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 ), and formerly as the tunny.
Atlantic bluefins are native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea. They have become regionally extinct in the Black Sea. The Atlantic bluefin tuna is a close relative of one of the other two bluefin tuna species, the Pacific bluefin tuna. The southern bluefin tuna, on the other hand, is more closely related to other tuna species such as yellowfin tuna and bigeye tuna, and the similarities between the southern and northern species are due to convergent evolution.
Atlantic bluefin tuna have been recorded at up to in weight, and rival the black marlin, blue marlin, and swordfish as one of the largest Percomorpha (though these species are vastly outweighed by the enormous Molidae species). 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 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 had 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. However, a UN proposal to protect the species from international trade was voted down (68 against, 20 for, 30 abstaining). Since then, enforcement of regional fishing quotas has led to some increases in population. the Atlantic bluefin tuna was moved from the category of Endangered to the category of Least Concern on the IUCN Red List of Threatened Species. However, many regional populations are still severely depleted, including western stocks which spawn in the Gulf of Mexico.
Most bluefins are captured commercially by professional fishermen using longlines, purse seines, assorted hook-and-line gear, heavy rods and reels, and harpoons. Recreationally, bluefins have 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, England, and Italy.
Taxonomy
The Atlantic bluefin tuna was one of the many fish species originally described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae, where it was given the binomial name Scomber thynnus.
It is most closely related to the Pacific bluefin tuna (T. orientalis), and more distantly to the other large tunas of the genus Thunnus – the southern bluefin tuna (T. maccoyii), the bigeye tuna (T. obesus) and the yellowfin tuna (T. albacares). The southern bluefin tuna was traditionally considered the closest relatives of the two northern species, but a recent study has suggested that the morphological similarities are the result of convergent evolution, and many are adaptations for cold water habitats. 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 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.
Description
thumb|right|Atlantic bluefin tuna migration
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 largest recorded specimen taken under International Game Fish Association rules was caught off North Lake, Prince Edward Island, an area renowned for huge Atlantic bluefin, and weighed and was long. The longest contest between man and tuna fish occurred near Liverpool, Nova Scotia in 1934, when six men taking turns fought a tuna for 62 hours. Both the Smithsonian Institution and the U. S. National Marine Fisheries Service have accepted that this species can weigh up to , though further details are lacking. Atlantic bluefin tuna reach maturity relatively quickly. In a survey that included specimens up to in length and in weight, none was believed to be older than 15 years. However, very large specimens may be up to 50 years old. 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. 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
Bluefins dive to depths of . They exhibit opportunistic hunting in schools of fish organized by size. Their white skeletal muscle allows for large contractions which aids burst swimming to ensure prey capture.
The species is host to over 70 parasites although none have been yet described as causing harm to the species. The tetraphyllidean tapeworm Pelichnibothrium speciosum is one parasite of the 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.
Atlantic bluefin tuna are eaten by a wide variety of predators. When they are newly hatched, they are eaten by other fishes that specialize in eating plankton, as well as by other plankton organisms like jellyfish. They may even be eaten by their own, with some populations of larvae demonstrating cannibalism. At that life stage, their numbers are reduced dramatically. Therefore, adult bluefin tuna spawn at specific locations and time of the moon cycle to minimize predation on their offspring. Those that survive face a steady increase in the size of their predators. Adult Atlantic Bluefin are not eaten by anything other than the very largest billfishes, toothed whales, and some open ocean shark species.
Life history
right|thumb|Larval Atlantic bluefin tuna
Bluefin tuna are oviparous, congregating together in large groups to spawn. Over several days, a female releases large numbers of eggs into the water where they are fertilized externally by male sperm. Female bluefins have been estimated to produce a mean of 128.5 eggs per gram of body weight, or up to 40 million eggs at a time. Eggs hatch into larvae two days after fertilization and become cannibalistic quarter-inch long fish by the end of a week. About 40% of larvae survive their first week, and about 0.1% the first year. Surviving bluefin tend to group together in schools according to size.
Atlantic bluefin tuna were traditionally known to spawn in two widely separated areas. Pop-up satellite tracking results generally confirm the belief held by many scientists and fishermen that although bluefin that were spawned in each area may forage widely across the Atlantic, the vast majority return to their natal area to spawn.
The spawning ground of the eastern stock of Atlantic bluefin is in the western Mediterranean, particularly in the area of the Balearic Islands. The spawning ground of the western stock is in the Gulf of Mexico. Because Atlantic bluefins group together in large concentrations to spawn, they are highly vulnerable to commercial fishing while spawning. This is particularly so in the Mediterranean, where the groups of spawning bluefins can be spotted from the air by light aircraft and purse seines directed to set around the schools.
In 2016, researchers suggested that a third spawning area exists in the Slope Sea, an area to the north and west of the Northeastern United States Continental Shelf. Subsequent research indicates that comparable concentrations of bluefin larvae are found in the Slope Sea and in the Gulf of Mexico.
A number of behavioral differences have been observed between the eastern and western populations, some of which may reflect environmental conditions. For example, bluefin in the Gulf of Mexico spawn between mid-April and mid-June, when the surface water temperature is between and , while bluefin in the Mediterranean spawn between June and August, when water is between and . In the Gulf of Mexico bluefin appear to correct for higher surface temperatures by diving, going deeper than when entering the Gulf and staying deeper than to spawn.
The western and eastern populations have been thought to mature at different ages. Bluefins born in the east are thought to reach maturity a year or two earlier than those spawned in the west. It has also been suggested that these apparent differences may reflect not-well-understood complexities of migration patterns and additional spawning areas such as the Slope Sea.
Commercial fishery
thumb|An adult Atlantic bluefin tuna on deck
After World War II, Japanese fishermen wanted more tuna to eat and to export for European and U.S. canning industries. They expanded their fishing range and perfected industrial long-line fishing, a practice that employs thousands of baited hooks on lines several kilometers long. In the 1970s, Japanese manufacturers developed lightweight, high-strength polymers that were spun into drift net. Though they were banned on the high seas by the early 1990s, in the 1970s, hundreds of kilometers 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.]]
Aquaculture
Tuna ranching 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. Ranching enables ranchers to exploit the unpredictable supply of wild-caught fish. Ranches across the Mediterranean and off South Australia grow bluefins offshore. According to OECD statistics, 35 thousand tons have been produced in 2018 with Japan accounting for about 50% of it, followed by Australia, Mexico, Spain and Turkey with smaller amounts. Large proportions 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, ranching is one of the most serious threats to the species. The slow growth and late sexual maturity of bluefin tuna 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 bluefins.
However, since bluefins require so much food per unit of weight gained, up to 10 times that of salmon, if bluefins 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 tonnes of small forage fish were removed from the oceans yearly, the majority to feed farmed fish.
Nonetheless, a number of lethal and sublethal impacts have been documented, including pericardial edema, defective cardiac function and cardiac abnormalities.
Conservation
Fisheries management organizations
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 7,500 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. 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. 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 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 was reviewed in November 2014, resulting in higher allowances listed below.
The latest stock assessment for Atlantic bluefin tuna reflected an improvement in the status for both western and eastern Atlantic/Mediterranean stocks. The Commission adopted new management measures that are within the range of scientific advice, are consistent with the respective rebuilding plans, and allow for continued stock growth. For the western stock, the TAC of 2,000 mt annually for 2015 and 2016 will provide for continued growth in spawning stock biomass and allow the strong 2003 year-class to continue to enhance the productivity of the stock. The TAC for the eastern Atlantic/Mediterranean stock was set at 16,142 t for 2015; 19,296 t for 2016; and 23,155 t for 2017.
In 2020, the UK government recognized the increasing incidence of bluefin tuna in UK waters in recent years and is funding ongoing research to understand the ecology of the species and devise an approach to its management.
Other organizations
In 2010, Greenpeace International added the northern bluefin tuna to its seafood red list. As of January 2022, the bluefin tuna remains on the list.
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 bluefins, and to the future of culinary traditions that depend on the species.
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.
Cuisine
thumb|Atlantic bluefin at a fish market in Marseille
Atlantic bluefin 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$247,000 at the Tsukiji fish market in Tokyo in 2008. This high price is considerably less than the highest prices paid for Pacific bluefin.
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.