The Atlantic bluefin tuna (Thunnus thynnus) is a type of tuna in the Scombridae family. It is also called the northern bluefin tuna (mainly when including Pacific bluefin as a subspecies), giant bluefin tuna (for individuals weighing more than 150 kg [330 lb]), and was once called the tunny.
Atlantic bluefin tuna live in the western and eastern Atlantic Ocean, as well as the Mediterranean Sea. They are no longer found in the Black Sea. The Atlantic bluefin tuna is closely related to the Pacific bluefin tuna. The southern bluefin tuna, however, is more closely related to other tuna species like yellowfin tuna and bigeye tuna. The similarities between the southern and northern bluefin tuna are due to convergent evolution.
Atlantic bluefin tuna can weigh up to 680 kg (1,500 lb). They are among the largest fish in the Percomorpha group, though they are much smaller than the Molidae species. For many years, the Atlantic bluefin tuna has been highly valued as a food fish. In addition to their use in food, their large size, speed, and strength as predators have earned admiration from fishermen, writers, and scientists.
The Atlantic bluefin tuna supports one of the world’s most valuable commercial fisheries. Medium and large fish are heavily targeted for the Japanese raw-fish market, where all bluefin species are highly valued for sushi and sashimi.
This commercial demand has caused severe overfishing. In October 2009, the International Commission for the Conservation of Atlantic Tunas reported that Atlantic bluefin tuna populations had dropped by 72% in the Eastern Atlantic and 82% in the Western Atlantic over the past 40 years. On October 16, 2009, Monaco recommended adding the Atlantic bluefin tuna to Appendix I of CITES, which would ban international trade. In early 2010, European officials, led by the French ecology minister, pushed for an international ban on bluefin tuna fishing. However, a UN proposal to protect the species was rejected (68 votes against, 20 for, 30 abstentions). Since then, regional fishing quotas have helped increase population numbers. As of September 4, 2021, the Atlantic bluefin tuna was reclassified from Endangered to Least Concern on the IUCN Red List of Threatened Species. However, many regional populations, including western stocks that spawn in the Gulf of Mexico, remain severely reduced.
Most Atlantic bluefin tuna are caught commercially by professional fishermen using longlines, purse seines, hook-and-line gear, heavy rods and reels, and harpoons. Recreational fishing has also made the bluefin tuna one of the most sought-after big-game species since the 1930s, particularly in the United States, Canada, Spain, France, England, and Italy.
Taxonomy
The Atlantic bluefin tuna was one of many fish species first described by Carl Linnaeus in his important 1758 10th edition of Systema Naturae. It was given the scientific name Scomber thynnus.
This species is most closely related to the Pacific bluefin tuna (T. orientalis) and more distantly related to other large tunas in the genus Thunnus, such as the southern bluefin tuna (T. maccoyii), the bigeye tuna (T. obesus), and the yellowfin tuna (T. albacares). For many years, the southern bluefin tuna was thought to be the closest relative of the two northern bluefin tuna species. However, a recent study suggests that the physical similarities between them are due to convergent evolution, meaning these traits developed independently as adaptations for living in cold water.
For many years, the Pacific and Atlantic bluefin tuna were considered the same species or subspecies and were called the "northern bluefin tuna." This name can sometimes cause confusion because the longtail tuna (T. tonggol) is occasionally referred to as "northern bluefin tuna" in Australia, New Zealand, and Fiji.
Bluefin tuna were often called the common tunny, especially in the UK, Australia, and New Zealand. The name "tuna," which comes from the Spanish word atún, became widely used in California in the early 1900s and is now used globally for all tuna species, including bluefin. In some languages, the red color of bluefin tuna meat is included in its name, such as atún rojo in Spanish and tonno rosso in Italian.
Description
The body of the Atlantic bluefin tuna is shaped like a diamond and strong. The head is cone-shaped, and the mouth is large. The head has a "pineal window," which helps the fish travel long distances across the ocean. The fish's color is dark blue on top, gray on the bottom, and has a golden shimmer on the body with bright yellow tail fins. Bluefin tuna can be identified by their short pectoral fins. Their livers have a special feature: they are covered with blood vessels arranged in a striped pattern. In other tunas with short pectoral fins, such vessels are either absent or found only along the edges.
Fully grown adult Atlantic bluefin tuna are about 2–2.5 meters (6.6–8.2 feet) long and weigh around 225–250 kilograms (496–551 pounds). The largest recorded fish, caught near North Lake, Prince Edward Island, weighed 679 kilograms (1,497 pounds) and was 3.84 meters (12.6 feet) long. A famous fishing contest took place near Liverpool, Nova Scotia, in 1934, where six people worked together for 62 hours to catch a tuna weighing 164–363 kilograms (361–800 pounds). Scientists from the Smithsonian Institution and the U.S. National Marine Fisheries Service have noted that this species may weigh up to 910 kilograms (2,010 pounds), though more details are unknown. Atlantic bluefin tuna reach adulthood quickly. A study of fish up to 2.55 meters (8.4 feet) long and 247 kilograms (545 pounds) in weight found none older than 15 years. However, very large fish may live up to 50 years.
The Atlantic bluefin tuna has powerful muscles that it uses to swim. These muscles are connected to two tendons that move the tail, which is shaped like a crescent, to propel the fish forward. Unlike many other fish, the body remains stiff while the tail moves back and forth, making swimming more efficient. The fish also has a highly effective circulatory system. It has one of the highest levels of hemoglobin in its blood, which helps deliver oxygen to its muscles. This is combined with a thin barrier between blood and water, allowing oxygen to be absorbed quickly.
To keep its core muscles warm for steady swimming, the Atlantic bluefin tuna uses a process called countercurrent exchange. Heat from the blood returning to the heart is transferred to the cool, oxygen-rich blood entering a special structure called a rete mirabile. While all tunas are warm-blooded, the Atlantic bluefin tuna has the most advanced ability to regulate its body temperature of any fish. This allows it to hunt in the cold, nutrient-rich waters of the North Atlantic.
Biology and ecology
Atlantic bluefin tuna can swim to depths of 1,006 meters (3,301 feet). These fish hunt small fish like sardines, herring, mackerel, and eels, as well as invertebrates such as squid and crustaceans. They often hunt in groups, organizing themselves based on the size of the fish they are targeting. Their white muscle tissue allows for strong, quick movements that help them swim rapidly to catch prey.
This species has over 70 types of parasites, but none are known to harm the tuna. One parasite is the tetraphyllidean tapeworm Pelichnibothrium speciosum. Since the tapeworm’s main host is the blue shark, which does not usually eat tuna, the Atlantic bluefin tuna is likely a dead-end host for this parasite.
Many animals eat Atlantic bluefin tuna. When they are newly hatched, they are preyed upon by other fish that eat plankton and by jellyfish. Some larvae even eat other larvae, which greatly reduces their numbers. To protect their young, adult bluefin tuna spawn in specific locations and at times linked to the moon’s cycle. As they grow, the size of their predators increases. Adult bluefin tuna are only eaten by the largest billfishes, toothed whales, and some open-ocean shark species.
Atlantic bluefin tuna reproduce by laying eggs. They gather in large groups to spawn, and females release many eggs into the water. These eggs are fertilized by males outside the body. A female bluefin tuna can produce about 128.5 eggs per gram of body weight, or up to 40 million eggs at once. Eggs hatch into larvae two days after fertilization and become small, cannibalistic fish about a quarter-inch long by the end of the first week. About 40% of larvae survive their first week, and about 0.1% survive their first year. Surviving bluefin tuna often form schools based on their size.
Traditionally, Atlantic bluefin tuna were known to spawn in two separate areas. Research using satellite tracking supports the idea that bluefin tuna born in each area may travel widely across the Atlantic but usually return to their birthplace to spawn. The eastern population spawns in the western Mediterranean, especially near the Balearic Islands. The western population spawns in the Gulf of Mexico. Because bluefin tuna gather in large groups to spawn, they are at high risk of being caught by commercial fishing. This is especially true in the Mediterranean, where spawning groups can be seen from the air, and fishing nets are used to surround them.
In 2016, researchers found evidence of a third spawning area in the Slope Sea, a region north and west of the Northeastern United States Continental Shelf. Later studies showed similar numbers of bluefin tuna larvae in the Slope Sea and the Gulf of Mexico.
Eastern and western bluefin tuna populations show some behavioral differences, possibly due to environmental conditions. For example, tuna in the Gulf of Mexico spawn between mid-April and mid-June when water temperatures range from 24°C (75°F) to 29°C (85°F). Tuna in the Mediterranean spawn between June and August when water temperatures range from 18°C (65°F) to 21°C (70°F). Gulf of Mexico tuna dive deeper than 500 meters (1,600 feet) when entering the Gulf and stay deeper than 200 meters (660 feet) to spawn.
It was once thought that eastern and western populations mature at different ages. Tuna born in the east may reach maturity one or two years earlier than those born in the west. These differences may also be influenced by migration patterns and other spawning areas, such as the Slope Sea.
Human interaction
According to Longo, "by the turn of the first millennium CE, a sophisticated bluefin tuna trap fishery had emerged. This trap fishery, called tonnara in Italian, madrague in French, almadraba in Spanish, and armação in Portuguese, forms an elaborate maze of nets that capture and corral bluefin tuna during their spawning season. Active for more than a thousand years, the traditional/artisanal bluefin tuna trap fishery has experienced a collapse in the Mediterranean and has struggled where it is still practiced."
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.
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 bluefins, usually discarding them after taking a picture. The bluefin sport fishing 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 bluefins cheaply, and started filling Japan-bound holds with tuna. Exposure to beef and other fatty meats during the U.S. occupation following World War Two 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 bluefins, 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 migratory habits of tuna 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.
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. 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 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.
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.
Global appetite for fish is the predominant threat to Atlantic bluefin. Overfishing continues despite repeated warnings of the current precipitous decline. 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.
The 2010 Deepwater Horizon oil spill released an estimated 4.9 million barrels of crude oil into the Gulf of Mexico during the spawning season of the Atlantic bluefin tuna. The oil is estimated to have affected roughly 3.1 million square miles, including more than 5 percent of the tuna habitat in the U.S. Exclusive Economic Zone. The spill occurred at a peak time for the fertilization of eggs and the development of larval bluefin tuna. Resulting short and long-term impacts on populations of Atlantic bluefin tuna and other pelagic species are difficult to determine, in part due to limitations in monitoring ability. Nonetheless, a number of lethal and sublethal impacts have been documented, including pericardial edema, defective cardiac function and cardiac abnormalities.
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.
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.