The Atlantic bluefin tuna (Thunnus thynnus) is a species of tuna in the family Scombridae. It is variously known as the northern bluefin tuna (mainly when including Pacific bluefin as a subspecies), giant bluefin tuna (for individuals exceeding 150 kilograms (330 lb)) and formerly as the tunny.
Atlantic bluefin are native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea. Atlantic bluefin have become extinct in the Black Sea. The Atlantic bluefin tuna is a close relative of the other two bluefin tuna species--the Pacific bluefin tuna and the southern bluefin tuna.
Atlantic bluefin tuna may exceed 450 kg (990 lb) in weight, and rival the black marlin, blue marlin and swordfish as the largest Perciformes. Throughout recorded history, the Atlantic bluefin tuna has been highly prized as a food fish. Besides their commercial value as food, the great size, speed, and power they display as apex predators has attracted the admiration of fishermen, writers, and scientists.
The Atlantic bluefin tuna has been the foundation of one of the world's most lucrative commercial fisheries. Medium-sized and large individuals are heavily targeted for the Japanese raw fish market, where all bluefin species are highly prized for sushi and sashimi.
This commercial importance has led to severe overfishing. The International Commission for the Conservation of Atlantic Tunas (ICCAT) affirmed in October 2009 that Atlantic bluefin tuna stocks have declined dramatically over the last 40 years, by 72% in the Eastern Atlantic, and by 82% in the Western Atlantic. On 16 October 2009, Monaco formally recommended Endangered Atlantic bluefin tuna for an Appendix I CITES listing and international trade ban. In early 2010, European officials, led by the French ecology minister, increased pressure to ban the commercial fishing of bluefin tuna internationally. European Union nations, who are responsible for most bluefin tuna overfishing, later abstained from voting to protect the species from international trade.
Most bluefin are captured commercially by professional fishermen using longlines, purse seines, assorted hook-and-line gear, heavy rods and reels, and harpoons. Recreationally, bluefin has been one of the most important big-game species sought by sports fishermen since the 1930s, particularly in the United States, but also in Canada, Spain, France and Italy.
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Taxonomy
The Atlantic bluefin tuna was one of the many fish species originally described by Linnaeus in the landmark 1758 10th edition of his Systema Naturae, where it was given the binomial name of Scomber thynnus.
It is most closely related to the Pacific bluefin tuna (Thunnus orientalis) and the southern bluefin tuna (Thunnus maccoyii), and more distantly to the other large tunas of the genus Thunnus - the bigeye tuna (Thunnus obesus) and the yellowfin tuna (Thunnus albacares). For many years, the Pacific and Atlantic bluefin tuna species were considered to be the same, or subspecies, and referred to as the "northern bluefin tuna". This name occasionally gives rise to some confusion, as the longtail tuna (Thunnus tonggol) can in Australia sometimes be known under the name "northern bluefin tuna". This is also true in New Zealand and Fiji.
Bluefin tuna were often referred to as the common tunny, especially in the UK, Australia, and New Zealand. The name "tuna", a derivative of the Spanish atún, was widely adopted in California in the early 1900s and has since become accepted for all tunas, including the bluefin, throughout the English-speaking world. In some languages, the red color of the bluefin's meat is included in its name, as in atún rojo (Spanish) and tonno rosso (Italian), amongst others.
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Description
The body of the Atlantic bluefin tuna is rhomboidal in profile and robust. The head is conical and the mouth rather large. The head contains a "pineal window" that allows the fish to navigate over its multiple thousands-of-miles range. The color is dark blue above and gray below, with a gold coruscation covering the body and bright yellow caudal finlets. Bluefin tuna can be distinguished from other family members by the relatively short length of their pectoral fins. Their livers have a unique 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.
Fully mature adult specimens average 2-2.5 m (6.6-8.2 ft) long and weigh around 225-250 kg (496-551 lb). The largest recorded specimen taken under International Game Fish Association rules was caught off Nova Scotia, an area renowned for huge Atlantic bluefin, and weighed 679 kg (1,497 lb) and 3.7 m (12 ft) long. The longest contest between man and tuna fish occurred near Liverpool, Nova Scotia in 1934, when six men taking turns fought a 361 kg (796 lb) tuna for 62 hours. Both the Smithsonian Institution and the National Marine Fish Service in North America have accepted that this species can weigh up to 910 kg (2,010 lb), though further details are lacking. Atlantic bluefin tuna reach maturity relatively quickly. In a survey that included specimens up to 2.55 m (8.4 ft) in length and 247 kg (545 lb) in weight, none was believed to be older than 15 years. However, very large specimens may be up to 50 years old.
The bluefin possesses enormous muscular strength, which it channels through a pair of tendons to its lunate-shaped caudal fin for propulsion. In contrast to many other fish, the body stays rigid while the tail flicks back and forth, increasing stroke efficiency. It also has a very efficient circulatory system. It possesses one of the highest blood hemoglobin concentrations among fish, which allows it to efficiently deliver oxygen to its tissues; this is combined with an exceptionally thin blood-water barrier to ensure rapid oxygen uptake.
To keep its core muscles, used for power and steady swimming, warm the Atlantic bluefin uses countercurrent exchange to prevent heat from being lost to the surrounding water. Heat in the venous blood is efficiently transferred to the cool, oxygenated arterial blood entering a rete mirabile. While all members of the tuna family are warm-blooded, the ability to thermoregulate is more highly developed in bluefin tuna than in any other fish. This allows them to seek food in the rich but chilly waters of the north Atlantic.
Biology and ecology
Bluefin dive to depths of 500 m (1,600 ft). They can reach speeds of 40 mph (64 km/h).
The Atlantic bluefin tuna typically hunts small fish such as sardines, herring, and mackerel, and invertebrates such as squid and crustaceans.
The tetraphyllidean tapeworm Pelichnibothrium speciosum parasitizes this species. As the tapeworm's definite host is the blue shark, which does not generally seem to feed on tuna, the Atlantic bluefin tuna likely is a dead-end host for P. speciosum.
Life history
Female bluefin are thought to produce up to 30 million eggs. Atlantic bluefin tuna spawn in two widely separated areas. One spawning ground exists in the western Mediterranean, particularly in the area of the Balearic Islands. The other important spawning ground of the Atlantic bluefin is the Gulf of Mexico. Pop-up satellite tracking results appear to confirm in large measure the belief held by many scientists and fishermen that although bluefin that were spawned in each area may forage widely across the Atlantic, they return to the same area to spawn.
Atlantic bluefin group together in large concentrations to spawn, and at such times are highly vulnerable to commercial fishing. This is particularly so in the Mediterranean, where the groups of spawning bluefin can be spotted from the air by light aircraft and purse seines directed to set around the schools. The western and eastern populations of Atlantic bluefin tuna are thought to mature at different ages. Bluefin born in the east are thought to reach maturity a year or two earlier than those spawned in the west.
Human interaction
Aquaculture
Tuna farming began as early as the 1970s. Canadian fishermen in St Mary's Bay captured young fish and raised them in pens. In captivity, they grow to reach hundreds of kilos, eventually fetching premium prices in Japan. Farming enables farmers to exploit the unpredictable supply of wild-caught fish. Ranches across the Mediterranean and off South Australia grow bluefin offshore. Annual revenues are $220 million. A large proportion of juvenile and young Mediterranean fish are taken to be grown on tuna farms. Because the tuna are taken from the wild to the pens before they are old enough to reproduce, farming is one of the most serious threats to the species. The bluefin's slow growth and late sexual maturity compound its problems. The Atlantic population has declined by nearly 90% since the 1970s.
In Europe and Australia, scientists have used light-manipulation technology and time-release hormone implants to bring about the first large-scale captive spawning of Atlantic and southern bluefin. The technology involves implanting gonadotropin-releasing hormone in the fish to stimulate fertile egg production and may push the fish to reach sexual maturity at younger ages.
However, since bluefin require so much food per pound of weight gained - up to 10 times that of salmon - if bluefin were to be farmed at the same scale as 21st-century salmon farming, many of their prey species might become depleted if farmed bluefin were fed the same diet as their wild counterparts. As of 2010, 30 million tons of small forage fish were removed from the oceans yearly, the majority to feed farmed fish.
Market entry by many North African Mediterranean countries, such as Tunisia and Libya in the 1990s, along with the increasingly widespread practice of tuna farming in the Mediterranean and other areas, such as southern Australia (for southern bluefin tuna), depressed prices. One result is that fishermen must now catch up to twice as many fish to maintain their revenues.
As food
The bluefin species are listed by the Monterey Bay Aquarium on its Seafood Watch list and pocket guides as fish to avoid due to overfishing.
This tuna is one of the most highly prized fish used in Japanese raw fish dishes. About 80% of the caught Atlantic and Pacific bluefin tunas are consumed in Japan. Bluefin tuna sashimi is a particular delicacy in Japan. For example, an Atlantic bluefin caught off eastern United States sold for US$15,400 at the Tsukiji fish market in Tokyo in 2008. This high price is considerably less than the highest prices paid for Pacific bluefin. Prices were highest in the late 1970s and 1980s.
Japanese began eating tuna sushi in the 1840s, when a large catch came into Edo [old Tokyo] one season. A chef marinated a few pieces in soy sauce and served it as nigiri sushi. At that time, these fish were nicknamed shibi -- "four days" -- because chefs would bury them for four days to mellow their bloody taste.
Originally, fish with red flesh were looked down on in Japan as a low-class food, and white fish were much preferred....Fish with red flesh tended to spoil quickly and develop a noticeable stench, so in the days before refrigeration, the Japanese aristocracy despised them, and this attitude was adopted by the citizens of Edo.
- Michiyo Murata
By the 1930s, tuna sushi was commonplace in Japan. After World War II, Japanese fishermen needed more fish to eat and to export for European and U.S. canning industries. They expanded their range and perfected industrial long-lining, a practice that employs thousands of baited hooks on miles-long lines. In the 1970s, Japanese manufacturers developed lightweight, high-strength polymers that were spun into drift nets. Though they were banned on the high seas by the early 1990s, in the 1970s hundreds of miles of them were often deployed in a single night. At-sea freezing technology then allowed them to bring frozen sushi-ready tuna from the farthest oceans to market after as long as a year.
The initial target was yellowfin tuna. Japanese did not value bluefin before the 1960s. By the late 1960s, sportfishing for giant bluefin tuna was burgeoning off Nova Scotia, New England, and Long Island. North Americans, too, had little appetite for bluefin, usually discarding them after taking a picture. Bluefin sportfishing's rise, however, coincided with Japan's export boom. In the 1960s and '70s, cargo planes were returning to Japan empty. A Japanese entrepreneur realized he could buy New England and Canadian bluefin cheaply, and started filling Japan-bound holds with tuna. Exposure to beef and other fatty meats during the U.S. occupation had prepared the Japanese palate for bluefin's fatty belly (otoro). The Atlantic bluefin was the biggest and the favorite. The appreciation rebounded across the Pacific when Americans started to eat raw fish in the late 1970s.
Prior to the 1960s, Atlantic bluefin fisheries were relatively small scale, and populations remained stable. Although some local stocks, such as those in the North Sea, were damaged by unrestricted commercial fishing, other populations were not at risk. However, in the 1960s, purse seiners catching fish for the canned tuna market in United States coastal waters removed huge numbers of juvenile and young Western Atlantic bluefin, taking out several entire year classes. Mediterranean fisheries have historically been poorly regulated and catches under-reported, with French, Spanish, and Italian fishermen competing with North African nations for a diminishing population. The fish's migratory habits complicate the task of regulating the fishery, because they spend time in the national waters of multiple countries, as well as the open ocean outside of any national jurisdiction.
Threats
Global appetites for fish, especially Japanese appetite for sushi, is the predominant threat to Atlantic bluefin. Bluefin aquaculture, which arose in response to declining wild stocks, has yet to achieve a sustainability, in part because it predominantly relies on harvesting and ranching juveniles rather than captive breeding.
Despite some concern, assessments from the 2010 Deepwater Horizon oil spill estimated that the population loss would not be significant, ranging from 0.4-4.0% of juveniles, which is within the range of annual variations.
Conservation
Overfishing continues despite repeated warnings of the current precipitous decline. In 2007, researchers from the International Commission for the Conservation of Atlantic Tunas (ICCAT)--the regulators of Atlantic bluefin fishing--recommended a global quota of 15,000 tonnes to maintain current stocks or 10,000 tonnes to allow the fisheries recovery. ICCAT then chose a quota of 36,000 tonnes, but surveys indicated that up to 60,000 tonnes were actually being taken (a third of the total remaining stocks) and the limit was reduced to 22,500 tonnes. Their scientists now say that 7500 tonnes are the sustainable limit. In November 2009, ICCAT set the 2010 quota at 13,500 tonnes and said that if stocks were not rebuilt by 2022, it would consider closing some areas.
In 2010, Greenpeace International added the northern bluefin tuna to its seafood red list.
On 18 March 2010, the United Nations rejected a U.S.-backed effort to impose a total ban on Atlantic bluefin tuna fishing and trading. The Convention on International Trade in Endangered Species (CITES) vote was 68 to 20 with 30 European abstentions. The leading opponent, Japan, claimed that ICCAT was the proper regulatory body.
In 2011, the USA's National Oceanic and Atmospheric Administration (NOAA) decided not to list the Atlantic bluefin tuna as an endangered species. NOAA officials said that the more stringent international fishing rules created in November 2010 would be enough for the Atlantic bluefin tuna to recover. NOAA agreed to reconsider the species endangered status in 2013. It was made a National Marine Fisheries Service Species of Concern, one of those species about which the U.S. government has some concerns regarding status and threats, but for which insufficient information is available to indicate a need to list the species under the U.S. Endangered Species Act.
In the summer of 2011, the Sea Shepherd Conservation Society led a campaign against supposedly illegal bluefin tuna fishing off the coast of Libya, which was under Muammar Gaddafi's regime at the time. The fishermen retaliated against Sea Shepherd's intervention by throwing various, small metal pieces at the crew. Nobody was injured due to the other side's actions during the conflict.
In November 2011, food critic Eric Asimov of The New York Times criticized the top-ranked New York City restaurant Sushi Yasuda for offering bluefin tuna on their menu, arguing that drawing from such a threatened fishery constituted an unjustifiable risk to bluefin, and to the future of culinary traditions that depend on it.
In November 2012, 48 countries meeting in Morocco for the International Commission for the Conservation of Atlantic Tunas voted to keep strict fishing limits, saying the species' population is still fragile. The quota will rise only slightly, from 12,900 metric tons a year to 13,500. The decision will be reviewed in 2014.
Source of the article : Wikipedia
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