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bluefin tuna
Tuna are relatives of billfish such as sailfish and marlin. There are 21 species of tuna. Five are important commercially: albacore tuna, bluefin tuna, little tuna, skipjack, and yellowfish tuna.

Tuna are mostly an open water fish found through temperate and tropical waters around the world. Unlike other fish, tuna are warm blooded. Their body temperatures are as much as 12 degrees C higher than the water they swim in. The need to keep their bodies at a high temperature for the muscles and reactions to be as strong and quick as they are.

Tuna change temperature when they are feeding. On feeding tuna, David Attenborough wrote: "Tuna usually operate in squadrons. A group has been observed carefully herding a shoal [school], driving it from behind and patrolling its flanks to keep it well packed. When they attack there is wholesale carnage. As they rip through the shoal, snatching the small fish with devastating precision and speed, the shoal itself panics. Fish shoot from the surface of the sea in hundred in an attempt to escape the snapping jaws below, like terrified impala leaping away from a team of rampaging lions.”

Fish Oil, Omega 3s and the Benefits of Eating Fish

Tuna is regarded as a healthy fish to eat. The Japanese have traditionally eaten a lot of fish and they are the longest living people in the world. In 2000, the American Heart Association recommended that people eat salmon or tuna twice a week. It was the first time the group recommended eating specific things rather than offering general guidelines. Studies have shown that eating oily fish can significantly reduce the chances of getting a stroke, developing prostrate or breast cancer, suffering from depression or being stricken by a sudden, unexpected death caused by a severely abnormal heart rhythms.

Eating fish has been linked with low cholesterol levels and low rates of heart disease.Oily fish such as salmon and tuna are in rich fish oils, which in turn are rich in omega-3 fatty acids, an essential nutrient in the human diet. There is evidence that omega-3 fatty acids prevent inflammation, clot formation and clogging of blood vessels with fat and cholesterol and that a lack of omega-3 fatty acids may play a role in a variety of maladies, including bipolar disorder, heat arrhythmia, high blood pressure, heart disease, kidney failure, irritable bowel syndrome and rhomboid arthritis.

yellowfin tuna
Omega-3 and omega-6 are essentially fatty acids that work together to promote good health, The human body can not make them so it is essential that people eat diet rich in them. Omega-3 are found in fish and certain oils such as canola and flaxseed and omega-6 are found in raw nuts and seeds. Omega-3 fatty acids are found in sardines, salmon, herring and some kinds of tuna. Shrimp, some kinds of tuna, haddock, clams, cod, and crab are low in Omega 3s. Omegas 3s are also found in wild game, nuts like almonds and walnuts and avocados. With the absence of Omega-3s the body uses saturated fat to make cell membranes. Cell membranes made with this type of fat are less elastic and a lack of elasticity can be dangerous to the heart.

Studies have also shown that eating fish rich in omega-3 fatty acids cuts the risk of stroke and cancer. A Finnish study published in the journal Neurology in August 2008, researchers found that older adults who regularly ate fish had a lower chance of developing subtle brain damage that contributed to stroke and dementia. A study by Dr. Mich Brown of Paterson Institute at Christie Hospital in Manchester England published in the journal Cancer in March 2006 found that eating foods rich omega-3 fatty acids could help prevent the spread of prostrate cancer.

Eating fish can reduce irregular heart beats. In a study conducted by Bingham and Women’s Hospital and Harvard Medical School, regularly eating tuna or broiled or baked “but not fried” fish reduces the risk of atrial fibrillation, a major cause of stroke and other problems. Scientists credit the Omega 3s in the fish.

There is also some research that suggests that mothers who eat Omega 3s while pregnant produce smarter children and that people who get angry easily can better control their tempers better if they eat them. A study by American and British researchers published in Lancet in 2007 reported that children of mothers who ate small amounts of fish when they were pregnant gave birth to children who had lower IQ and academic test scores and had more social and behavioral problems than children of mothers who ate 12 or more ounces of fish per week. Other studies have indicated that women who don’t get enough omega-3 fatty acid are more likely to deliver babies too early and at a low birth weight.

Hydrodynamic Tuna and Oxygen

tail of a bigeye tuna
Tuna have hydrodynamically-shaped bodies that allow them to swim very fast. Their snouts are pointed like a supersonic plane; their rear part tapers gently; and they have a crescent-shaped tail. The surface of their eyes are contoured so they don’t bulge and interrupt the streamlined surface. The crescent-shaped tail of tuna, marlins, swordfish and sailfish, creates a thrust similar to that of the wings of birds.

Tuna are covered by specially-modified scales just behind the head that act like the spoilers on race cars. They produce a slight turbulence around the widest part of the body, which reduces drag on the rear of the body. When swimming at high speed, the fins slot into special groove so the flow of water is not obstructed.

To study how tuna achieve their great speeds in a medium 800 times denser than water scientists at MIT invented Robotuna. The robot has a stainless steel spine with eight vertebrae connected with ball-bearing joints that are moved with a pulley and cable system. Another tuna-like robot created at Cambridge, called VCUUV, cost $1 million to make.

Tuna draw heat from the movement of their own muscles. This gives them lots of energy but it also means they need a lot of oxygen too. Swimming at great speeds requires a great deal of energy and oxygen. Tuna obtain the oxygen they need by swimming with their mouth's open, forcing jets of water over the large gills. This means they have to swim continuously at relatively high speeds to breath. If they don’t they die.

Tuna and Dolphins

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In the late 1980s it was estimated that over a million dolphins a year were being killed by fisherman trying to catch tuna, squid and other food fish. Most of the dolphins died by getting entangled in drift nets up to 40 miles long or encircled by purse seine nets, used to capture schools of tuna.

Tuna fishermen have traditionally looked for schools of dolphins because yellowfin tuna tend to congregate underneath them. Scientists speculate the tuna do this because the dolphins protect them from predators and help them locate food. The confusion of being trapped in a seine net often forces the school of dolphins to sink to the bottom of the net in a helpless pile. Dolphins could easily prevent themselves from being trapped by purse string nets that they could easily jump over but they don?t. Fortunately drift nets are no longer used and techniques have devised to drive the dolphins from the seine nets without affecting the tuna catch.┺

Changes introduced in U.S. fisheries in the 1990s reduced the numbers of dolphins accidently caught by a third. The United States introduced the dolphin-safe label which placed on cans of tuna in which tuna were caught using methods that minimized the harm to dolphins. Starkist, Chicken of the Sea and Bumble Bee all went along. In 2002, the Bush administration changed the definition of dolphin-safe to allow the encircling of dolphins to catch tuna in the name of free trade and globalization and a concession to the Mexican fishermen who want access to the American market.

In the mid 2000s it was estimated that 300,00 dolphins, porpoises and whales were still being killed by fishing nets each year, with dolphins in the Philippines, India and Thailand being under the greatest threat. Many of those killed in the open sea are killed by gill nets. Deaths can be reduced by using simple, low cost safety measures such as making slight modifications to fishing gear.

Skipjack, Yellowfin and Bigeye Tuna

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Most canned tuna and the grilled tuna and tuna salads served in restaurants uses skipjack and yellowfin tuna. Skipjack tuna reach lengths of two and three feet. They live in tropical and subtropical waters. One of the most heavily fished of all fishes, they are found in huge schools near the surface that are scooped up with purse seine nets and caught with hooks and lines. Skipjack tuna stock are still healthy.

Yellowfin tuna is widely consumed in Japan in sashimi and sushi. Around 20,000 to 38,000 tons of the annual catch of 100,000 to 150,000 tons of the fish caught in central and western Pacific is consumed in Japan. Because overfishing of the fish is regarded as a serious problem fishing experts have called for a 30 percent reduction of the yellowfin tuna catch,

Bigeye tuna is widely sold as supermarket sashimi in Japan, where it is popular because of its relatively cheap price and a taste that is similar to bluefin tuna. Declining supplies of bluefin tuna have increased demand for bigeye tuna.

In 2007, the worldwide trade network TRAFFIC, which is partly run by the WWF, said that some tuna stocks were on the verge of collapse. There has been some discussion of reducing quotas of the fish in the Indian Ocean and the Central and Western Pacific Ocean where there are concerns about overfishing.

In December 2008, the Western and Central Pacific Fisheries Commissions (WCPFC) agreed to cut catches of bigeye tuna, in part of the Pacific Ocean by 10 percent in each of the next three years. Environmentalists criticized the decision for being too little too late. They wanted an immediate reduction of 30 percent. in June 2009, the Inter-American Tropical Tuna Commission agreed to cut quotas of bigeye tuna in the eastern Pacific Ocean by 9 percent in 2011 from 2007 levels.

Japan and Taiwan have the largest bigeye fishing fleets, Micronesia, the Marshall Islands and Maldives have large artisan fleets and provide processing for large vessels.


yellowfin tuna
Bonito is a name given to various species of medium-sized, predatory fish in the Scombridae family. 1) Bonito most commonly refers to species in the genus Sarda, including the Atlantic bonito (Sarda sarda) and the Pacific bonito (Sarda chiliensis lineolata). 2) In Japanese cuisine, bonito refers to the skipjack tuna (Katsuwonus pelamis), which, in Japan, is called by its local name, katsuo. 3) Bonito can generally refer to any of various scombroid fish related to, but smaller than, tuna. [Source: Wikipedia]

Katsuo is used extensively in Japanese cusine. Aside from its prevalance as in raw preparation (e.g. sushi and sashimi), it is also smoked and dried to make katsuobushi, an important ingredient in dashi (a type of common Japanese fish stock). It is also a key ingredient in shiokara.

Pacific and Atlantic bonito meat has a firm texture and a darkish color. The bonito has a moderate fat content. The meat of young or small bonito can be of lighter color, close to that of skipjack tuna, and is sometimes used as a cheaper substitute of skipjack, especially for canning purposes. Bonito may not be marketed as tuna in all countries, however. In Spain, tuna (Thunnus alalunga) is sometimes referred as Bonito del Norte. The Atlantic bonito is also found in the Black Sea. Called palamut in Turkish, it grows to a size of 65 cm, and is a popular seasonal choice.

Limits on Bonito Fleets

In November 2010, the Yomiuri Shimbun reported that the Japanese government is considering proposing a limit to the number of fishing boats that use encircling nets to catch bonitos, whose stocks are regulated by the Western and Central Pacific Fisheries Commission (WCPFC). Tokyo is mulling the proposal because the catch of bonitos that swim north to Japanese waters has been on the decline as the number of fishing boats with encircling nets, or seines, are increasing rapidly near the equator in the Pacific Ocean. Many such boats from Taiwan, China and other countries are working in the area to capitalize on a growing bonito boom in the United States and European countries, where the fish had not been consumed on a major scale in the past. [Source: Yomiuri Shimbun, November 2, 2010]

The Japanese fisheries industry and its related businesses are increasingly anxious about the situation. Bonito is a familiar part of the Japanese diet in dishes such as sashimi and the lightly seared fish meat called tataki. Bonitos come to Japan from areas near the equator, riding on the Kuroshio current.

In Japan, bonitos have been caught by a traditional single-hook fishing method since the Edo period (1603-1867). The bonito catch using this method off Japanese shores exceeded 100,000 tons in 1991, but dropped to 30,475 tons last year. The Fisheries Agency attributes the fall to the sharp increase of fishing boats with seines that sweep up very young fish along with mature ones. According to the agency, the number of fishing boats with such nets near the equator increased by 64 to 221 in the past 10 years. Nearly 70 percent of the new boats are from Taiwan and China, the agency said.

Skipjack tuna shoal

According to the agency, the increase in fishing boats with encircling nets is partly due to the economic strategies of Fiji and other Pacific island nations. Since the 1980s, the island nations have encouraged other countries' fishing boats to operate in their exclusive economic zones. In exchange, the countries earned foreign currency through fishing fees and registration fees. In the United States and European countries, canned bonito is increasingly popular among a health-conscious public. Therefore, fishermen from Taiwan, China and other countries have flocked to equatorial fishing grounds to catch bonitos for export.

Though Japanese fishing boats with encircling nets have also entered those areas, the government has limited the number of Japanese boats to 35 for more than 10 years to prevent the bonito resources from running out. A long-term decline in the bonito catch has dealt a severe blow to domestic fishery operators. Hideo Kamimaki, 58, who runs a single-hook fishing operation in waters off Kesennuma, Miyagi Prefecture, said: "Twenty years ago, we saw a large school of bonitos swimming as if they were a river. But now, the volume looks like only one-tenth of what they were." According to the Fisheries Agency, the number of single-hook fishing operators in Japan decreased from 105 in 1998 to 58 in 2009 year because of financial difficulties. A 71-year-old fish wholesaler in Tokyo's Tsukiji market said: "Bonitos caught by single-hook fishing are very good in quality as they are free of blemishes on the body. But we've seen falling volume of shipments lately."

At the 10th meeting of the Conference of the Parties to the Convention on Biological Diversity, or COP10, in Nagoya last month, Ajinomoto Co., an umami seasoning manufacturer, demanded that bonito resources be preserved. Ajinomoto produces umami broth made from bonito. Aiko Yamauchi, WWF Japan fisheries officer, said international management systems to check the haul of bonito, like those to monitor bluefin tuna, should be introduced before bonito is overfished.


Bluefin Tuna

Bluefin tuna 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 Characteristics and Behavior

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 1,496 pounds. Bluefin tuna are sometimes seen in schools of 4,500 members. During the afternoon they like to bask in the sun.

Bluefin tuna are predators. They feed on mackerel, herring and other fish. A school of tuna can attack as a group, traveling at 40mph. 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 and killer whales.

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 Breathing and High-Speed Swimming

Described by some people as "Porsches of the sea," bluefin tuna can accelerate faster than a Porsche 911, reach speeds of 40 mph, cruise 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.

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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.

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.

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.

Bluefin tuna are warm blooded like a mammals, an unusual characteristic for a fish. Able to roam between the Arctic and the tropics, they can maintain a body temperature of 86̊F in icy 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.

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.

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.

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Atlantic bluefin tuna
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 Species and Stocks

There are four different bluefin tuna species: the Pacific, the southern, and two distinct species on the Atlantic (See Below).

The southern blue fin is found in warm waters of the Southern Hemisphere. It swims in waters off Australia and New Zealand and elsewhere and spawns in the Timor Sea south of Java, Indonesia. Many hang out in the Australian Blight or in waters off Tasmania and migrate northward to the Timor Sea via western Australia.

Pacific Bluefin Tuna

Pacific bluefin tuna are slightly smaller than their Atlantic cousins. They are closely related to the Atlantic bluefin and for years werw considered to be the same subspecies. Many Pacific bluefin are born in the Sea of Japan and then migrate thousands of miles to east. Pacific bluefin tuna that breed and spawn in seas off Taiwan migrate northwards towards Japan or to the west coast of the United States. Tagged Pacific bluefins have been observed migrating between Japan and California and back and swimming to depths of 1,000 feet.

Bluefin tuna that were first tagged in Tokyo Bay were spotted in Hokkaido three month later and off California six months after that. Another that was monitored for years swam from Baja California to Japan and back, logging more than 18,000 nautical miles. One tagged tuna swam back and forth across the Pacific Ocean three times in a year.

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Pacific bluefin tuna
Some Pacific bluefin spawns in the waters between the Philippines and southern Japan, then migrates more than 6,000 miles to Baja. Peter Bridson, aquaculture research manager at the Monterey Bay Aquarium, told the Los Angeles Times, "The tuna come over to Mexico when they're 1 to 2 years old, and then when they're 5 to 6 years old they start migrating back over toward the other side of the Pacific to reproduce. [Source: Adam Yamaguchi and Zach Slobig, Los Angeles Times, July 21, 2011]

Though the Atlantic species is under the greatest threat, concerns also loom over the Pacific bluefin. Before the census started, the migration of the Pacific bluefin tuna had not been monitored mu But by tagging a 15-kilogram tuna, scientists found that it crossed the Pacific three times in just 600 days, according to Stanford University’s Barbara Block.

Atlantic Bluefin Tuna Stocks

There two separate Atlantic stock of bluefin tuna that are genetically distinct: one harvested mainly by European nations that roams the eastern Atlantic and breeds in the Mediterranean; and another that is harvested mainly by the United States, Canada and Japan that roams the western Atlantic and breeds in the Gulf of Mexico. About 20 percent of fished bluefins have traditionally come from the Mediterranean.

ensnared tuna
Many Atlantic bluefin, migrate about 6,000 kilometers between North America and Europe. Fish of both Atlantic species migrate regularly across the Atlantic, traversing the entire ocean in as little as a month and a half. For much of the year the two species mix before heading off to separate spawning areas. They do not interbreed. The mixing had led some to believe that there was one vast Atlantic stock rather than an eastern Atlantic stock and western Atlantic stock. It has also led some to believe the Mediterranean stocks are in better shape than they actually are.

Migration patterns of Atlantic bluefin can be quite complex. Two fish tagged within a couple of minutes of each other off the coast of Ireland were 3,000 mile apart eight month later. One was northeast of Cuba and the other was off the coast of Portugal. One bluefin tuna tagged off the Bahamas was captured 50 days later in Norway after making a journey of 6,200 miles. Around breeding season many bluefins gather in the middle of the Atlantic between Bermuda and the Azores.

Most of the Atlantic bluefins studied migrated between their breeding ground in the Gulf of Mexico and feeding grounds in the central and eastern Atlantic. One thing that researchers found is that tuna that traditionally migrated to places where fishing regulations were strict and quotas were low also migrated across the ocean to places where regulations were ignored and quotas are high. In breeding grounds in the Gulf of Mexico where bluefin fishing is not allowed bluefin are caught by fisherman going after other species of tuna.

The catch of blue fin tuna in the western Atlantic was 5,000 tons in 2001, one tenth the amount taken in the 1970s.

Studying Bluefin Tuna

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tuna caught in 1898
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. 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.

See Studying Fish

Image Sources: Wikimedia Commons, National Oceanic and Atmospheric Administration (NOAA)

Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated January 2012

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