Thunnus orientalis (Pacific bluefin tuna)
Bluefin tuna is the common name for three of the eight species of tuna in the Thunnus genus (family Scombridae): the Pacific bluefin tuna (Thunnus orientalis), the northern bluefin tuna or Atlantic bluefin tuna (T. thynnus), and the southern bluefin tuna (T. maccoyii). They share with other tuna such characteristics as two dorsal fins that are depressible into grooves, finlets behind the second dorsal fin, pelvic fins with six rays placed behind the pectoral fins, a deeply forked tail, and a body that is very narrow right before the tail. These highly migratory, large, pelagic fish are fast swimmers and have a heat exchange system that allows them to elevate their body temperature well above ambient water temperatures; they can tolerate a very broad range of temperatures and inhabit a broad thermal niche.
Pacific bluefin tuna are found in both the western and eastern Pacific Oceans, largely in temperate waters in the North Pacific but their range also includes tropical waters in the southern hemisphere. The northern bluefin tuna is native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea, extending southward to northern Brazil and the Canary Islands. The southern bluefin tuna is found in open southern hemisphere waters of all the world's oceans, mainly between 30°S and 50°S, to nearly 60°S.
Bluefin tuna are important both ecologically and commercially. Ecologically, they are an integral part of marine food chains, preying (depending on whether young larvae, juveniles, or adults) on such organisms such as rotifers, crustaceans, other fish (from larvae to adult), mollusks (such as squid), and macroalgae, and being preyed upon by sharks, large predator fishes, and marine mammals. Commercially, they serve as food for humans, and bluefin tuna also are a popular target for sportsfishing.
The bluefin tuna of all three species—northern, southern, and Pacific—are overfished throughout the world. They are hooked on longlines or illegally netted everywhere they swim. Creating effective fishing policies for bluefin tuna is difficult since they are highly mobile and swim through the territorial waters of many different nations. Both the Atlantic bluefin tuna and the southern bluefin tuna are critically endangered species. There are regulatory bodies among the fishing nations that try to limit the catch of these fisheries; however, catch limits are often set higher than recommended by researchers, and an effort in 2010 to ban international trade in Atlantic bluefin tuna via CITES was soundly defeated.
Overview and description
As tuna, bluefin tuna belong to the tribe Thunnini in the subfamily Scombrinae and the family Scombridae (the mackerel family). There are 14 extant species in this tribe (collectively known as tunas), organized into five genera—Thunnus, Euthynnus, Allothunnus, Auxis, and Katsuwonus. The bluefin tuna comprise three of the 8 extant species in the Thunnus genus: Thunnus orientalis (Pacific bluefin tuna), T. thynnus (Pacific or Atlantic bluefin tuna), and T. maccoyii (southern bluefin tuna). The Scombrie family overall has about 51 species organized into 15 genera.
Members of the Scombridae family have two dorsal fins that are depressible into grooves. There are five to 12 finlets behind the second dorsal fin and also after the anal fins. The first dorsal fin has nine to 27 rays, the pelvic fins have six rays and are placed behind the pectoral fins, which are high on the body. Tunas have a deeply forked or crescent-shaped tail, and the body is very narrow right before the tail. Members of the subfamily Scombinae are characterized by scales that are cycloid and minute or are absent.
As a general description, bluefin tuna have a pointed snout, small eyes, and the dorsal, pelvic, and pectoral fins fit into slots, which helps to reduce drag as they swim. There is a small space between the two dorsal fins and they have short pectoral fins. The anal fin starts far behind the second dorsal fin.
The three species of bluefin tuna differ from some other members of the Thunnus genus by the fact that they all have striations on the ventral surface of the liver. For example, there are no such striations in the blackfin tuna (T. atlanticus), yellowfin tuna (T. albacares), or longtail tuna (T. tonggol).. However, the other two species in the Thunnus genus—the bigeye tuna (T. obesus) and the albacore (T. alalunga)—also have the presence of striations on the ventral surface of the liver.
One notable difference from other members of Thunnus is that the bluefin tuna all have very short pectoral fins, shorter than the other species. In other tunas with short pectoral fins, the striations on the ventral surface of the liver (from the blood vessels) are either not present or present in small numbers along the edges.
Bluefin tuna, in general, are the largest of the tunas. In particular, T. thynnus, the Atlantic tuna or northern tuna, is notable for its size and is also known as the "giant bluefin tuna." It achieves a length of up to 4.2 meters and is capable of reaching well over 450 kilograms (992 lb) in weight, rivaling the black marlin and blue marlin as the largest bony fish. The smallest of the bluefin tuna is the southern bluefin tuna, which reaches up to 2.5 meters (8.2 feet) and weighs up to 400 kilograms (882 pounds).
The bluefin tuna species have different distributions in general, with some overlap between T. maccoyii and T. orientalis. T. maccoyii is found solely in the southern hemisphere, being widely distributed below about 30°S in all oceans. T. orientalis is mainly found in the northern hemisphere, in temperate waters, but has been spotted as far south as Australia and New Zealand, as well as near the Galapagos Islands. T. thynnus is found in the Atlantic Ocean and the Mediterranean Sea.
Anatomical differences between bluefin species
The northern bluefin tuna (Thunnus thynnus) and the Pacific bluefin tuna (T. orientalis) are very similar and were for a long time considered subspecies of the same species, T. thynnus, with the Pacific bluefin tuna known as T. thynnus orientalis and the Atlantic or northern bluefin tuna as T. thynnus thynnus. On the other hand, the southern bluefin tuna is much more easily distinguishable.
The southern bluefin tuna was originally differentiated from the Pacific bluefin tuna, with which it range overlaps, based on the position of the first ventrally directed parapophysis, which is on the 8th vertebrae in T. orientalis and on the 9th vertebrae in T. maccoyii. Another character used to differentiate these two is the presence of a muscular protrusion in the dorsal abdominal cavity (a "bust"), which is present in both T. maccoyii and T. thynnus, but not in T. orientalis; it has been confirmed as a reliable character in distinguishing T. maccoyii and T. orientalis in large fish in the field.
Distinguishing the northern bluefin tuna from the Pacific bluefin tuna is more difficult, with most characters indistinguishable. Godsil and Holmberg note five characteristics in which they can be differentiated, however. First, there are generally differences in the shape and length of the air bladder, with T. thynnus having a normal air bladder that extends almost the full length of the body cavity, while T. orientalis has a irregular air bladder, often pear-shaped and generally covering only the front half of the abdominal cavity. Secondly, the northern or Atlantic species has no arterial trunk that connects the No. II and No. III branches of the coeliac mesenteric artery, whereas the Pacific species has the presence of this connecting network. Third, there is a difference in the pelvic girdle in the two forms. Fourth, the gill raker count differs, with T. thynnus typically having a total count of from 39 to 41, whereas T. orientalis has from 32 to 39, with the main difference in the number of the lower limb of the arch (25-28 in the former, and 21-25 in the later). Finally, the most conspicuous difference was found in the shape of the dorsal wall of the body cavity, with this difference particularly striking and consistent in the large fish.
Bluefin tuna have very effective lateral heat exchangers, which allows bluefin tuna to conserve metabolic heat, invade cooler waters, tolerate a wide temperature range, and swim faster. The bluefin tuna’s heat exchange system works so well that it can elevate the bluefin’s body temperatures to more than 20°C above ambient water temperatures. .
Essentially, the heat exchange system means that the outgoing veins carrying warm, carbon dioxide-laden blood toward the gills pass the heat over to incoming arteries carrying cold oxygenated blood from the gills. More specifically, tunas possess organs near their muscles called retia mirabilia that consist of a series of minute parallel veins and arteries that supply and drain the muscles. As the warmer blood in the veins returns to the gills for fresh oxygen it comes into close contact with cold, newly oxygenated blood in the arteries. The system acts as a counter-current heat exchanger and the heat from the blood in the veins is given up to the colder arterial blood rather than being lost at the gills. The net effect is an increase in temperature.
All members of the tuna family have the ability to thermoregulate; however, this ability is more highly developed in bluefin tuna than in any other fish. The Atlantic bluefin tuna can tolerate ambient temperature ranges from 2.8°C to 31.0°C, allowing them to spawn in warm water during the summer and forage in cool waters during the summer, giving them the "broadest thermal niche of all species of the family Scombridae."
The heat exchange system also keeps the swimming muscles warm by conserving heat, allowing bluefin tuna to function more efficiently, have extra power, and speed. Bluefin tuna have been clocked in excess of 30 miles per hour (48 km/h) during 10 to 20 second sprints, enabling them to hunt squid, herring, mackerel, and so forth that slower predators cannot capture.
Bluefin tuna also possess one of the highest blood hemoglobin concentrations among fish, which allows them to efficiently deliver oxygen to their tissues; this is combined with their exceptionally thin blood-water barrier to ensure rapid oxygen uptake.
Pacific bluefin tuna
Thunnus orientalis, the Pacific bluefin tuna, spawn in the Western Pacific between Okinawa and the Philippines and probably the Sea of Japan/East Sea, then migrate over six thousand nautical miles (11,100 kilometers) to the Eastern Pacific, and eventually return to their birth waters to spawn again. Although T. orientalis is primarily found in the northern hemisphere, specimens have been reported as far south as Australia, New Zealand, and the Galapagos Islands, where their range overlaps with the T. maccoyii, the southern bluefin tuna.
The Pacific bluefin tuna is one of the biggest and fastest fish in the Pacific Ocean. Their streamlined bodies reduce water resistance and conserve energy for trans-Pacific migrations; retractable fins also allow a freer flow of water when navigation does not require their use.
Bluefin tuna mature slowly, reaching sexual maturity at about 5 years of age, with a maximum lifespan believed to be about 25 years. Pacific bluefin tuna have been recorded to reach 9 feet (2.7 meters) in fork length and can weigh over 1,000 pounds (454 kg). They eat smaller fish, krill, pelagic red crab, mackerel, and squid.
Pacific Bluefin Tuna are overfished throughout their range and the status of their stocks are considered to be of high conservation concern. They are hooked on long lines or illegally netted where they swim, and many young bluefin are captured before they reproduce to be fattened to full size in open net pens for the market. Creating effective fishing policies is difficult because they are migratory, swimming through the territorial waters of many different nations. Data about their movements and high levels of international cooperation are needed to ensure sustainable populations. They have been placed on various endangered species lists and most seafood sustainability guides recommend consumers choose alternatives to bluefin tuna.
Northern bluefin tuna
The northern bluefin tuna (Thunnus thynnus), also known as the Atlantic bluefin tuna and giant bluefin tuna, is native to both the western and eastern Atlantic Ocean, as well as the Mediterranean Sea. In the Western Atlantic Ocean, they range from Labrador in Canada to northern Brazil, including the Caribbean Sea and the Gulf of Mexico. In the eastern Atlantic Ocean, they are found from the Lofoten Islands off of Norway to the Canary Islands and Mauritania in the south, including the Mediterranean Sea.
While many accounts list a subpopulation in the southern part of the Black Sea, where they were once common, they are now believe to be extinct in the Black Sea. Di Natale notes in a 2010 paper that "The bluefin tuna finally disappeared completely from the Black Sea, even from Turkish waters close to the Bosphorus, in 1988." It is also extinct from the Caspian Sea.
The body of the northern bluefin tuna is rhomboidal in profile and robust. The head is conical and the mouth rather large. The color is dark blue above and gray below with a gold coruscation covering the body and bright yellow caudal finlets. Fully mature adult specimens average 2–2.5 meters (6 ft 7 in to 8 ft 2 in) long and around 350 kilograms (770 lb) in weight. The species can reach a maximum length of about 4.3 meters (14 ft). The largest recorded specimen taken under International Game Fish Association rules was caught off Nova Scotia, an area renowned for huge Atlantic bluefin, by Ken Fraser and weighed 679 kilograms (1,497 lb).
The bluefin possesses enormous muscular strength, which it channels through a pair of tendons to the sickle shaped tail. In contrast to many other fish, the body stays rigid while the tail flicks back and forth, increasing stroke efficiency. Bluefin dive to depths of 1000 meters.
Throughout recorded history, the Atlantic bluefin tuna has been highly prized as a food fish. Bluefin have been a valuable commercial catch from the time of the ancient Greeks and Phoenicians to the modern era. According to archaeological findings, the northern bluefin tuna was first exploited over 11,200 years ago in the Mediterranean Sea.
Besides their commercial value as food, their great size and the speed and power they display as predators has attracted the admiration and respect of both ancient and modern fishermen, as well as writers, sport anglers, and scientists. The longest contest between man and tuna fish occurred near Liverpool, Nova Scotia in 1934, when 6 men, taking turns, fought the 795-lb tuna in a terrific battle that lasted sixty-two hours.
The northern bluefin tuna typically hunts small fish and invertebrates, such as sardines, herring, eels, mackerel, squid, and crustaceans, with larvae feeding on such organisms as rotifers, brine shrimp, and other fish larvae. They can use their fast speed to chase down prey and use modified filter feeding to catch smaller, slow moving organisms. Predators of the tuna include sharks, marine mammals (killer whales, pilot whales), and large predatory fishes. Bluefin tuna also are host to at least 72 parasites.
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, with males and females producing eggs and sperm synchronously nd resulting in many individuals mating at the same time (broadcast spawning). The tuna are highly vulnerable to commercial fishing at such times. 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.
Female bluefin produce up to 10 million eggs during each spawning period. No parental care is provided. The bluefin tuna become sexually mature between 4 and 8 years old. The western and eastern populations of Atlantic bluefin tuna are thought to mature at different ages. It is thought that bluefin born in the east reach maturity a year or two earlier than those spawned in the west.
Atlantic bluefin tuna can live for 30 years, but due to heavy fishing mortality and predation, few known specimens grow to a mature age.
Threats and conservation
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. The bluefin tuna is particularly prized by Japanese sushi consumers for its fatty belly flesh.
The bluefin tuna (including Pacific and Southern fisheries) supports a $7.2 billion industry. The highest prices in the Japanese market tend to be for Pacific bluefin tuna caught in Japanese waters, but high-grade Atlantic bluefin also fetch high prices. For example, in January 2010, a 510 pound (232 kg) bluefin sold for nearly $180,000 at auction in Tokyo, Japan.
The commercial importance of the Altantic bluefin tuna has led to severe overfishing of both the Atlantic and Mediterranean stocks. Proposals to ban international trade in Atlantic bluefin tuna, however, were soundly defeated (68 to 20, with 30 abstentions) by the delegates of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which had gathered March 2010 in Doha, Qatar. Japan had argued that ICCAT should be responsible for regulating the fishery, not the United Nations via CITES. The United States advocated for the ban on international trade in bluefin tuna, while several European Union nations, who have fisheries considered responsible for much of the overfishing, abstained from voting, and many fishing nations voted against the proposal being concerned about this being the first intrusion by CITES into a major commercial fishery.
Bluefin are captured for the commercial market by professional fishermen using purse seine gear, assorted hook-and-line gear, most importantly the longline, and in certain areas by harpooners. Atlantic bluefin are also taken commercially by heavy rod and reel gear. Since the 1930s the bluefin tuna has long been one of the most important big-game species sought by sports fishermen, particularly in the United States, but also in Canada, Spain, France, and Italy.
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 decimated by unrestricted commercial fishing, other populations were not at risk. However, in the 1960s purse seiners catching fish in United States coastal waters, largely for the canned tuna market, removed huge numbers of juvenile and young Western Atlantic bluefin, taking out several entire year classes.
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. However, this method requires the taking of juvenile and young Mediterranean fish to be grown on the tuna farms. Because the tuna are taken before they are old enough to reproduce, tuna farming is a serious threats. The bluefin's slow growth and late sexual maturity compound its problems. In 2009, aquaculturists succeeded in breeding bluefin in captivity and keeping them alive through their development from larvae to fingerlings to young juveniles.
Overfishing continues today despite repeated warnings. In 2007, researchers from ICCAT, the regulators of Northern bluefin fishing, recommended a global quota of 15,000 metric tons (15,000 long tons; 17,000 short tons). ICCAT then chose twice that number, later dropping it to 22,500 metric tons (22,100 LT; 24,800 ST). Their scientists now say that 7,500 metric tons (7,400 LT; 8,300 ST) is the sustainable limit. In November, 2009 ICCAT set the 2010 quota at 13,500 metric tons (13,300 LT; 14,900 ST) 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, a list of fish that is commonly purchased in supermarkets and yet have a very high risk of coming from unsustainable fisheries.
Southern bluefin tuna
The southern bluefin tuna (Thunnus maccoyii) is found in open southern hemisphere waters of all the world's oceans, mainly between 30°S and 50°S, to nearly 60°S. Its range includes waters off the coast of Australia, New Zealand, South Africa, Madagascar, Brazil, Argentina, and Uruguay. They are only rarely found in the eastern Pacific. It is a large (up to 2.5 meters or 8.2 feet in length and 400 kilograms or about 882 pounds), streamlined, fast swimming, pelagic fish with a long, slender caudal peduncle and relatively short dorsal, pectoral and anal fins. The body is completely covered in small scales. The body color is blue-black on the back and silver-white on the flanks and belly, with bright yellow caudal keels in adult specimens. The first dorsal fin's color typically is gray with a yellow tinge, the second dorsal is red-brown, and the finlets are yellow with a darker border.
Southern bluefin tuna spawn between September and April each year in the only known spawning grounds in the Indian Ocean, between the north-west Coast of Australia and Indonesia, southeast of Java, Indonesia. Age and size at maturity is uncertain, but believed to be no younger than at age 8 and around 1.5 meters in length. Mature females produce several million eggs during a spawning period. The eggs are estimated to hatch within two to three days, and over the next two years attain sizes of approximately 15 kilograms. The southern bluefin tuna is an opportunistic feeder, preying on a wide variety of fish, crustaceans, cephalopods, salps, and other marine animals. Southern bluefin tuna live up to 40 years in age.
Threats and conservation
The onset of industrial fishing in the 1950s, in conjunction with ever improving technologies such as GPS, fishfinders, satellite imagery, refrigeration techniques, and so forth, and the knowledge of migration routes, has led to the exploitation of southern bluefin tuna across its entire range. With a demanding global market taking its tool, the total population of southern bluefin tuna declined by 2007 to about eight percent of the levels prior to the increase of industrial fishing in the 1950s.
The southern bluefin tuna is now classified as Critically Endangered on the IUCN Red List of Threatened species. In 2010, Greenpeace International added the southern bluefin tuna, along with the northern bluefin tuna, to its seafood red list of species common in supermarkets and likely to having come from unsustainable fisheries.
Increasing concerns about sustainability in the mid 1980s did lead the main nations fishing for the southern bluefin tuna to by imposing strict quotas to their fishing fleets, although no official quotas were put in place. In 1994, the then existing voluntary management arrangement between Australia, Japan, and New Zealand was formalized when the Convention for the Conservation of Southern Bluefin Tuna came into force. The Convention created the Commission for the Conservation of Southern Bluefin Tuna (CCSBT). Its objective was to ensure, through appropriate management, the conservation and optimum utilization of the global southern bluefin tuna fishery. South Korea, Taiwan, Indonesia, South Africa, and the European Union have since joined and the Philippines is cooperating with the Commission. The CCSBT is headquartered in Canberra, Australia.
In 2011, the CCSBT adopted a management procedure to guide the setting of total allowable catch (TAC) to ensure restocking of the bluefin tuna population. The TAC is set in three-year periods, with the global TAC for 2021 to 2021 set at 17,647 tonnes (metric tons). Australia currently has the highest "effective catch limit" with 6,238 metric tons, followed by Japan (6,197), Republic of Korea (1,257), Fishing Entity of Taiwan (1,257), New Zealand (1,102), Indonesia (1,223), South Africa (455), European Union (11), and Philippines (45) metric tons.
The quota system has actually increased the value of the catch, as fisherman that once earned $600 a ton selling fish to canneries began making more than $1,000 per fish, selling them to buyers for the Japanese market.
The rapidly declining fishery led to Australian tuna fishers investigating the potential for value-adding their catch through aquaculture. All southern bluefin tuna ranching occurs in a small region offshore of Port Lincoln, South Australia.
Juvenile tuna to be raised are mainly caught on the continental shelf in the Great Australian Bight region from December to around April each year, weighing on average 15 kilograms, a size they had reached after roughly two years in the wild. The tuna that are located are purse seined, and then transferred through underwater panels between nets to specialized tow pontoons. They are then towed back to farm areas adjacent to Port Lincoln at a rate of about 1 knot; this process can take several weeks. Once back at the farm sites, the tuna are transferred from the tow pontoons into 40-50 meter diameter farm pontoons. They are then fed fish (pilchards or sardines, mackerel, etc.) and squid six days per week, twice per day and "grown out" for three to eight months, reaching an average of 30 to 40 kilograms. At harvest time, the fish are gently guided into a boat (any bruising lowers the price) and killed and flash frozen and predominantly put on Tokyo-bound planes.
Initially, the notorious difficulties in closing the life cycle of this species dissuaded most from farming them. However, in 2007, using hormonal therapy developed in Europe and Japan (where they had already succeeded in breeding Northern Pacific bluefin tuna to third generation) to mimic the natural production of hormones by wild fish, researchers in Australia managed for the first time to coax the species to breed in landlocked tanks. This was done by the Australian aquaculture company, Clean Seas Tuna Limited, which collected its first batch of fertilized eggs from a breeding stock of about 20 tuna weighing 160 kilograms. They were also the first company in the world to successfully transfer large SBT over large distances to its onshore facilities in Arno Bay, which is where the spawning has taken place.
Among concerns relative to aquaculture is the inefficient feed conversion ratios (feed to tuna growth) of approximately 10:1 or higher, a consequence of the carnivorous diet and high metabolic costs of the species. Removing tuna from the wild before they have spawned is another obvious impact, which hopefully the closed life cycles at Clean Seas will alleviate some of the pressure on declining stocks. Tuna farms also are point sources of solid waste onto to the benthos and dissolved nutrients into the water column. Most farms are more than a kilometer off the coast, thus the deeper water and significant currents alleviate some of the impact on the benthos.
Other environmental impacts include the use of chemicals on the farms, which can leach into the surrounding environment. These include antifoulants to keep the cages free from colonial algae and animals, and therapeutants to deal with disease and parasitism. Toxicants such as mercury and PCB's (polychlorinated biphenyls) can build up over time, particularly through the tuna feed, with some evidence of contaminants being more elevated in farmed fish than in wild stocks.
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- Atlantic Bluefin Tuna, Thunnus thynnus The MarineBio Conservation Society
- Commission for the Conservation of Southern Bluefin Tuna (CCSBT)
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