Tuna

• 1. Introduction

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  Tuna is a saltwater fish of the tribe Thunnini, and its members hardly look like relatives when measured side by side. The smallest, the bullet tuna, tops out at about 50 centimetres and 1.8 kilograms. The largest, the Atlantic bluefin, can reach 4.6 metres and 684 kilograms. Its average length is 2 metres, and it is believed to live as long as 50 years.

  In 2019, a single bluefin of 278 kilograms was sold at an opening auction in Tokyo for 333.6 million yen, roughly 3.1 million US dollars. A fish that fetches that much is more than a meal. What does its body do that other fish cannot? How did it get its name, and how many kinds are there? And what has the world's hunger for it done to the oceans it lives in?

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• 2. The Warm Swimmer

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  The yellowfin tuna can swim at up to 75 kilometres an hour, and part of the reason is heat. Tuna, opah and mackerel sharks are the only fish that keep their bodies warmer than the surrounding water. A bluefin can hold a core temperature between 25 and 33 degrees Celsius in water as cold as 6 degrees. Unlike mammals and birds, tuna do not keep that temperature within a narrow range.

  The trick is a tangle of veins and arteries in the body's periphery called the rete mirabile, which means "wonderful net". Blood leaving the muscles carries heat, and the net passes nearly all of it across to the blood heading back out. This counter-current exchange limits the chill of the surface. It keeps the skeletal muscles, eyes and brain warm, which supports faster swimming with less energy spent. It also lets tuna live in cooler waters than most fish can tolerate.

  The heart is the exception, because it runs at the temperature of the sea. It receives cooled blood, and its own supply comes directly from the gills.

  Tuna flesh is pink to dark red, unlike the white flesh of most fish. The colour comes from myoglobin, an oxygen-binding molecule that tuna carry in far greater amounts than most fish. Oxygen-rich blood helps deliver energy to those muscles.

  The shape is built for speed. Two closely spaced dorsal fins sit on the back, and the first is supported by spines and can fold flat into a groove. Seven to ten yellow finlets run back toward a crescent-shaped tail. The thin base of the tail carries three stabilizing keels on each side. The back is dark metallic blue and the underside is silvery with an iridescent shine, which hides the fish from above in deeper water.

  Even a body this refined meets a hard limit. At high speed, collapsing bubbles called cavitation form a vapor film around the fins and stop the fish from going faster. Dolphins feel those bubbles as pain, but tuna have bony fins without nerve endings. Lesions found on tuna are consistent with cavitation damage.

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• 3. Names and Kin

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  The English word tuna travelled through Spanish atún and Andalusian Arabic at-tūn before reaching English. Behind the Arabic stands the Middle Latin thunnus. That in turn comes from the Greek θύννος, a name used for the Atlantic bluefin. The Greek traces to θύνω, thýnō, meaning "to rush, dart along". An older English alternative is "tunny".

  In Japan, canned tuna can be called シーチキン, literally "sea chicken". The same idea appears in English as the nickname Chicken of the Sea.

  Biologists group the tunas into the tribe Thunnini, which holds 15 species in five genera. The family is Scombridae, the mackerels. The genera are Allothunnus, the slender tunas, and Auxis, the frigate tunas. Then come Euthynnus, the little tunas, and Katsuwonus, the skipjacks. Last is Thunnus, the albacores and true tunas. Thunnus splits into a bluefin group and a yellowfin group.

  The slender tunas are the most primitive of the lot. The nearest relatives of all the tunas are the bonitos of the tribe Sardini.

  The skipjack, Katsuwonus pelamis, reaches 1.1 metres and a maximum of 34.5 kilograms. It is one of seven tunas outside the genus Thunnus. All seven are listed as least concern by the IUCN.

  The true tunas held a surprise for scientists. Until recently, Atlantic bluefin and Pacific bluefin were thought to be subspecies of one species. In 1999, Collette showed from molecular and body-shape evidence that they are distinct species. That finding changed how the bluefin catch could be counted.

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• 4. Nets and Hooks

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  Between 1940 and the mid-1960s, the yearly world catch of the five main market tunas rose from about 300 thousand tons to about 1 million. Most of it came in on hook and line. Then purse-seine nets took over, and catches climbed to more than 4 million tons a year.

  About 68 percent of that catch comes from the Pacific Ocean. The Indian Ocean supplies 22 percent, and the Atlantic and Mediterranean together supply the remaining 10 percent. By species, skipjack makes up about 60 percent, followed by yellowfin at 24 percent, bigeye at 10 percent and albacore at 5 percent. Bluefin is the remainder. Purse-seines take about 62 percent of world production, longline about 14 percent, and pole and line about 11 percent.

  Some methods are very old. The Phoenicians used a maze of nets called Almadraba to trap Atlantic bluefin, and it is still used in Portugal, Spain, Morocco and Italy. In Sicily the same method is called Tonnara.

  Tuna are also sought for sport. Big ones put up a fierce fight, and they have injured anglers and damaged gear.

  In November 2011, a fisherman in Massachusetts hauled in an 881-pound tuna by accident in a dragnet. United States law allows tuna to be taken by rod and reel, and the federal authorities impounded the fish. The net had also damaged it, and it sold for just under 5,000 dollars.

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• 5. Dolphins and Whales

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  In the eastern Pacific Ocean, dolphins swim above schools of yellowfin tuna. Albacore do not join them. Tuna are believed to gather beneath dolphins for protection from sharks, which prey on tuna.

  Fishing vessels once exploited this habit. They searched for dolphin pods and encircled them with nets to catch the tuna below. Dolphins became entangled, and many were injured or killed. Public outcry and new regulations, now monitored by NOAA, pushed the industry toward methods built around lines rather than nets.

  The protection is not absolute. There are neither universal independent inspection programs nor verification of dolphin safety. Consumers Union has said that claims of "dolphin safe" tuna deserve little credence.

  The change had a cost. Fishermen stopped following dolphins and began working around floating objects called fish aggregation devices, or FADs, which attract many other organisms. That shift has caused greater bycatch, including sharks, turtles and other oceanic fish.

  Whales entered the tuna debate in 2005. The Pacific island nation of Nauru defended its vote at the International Whaling Commission against Australian criticism. It argued that some whale species could devastate its tuna stocks, and that its food security and economy depend heavily on fishing. Nauru does not permit whaling in its own waters.

  In 2010 and 2011, Nauru backed Australian proposals to ban tuna purse-seining near marine mammals across the western Pacific. The Western and Central Pacific Fisheries Commission agreed to that measure at its eighth meeting in March 2012.

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• 6. Japan's Appetite

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  Japan is the biggest tuna-consuming nation, and its auction halls show it. A bluefin of 202 kilograms sold at Tokyo's Tsukiji fish market in 2001 for 20.2 million yen. In 2012, a 269 kilogram fish brought 56.49 million yen. In 2013, a 221 kilogram fish sold for 155.4 million yen, about 1.76 million US dollars. New single-fish price records were set in each of 2010, 2011, 2012-2013 and 2019.

  Then the market flinched. The opening auction price for 2014 fell to less than 5 percent of the year before. The earlier price had drawn complaints that it was "way out of line".

  Japan also leads in farming research. It first hatched and raised bluefin tuna in captivity in 1979. In 2002 it completed the full reproduction cycle, and in 2007 it raised a third generation. The farm-bred fish is called Kindai tuna, a contraction of Kinki University, or Kinki daigaku in Japanese.

  Farming has spread. Australia has former fishermen raising southern bluefin tuna in net pens and feeding them bait fish. Farming of Atlantic bluefin is beginning in the Mediterranean, North America and Japan. In 2009, Hawaii approved permits for the first United States offshore farming of bigeye tuna, in water 1,300 feet deep.

  That same year, the Australian company Clean Seas, which had help from Kinki University, bred southern bluefin in captivity. Time magazine awarded it second place in the World's Best Invention of 2009.

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• 7. On the Plate

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  Canned tuna was first produced in Australia in 1903, and it quickly became popular. Australian rules once required cans to hold at least 51 percent tuna, but those rules were dropped in 2003. In the early 1980s the fish in Australian cans was most likely southern bluefin. By 2003 it was usually yellowfin, skipjack or tongol.

  In the United States, canned tuna became more plentiful in the late 1940s. In 1950, 8,500,000 pounds were produced, and the Department of Agriculture classed it a "plentiful food". American cooks use 52 percent of it for sandwiches, 22 percent for tuna salads, and 15 percent for casseroles and boxed meal kits such as General Mills's Tuna Helper.

  The cans carry colour labels. "Light" means greyish pink flesh and "white" means light pink. In the United States, only albacore can legally be sold as white meat tuna. The luxury end is ventresca, from the fatty belly of bluefin, which also appears in sushi as toro.

  Fresh tuna reached British kitchens when supermarkets began flying in steaks in the late 1990s. By 2009, celebrity chefs regularly used it in salads, wraps and char-grilled dishes. In Japan, tuna is often eaten raw as sushi or sashimi.

  Plain canned light tuna in oil is 29 percent protein, 8 percent fat and 60 percent water. A 100 gram portion provides 200 calories and is rich in vitamin D and phosphorus.

  Mercury is the shadow over the table. In March 2004, the United States FDA advised pregnant women, nursing mothers and children to limit tuna and other predatory fish. The Environmental Protection Agency suggests one 6 ounce can of light tuna a week for people under 110 pounds, and two cans for heavier people.

  In 2007, reports found that canned yellowfin held significantly more mercury than skipjack. Consumers Union and other groups then advised pregnant women to avoid canned tuna. In 2009, a California appeals court upheld a ruling that canned tuna needs no warning label, because the methylmercury is naturally occurring.

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• 8. Fragile Stocks

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  In 2006, Australia accused Japan of taking 12,000 to 20,000 tonnes of southern bluefin a year, against an agreed 6,000 tonnes. The overfishing could have been worth as much as 2 billion US dollars. The WWF warned that "Japan's huge appetite for tuna will take the most sought-after stocks to the brink of commercial extinction unless fisheries agree on more rigid quotas". Japan's Fisheries Research Agency replied that Australian and New Zealand tuna companies under-report their catches and ignore the mandated limits.

  In January 2007, the five main tuna management bodies met together for the first time, in Kobe, Japan. Delegates from some 60 countries or areas drafted an action plan. It called for certificates of origin to block illegal fishing and for more transparency in setting regional quotas.

  Greenpeace International put albacore, bigeye, Pacific bluefin, Atlantic bluefin, southern bluefin and yellowfin tuna on its seafood red list in 2010. It described them as fish commonly sold in supermarkets with a very high risk of coming from unsustainable fisheries.

  The picture varies by species. Bluefin are widely accepted as severely overfished, and some stocks risk collapse. The International Seafood Sustainability Foundation judged Indian Ocean yellowfin, bigeye in the eastern and western Pacific, and North Atlantic albacore to be overfished. In April 2009, no skipjack stock was considered overfished.

  The BBC documentary South Pacific, first aired in May 2009, warned that all tuna populations in the Pacific could collapse within five years at the current rate of fishing. A Pacific Community report released in January 2012 backed the concern. It recommended that all tuna fishing be reduced or held to current levels, and that limits on skipjack be considered.

  The sea itself is now part of the problem. In the Indian Ocean, rapid warming has reduced marine phytoplankton. Bigeye catch rates there have fallen abruptly over the past half century, mostly because of increased industrial fishing, with warming adding further stress.

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