Bluefin Tuna: Characteristics, Behavior, Hunting and Mating | Sea Life, Islands and Oceania

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BLUEFIN TUNA

Bluefin tuna (Scientific name: Thunnus orientalis) are one the most valuable fish in the sea. Prized for making sushi and sashimi in Japan, they appeared in the caves of ancient people, were placed on coins by the ancient Romans, painted by Salvador Dali and described by fishmongers in Tokyo as Catherine Zeta-Jones of the sea. At Nishinomiya Shrine in Nishinomiyama Hyogo Prefecture in Japan people press coins onto a frozen tuna and pray to Ebisui, the deity of wealth and commerce.

Elizabeth Kolbert wrote in The New Yorker, “The Atlantic bluefin tuna is shaped like a child’s idea of a fish, with a pointy snout, two dorsal fins, and a rounded belly that gradually tapers toward the back. It is gunmetal blue on top, and silvery on the underside, and its tail looks like a sickle. The Atlantic bluefin is one of the fastest swimmers in the sea, reaching speeds of fifty-five miles an hour. This is an achievement that July 2, 2011scientists have sought to understand but have never quite mastered; a robo-tuna, built by a team of engineers at M.I.T., was unable to outswim a real one. (The word “tuna” is derived from the Greek thuno, meaning “to rush.”) Atlantic bluefins are voracious carnivores — they feed on squid, crustaceans, and other fish — and can grow to be fifteen feet long. [Source: Elizabeth Kolbert, The New Yorker, August 2, 2010]

Bluefin tuna live in saltwater or marine environments usually in the open ocean far from land at depths of zero to 550 meters (1804 feet). The generally inhabit the epipelagic (open ocean) region, but occasionally come close to shore. They have tolerance to a wide temperature range (2° to 30°C, 36° to 86°F) and thus can inhabit waters at a variety of depths and a variety of latitudes. [Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]

Websites and Resources: “Netting Billions 2020: A Global Tuna Valuation, Pew Charitable Trusts”, October 6, 2020 pewtrusts.org; Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems

Bluefin Tuna and Human History

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tuna caught in 1898
Kenneth Brower wrote in National Geographic: “The three species of bluefin — the Atlantic, Pacific, and southern — have divided the world’s oceans among themselves, and they roam all planetary seas except the polar. The bluefin is a modern fish, yet its relationship with humanity is ancient. Japanese fishermen have caught Pacific bluefin for more than 5,000 years. The Haida of the Pacific Northwest have hunted the same species for at least as long, based on the evidence of bluefin bones in their middens. Stone Age artists painted Atlantic bluefin tuna on the walls of Sicilian caves. [Source: Kenneth Brower, National Geographic, March 2014]

Iron Age fishermen — Phoenician, Carthaginian, Greek, Roman, Moroccan, Turkish — watched from promontories for the arrival of bluefin schools at their Mediterranean spawning grounds. “Bluefin helped build Western civilization,” Stanford University professor Barbara Block, a preeminent scholar of this fish, told me. “Across all the Mediterranean, everybody netted giant tuna. The bluefin have annual migrations in through the Strait of Gibraltar, and everyone knew when they came. In the Bosporus there were 30 different words for bluefin. Everyone put out net pens that had different names in the different countries. Penning created cash. Bluefin were traded.

The coins of Greece and Celtic coins, they had giant bluefin on them.“The king of all fish,” Ernest Hemingway reported in the Toronto Star Weekly in 1922, after seeing Atlantic bluefin off Spain. Carl Linnaeus, the father of modern scientific classification, named the Atlantic bluefin in 1758. Linnaeus often resorted to repetition in flagging superlative animals. Gulo gulo he named the wolverine, king of the weasels. Bison bison he named the bison, king of the prairie. Thunnus thynnus he named the Atlantic bluefin: tuna of tunas.

But a few generations people didn’t think much of the fish when others were plentiful. A fisherman in Nova Scotia told Brower. “We didn’t fish tuna,” he says of his father’s generation. “Tuna fishing was more of a sport. Years ago they used to call it ‘horse mackerel.’ It was cat food back then, or fertilizer.” Now that the sushi market brings in big bucks bluefin tuna are only way some fishermen can earn a living — a good living of they catch the right fish.

““In January 2013 a single bluefin tuna sold in Tokyo for $1.76 million. The outrageous price was part publicity stunt, part Japanese ritual: The first tuna on the auction market each year is subject to a bidding war that’s over the top, even by Japanese standards. Yet even the normal price of one medium-size bluefin — between $10,000 and $20,000, depending on quality — is a startling measure of how much 21st-century Japanese have come to treasure maguro, bluefin sushi. It is a measure, too, of what the bluefin tuna is up against if more than a handful are to see the 22nd century.

Bluefin Tuna Characteristics

Bluefin Tuna can reach lengths of 4.3 meters. They reach a weigh of about 20 kilograms at age three and reach 130 kilograms and extend to over two meters at age eight. The largest bluefin tuna ever caught with a rod and reel was a 32-year-old beast that weighed 678 kilograms (1,496 pounds).

20120521-bluefin tunaOsaka_Kaiyukan_Aquarium).jpg Bluefin tuna are endothermic (use their metabolism to generate heat and regulate body temperature independent of the temperatures around them), which unlike nearly all other fish. Their average weight is 60 kilograms (132 pounds) and their average length is 150 centimeters (59 inches). They have a relatively high basal metabolic rate, ranging from 0.235 to 0.498 cubic centimeters of oxygen per gram per hour. This attribute allows them to tolerate a wide range of water temperatures. Sexual dimorphism (differences between males and females) is not pronounced: Both sexes are roughly equal in size and look similar. Atlantic bluefin tuna are the largest of three tuna groups.[Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web: Bluefin tuna have black or dark blue dorsal sides with grayish-green iridescence and silver or gray spots on their bellies. Small yellow "fins" that are edged in black begin behind the second dorsal fin and continue to the tail. They have relatively small pectoral fins, and small eyes relative to other tuna species. They have a homocercal, lunate (crescent-shaped) tail that allows for great speed over long distances. /=\

Adult males and females vary in length from 100 centimeters to 300 centimeters. It takes individuals five years to mature, generally averaging about 150 centimeters in length with a weight of around 60 kilograms. Bluefin tuna never stop growing, but growth slows over time. Males and females have about the same growth rate. In the wild, bluefin tuna live around 15 years, but the longest lifespan recorded is 26 years. They do not do well in captivity for long periods of time. /=\

Bluefin Tuna High-Speed Swimming

Described by some people as "Porsches of the sea," bluefin tuna can accelerate faster than a Porsche 911, reach speeds of 65 kilometers per hour (40 mph), cruise 250 kilometers (150 miles) a day and dive to depths of 2,400 feet and stay in those depths for two hours. Their bodies have a streamlined teardrop shape. Even their eyeballs are flush with their bodies to cut down on drag. The small "finlets" at the rear of their bodies are thought to prevent drag by creating a slight turbulence the same way dimples on a golf ball do.

The tail fin of the bluefin tuna propels the fish forward and pectoral fins control posture. The tail fins in young fish grow faster than the abdominal and pectoral fins. This means young fish can travel very fast but have difficulty turning. Bluefin tuna fins retract into groves in the body when swimming at fast speeds. When it approaches prey the fins are raised so the fish can maneuver better. Unlike most fish who move their bodies from side to side, bluefin keep their bodies stiff and get most of their power from moving their tail back and forth.

Describing swimming with a trapped school of bluefin tuna, Fen Montaigne wrote in National Geographic, “Their backs were battleship gray topped with a saw-toothed line of small yellow dorsal fins. Their sided had the look of battered chrome and steel; some bore the streak of an electric blue line. The large fish, weighing more than 500 pounds, were at least eight feet long....One giant bluefin — some 400 pounds heavier and two feet longer than most of the others — caught my eye. It was not swimming endlessly with the school in a clockwise gyre. Instead, it darted in different directions, sudden and aggressive, nearly brushing against me as it scanned me with large, black, disk-shaped eyes.”

Bluefin Tuna and Great White Shark — the Sea’s Fastest Long Haul Swimmers

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Atlantic bluefin tuna

A research team led by Yuki Watanabe, an associate professor of marine zoology at the National Institute of Polar Research, compared the long-distance cruising speeds of 46 species of fish. in a paper published in April 2016 on the Proceedings of the U.S. National Academy of Sciences. It showed that a 428-kilogram great white shark swam the fastest at 8.1 kilometers per hour (kph (5 miles per hour (mph)), followed by a 240-kilogram bluefin tuna at 7.2 kph (3.5 miles per hour). [Source: Earth.com, January 16, 2017]

This compares with 3.1 kph (1.9 miles per hour) of a 2.2-ton whale shark, a speed not unexpected considering its huge body. Sunfish and salmon were the slowest. An 87-kilogram sunfish swam at 2.2 kph (1.4 miles per hour) and a 3.3-kilogram salmon at 2.7 kph (1.7). The scientists used small monitoring systems attached to fish. The high speeds of the sharks and tuna are attributed to their unique body system that has evolved to keep their body temperatures relatively high.

According to Earth.com: “The shark is a cartilaginous fish while tuna have a bony skeleton. However, great white sharks and bluefin tuna have a common anatomy that makes them faster than other fish. Both have dark muscles where many blood vessels meet. Fish with lower body temperatures have very few blood vessels close to the surface of their bodies. These species of sharks and tuna also have a common structure called a rete mirabile. This a complex system of veins with cold blood and arteries with warm blood lying close together to prevent heat from escaping the body. As hunters of the outer seas where they are less concealed and more vulnerable to attack, speed is critical to the survival of great white sharks and bluefin tuna. The resemblance between these fish is a result of “convergence,” which is the evolution of unrelated animals to develop similar body characteristics in a common environment.

Bluefin Tuna Breathing and Warm Bloodedness

Unlike most fish, bluefin tuna are warm-blooded. This helps explain why they are such powerful swimmers but seems to be a disadvantage when the fish are in warm water. The deep red color of tuna flesh comes from myoglobin in the fish's muscles. Myoglobin is similar to hemoglobin. It helps carry oxygen to the muscles and helps the fish swim faster.

Bluefin tuna have rigid skulls built for speed and are unable to widen their jaws and suck and squeeze water to breath through their gills like other fish. Instead they have to keep swimming to keep water flowing through their gills the way some sharks do. If they don’t swim fast enough they suffocate. They also have very large gills to help them draw more oxygen from the water.

Being warm blooded like a mammal allows bluefin tuna to roam between the Arctic and the tropics, they can maintain a body temperature of 30̊C (86̊F) in icy 4̊C (39̊F) waters with the help of an intricate network of tiny blood vessels that circulate blood brought from the warm interior of the fish to its exterior. They can also speed up their like mammals when exerting themselves.

Bluefin Tuna Behavior and Perception

marinated marguro

Bluefin tuna are diurnal (active mainly during the daytime), motile (move around as opposed to being stationary), migratory (make seasonal movements between regions, such as between breeding and wintering grounds), and social (associates with others of its species; forms social groups). [Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]

Bluefin tuna are sometimes seen in schools of 4,500 members. During the afternoon they like to bask in the sun. Pacific bluefin tuna tend to swim in schools with other tuna, including closely-related species like Atlantic bluefin tuna and southern bluefin tuna. Bluefin tuna among the most mobile of all tuna species. Scientists who tagged and documented over 250 tuna found that individuals didn’t have home ranges but they did belong to populations that kept home ranges seasonally, and migrated in schools. They do not actively defend their territory.

Pacific bluefin communicate with vision, touch and chemicals usually detected by smelling and sense using vision, touch and chemicals usually detected with smell. According to Animal Diversity Web: They use their eyes to locate prey. Egg fertilization occurs externally, so they also rely on vision to locate eggs. Although they have poor vision, their eyes have rods and cones, allowing them to see color and even shades of gray. By the time larvae are five days old (ca. 4.5 millimeters long), they are reported to be photopositive (attracted to light). Olfactory senses differentiate chemicals in the water, so Pacific bluefin tuna aren’t fully reliant on sight. Taste buds begin to develop in larvae about 10 days after hatching, and are complete by Day 33. /=\

Bluefin Tuna Hunting

Bluefin tuna are ferocious and cunning predators. They feed primarily on herring and mackerel such as jack mackerel and Pacific mackerel, but also consume anchovies, sardines, squid, Pacific saury, rock fish, crustaceans, lanternfish, and pelagic red crab.

Bluefin tuna are opportunistic feeders, meaning they do not have specific feeding patterns and feed based on prey abundance. While they are mostly carnivorous, they also consume algae. Up to a year of age they predominantly consume small squids and zooplankton such as Daphnia. After about one year they start eating a wide range of fish and other algae. Their endothermic metabolism and their physical design allows them to swim at high speeds and catch prey. The ability to see shades of gray gives them an advantage while hunting in areas with low light. They use their sense of touch to locate what is around them and potentially alert about changes in the environment. [Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]

A school of tuna can attack as a group, traveling at 40 mph. Bluefin tuna often hunt for prey where cold and warm water meet and nutrients attract plankton which feed on a variety of fish on up through the food chain. They often make deep dives for prey around sunrise and sunset. The only predators observed feeding on bluefin tuna are mako sharks, great white sharks, orcas (killer whales) and humans. The tuna;s' coloration allows them to blend with the water and be hard to make out from a distance. Their color patterns provide natural camouflage in the oceanic environment.

Describing a quick bluefin midday snack, Kenneth Brower wrote in National Geographic: “One moment the undersea is featureless blue... The next moment the ocean is full of giant, bomb-shaped bluefin tuna, the largest measuring 14 feet long and weighing three-quarters of a ton. In the sea’s refracted sunlight, their pale flanks flare and scintillate like polished shields. Their fixed fins — the long, curved anal fin and the second dorsal — flash like sabers. Their quick-sculling tail fins drive the formation forward at ten knots, with sprints to 25, a ceaseless, staccato beat. And just as suddenly they are gone. The ocean is empty again. Here and there a small galaxy of scales marks where a bluefin swallowed a herring. The victim’s scales swirl in the turbulence of the departed tuna, now bearing off at high speed.[Source: Kenneth Brower, National Geographic, March 2014]

Bluefin Tuna Mating, Reproduction and Offspring

Bluefin tuna are oviparous (young are hatched from eggs) and iteroparous (offspring are produced in groups). They engage in seasonal breeding and external reproduction in which sperm from the male fertilizes the female’s egg outside her body and employ broadcast (group) spawning, the main mode of reproduction in the sea. It involves the release of both eggs and sperm into the water and contact between sperm and egg and fertilization occur externally. [Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]

Bluefin tuna are polygynandrous (promiscuous), with both males and females having multiple partners. There is pre-fertilization provisioning but no parental involvement after birth. The egg has nutrition to sustain adequate larvae development. Swimming to designated spawning areas and the act of mating itself are main parental investment in the raising of their young. .

Bluefin tuna reach sexual maturity around the age of eight years old. Studies have shown that their bodies warm as they engage in courtship and thus they prefer to stay in warm waters which is why tuna in the Atlantic seek out the warm waters in the Gulf of Mexico and the Mediterranean during the spawning season.

Describing spawning bluefin tuna in the Mediterranean Sea, Fen Montaigne wrote in National Geographic, “Slashing through the sea, planing on their sides and exposing their massive silver-colored flanks, the large females each expel tens of millions of eggs, and the males emit clouds of milt. From the air, on a calm day, this turmoil of reproduction — the flashing of fish, the disturbed sea, the slick of spawn and sperm — can be seen from miles away by spotter planes.”

Bluefin Tuna Development and Growth

Pacific bluefin tuna spawning typically takes place in the Pacific Ocean in waters between 23.5° and 29.5°C (74.3 to 55°F). After they hatch juveniles migrate to waters of varying temperature. The average time for eggs to hatch is about 38 hours, with a range of 22 to 48 hours.. Young are independent at birth and juveniles swim alone or in schools. Sexual maturity varies from three to nine years, but both males and females typically reach maturation at around age five. There is documentation of one individual that sexually active until near the time of his death at age 26 years old. [Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]

Kawamura et al. (2003) reported the length of newly-hatched larvae is three millimeters. They reach 3.9 millimeters by Day 2, four millimeters by Day 3, 5.6 millimeters by Day 8 and 16 millimeters by Day 16. At 30 days these juveniles are five centimeters long. At one year old, they reach an average fork length of 58 centimeters. Fork length is the length of an individual fish from the tip of its snout to the end of its middle caudal fin rays). They reach around one meter fork length by the end of year three. ./=\

Their development and life cycle is characterized by indeterminate growth (they continue growing throughout their lives). Growth is quick at the start.During years two to three, the development slows down. Growth occurs mostly between one and three years, after which the growth rate slows down, eventually hitting steady growth. They grow indeterminately but very slowly by the time they have reached 230 centimeters in total length.

Studying Bluefin Tuna

Even though bluefin tuna are such a valuable food fish little is known about them. It is not known, for example, exactly where many of them feed or breed. Bluefin tuna are studied using a number of different techniques. Chemical markers in their urine and bones — which reflect the waters the fish have been swimming in — are analyzed to figure out where the fish have seem. Studies on migration and feeding sometimes employ stable isotope analyses. Analysis of carbon and oxygen isotopes allows scientists to determine whether tuna in the Atlantic are from the Mediterranean stock or the Gulf of Mexico stock.

Bluefin tuna are tagged just below their dorsal fin with microprocessors and transmitters that releases from the fish at a designated time and transmit data to satellites. Similar devices have been used to study the migration patterns of great white sharks and leatherback turtles. Tags placed on tuna, sharks and seabirds measure death and record levels of ambient lights that can be translated into longitude and latitude to study migration patterns. Electronic tags were placed on 900 of bluefin tuna in the Atlantic by a team led by Barbara Block of Stanford University and the Monterey Bay Aquarium to track their migration patterns, diving behavior and body and water temperature of the fish. Some tags pop up and their data has been retrieved by satellite. Others are implanted in the fish and anyone who returns them receives a $1,000 reward.

Describing a tagging session led by Steve Wilson, a Stanford University researcher at the Tuna Research and Conservation Center (TRCC) in Monterey, California, Kenneth Brower wrote in National Geographic: “Wilson and his team worked efficiently and fast, like a crew swarming an underwater racing machine at a pit stop. A wet black cloth went over the eyes as blindfold. A green hose went in the mouth and began pumping seawater past the gills. A roll of measuring tape flew over the fish, tossed from one man to another. The tape was laid flush against the body between the tip of the nose and the point where the tail fin forked. This measurement, the curved fork length, or CFL, in this fish was 300 centimeters, just short of ten feet. CFL is an accurate predictor of a tuna’s They weigh 1,226 pounds in this case, nearly twice Gillis’s original estimate. It was the third biggest bluefin ever tagged by the team in nearly 20 years of work. [Source: Kenneth Brower, National Geographic, March 2014]

“Straddling the rear of the fish, Wilson drove in a titanium dart to anchor a satellite tag just forward of the second dorsal fin. Four team members took up positions at the corners of the blue mat and lifted. Clearing the deck, the mat became a hammock. Straining under the burden of the fish, the four men walked a semicircle of tiny mincing steps, rotating the fish 180 degrees to bring it around to face the tuna door. From the scimitar of the anal fin, Schallert snipped a sliver for DNA analysis. Then the two men at the tail hoisted their end of the mat. The tuna plunged through the doorway and back into the gulf, raising a splash like a horse diving off a pier. Two flicks of its tail fin and it was gone.

“On his laptop the night before, Wilson had programmed the satellite tag on this fish to pop off on June 1 of the next year. Nine months and two weeks from this day, in whatever time zone the bluefin happened to be, the tag would send an electric current through the metal pin attaching it to the leader and dart in the fish. The electrolyzed pin would begin to corrode. Within a few hours it would sever. A bulb on top of the tag is made of foam that’s incompressible and therefore buoyant at any depth. The tag would rise through the ocean. On breaking the surface, it would begin uploading the encoded secrets of this bluefin — its travels, its seasons, its dive patterns — to Argos satellites orbiting overhead.

See Studying Fish

Image Source: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023