Pelichnibothrium speciosum has been found to parasitize this species (Scholz et al. 1998). As the tapeworm's definite host is the blue shark which does not generally seem to feed on tuna[citation needed], it is likely that the northern bluefin tuna is a dead-end host for P. speciosum. This fish is also a host to 72 parasites, 9 being host specific.
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[edit] Physiology
[edit] Thermoregulation
In order to keep its core muscles (used for power and steady swimming) uses countercurrent heat exchange to prevent internal heat from being lost to the surrounding water. Arteries branch out from the core of the body toward the surface tissue. The heat in the blood moving through the arteries is transferred to the venous blood coming back from the tissue closer to the surface, which is cooled by the water temperature. This keeps the core muscles warm so that they can function efficiently.[1]
[edit] Circulation
Bluefin tuna have a very efficient circulatory system. They have some of the highest amount of hemoglobin per unit of blood among fish. This allows them to maintain high rates of oxygen delivery to their tissues. This is combined with their exceptionally thin blood-water barrier to ensure rapid uptake of oxygen.[1]
[edit] References
- Clover, Charles. 2004. The End of the Line: How overfishing is changing the world and what we eat. Ebury Press, London. ISBN 0-09-189780-7
- Safina (1996). Thunnus thynnus. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 12 May 2006.
- "Thunnus thynnus". FishBase. Ed. Ranier Froese and Daniel Pauly. January 2006 version. N.p.: FishBase, 2006.
- Scholz, Tomáš; Euzet, Louis & Moravec, František (1998): Taxonomic status of Pelichnibothrium speciosum Monticelli, 1889 (Cestoda: Tetraphyllidea), a mysterious parasite of Alepisaurus ferox Lowe (Teleostei: Alepisauridae) and Prionace glauca (L.) (Euselachii: Carcharinidae). Systematic Parasitology 41(1): 1–8. doi:10.1023/A:1006091102174 (HTML abstract)
- ^ a b Hill, Richard W.; Gordon A. Wyse, Margaret Anderson (2004). Animal Physiology. Sinauer Associates, Inc.. ISBN 0-87893-315-8.
[edit] External links
- Bluefin Tuna at Seafood Watch
- Greenpeace article on tuna at Greenpeace
- MarineBio article on tuna at MarineBio
- brochure on bluefin tuna tagging at Tag-a-Giant Foundation
