My Meals are on Wheels

Fish of the Week – Tuna

A tuna is a saltwater finfish that belongs to the tribe Thunnini, a sub-grouping of the mackerel family (Scombridae) – which together with

Tunas (from top): albacore, Atlantic bluefin, skipjack, yellowfin, bigeye

the tunas, also includes the bonitos, mackerels, and Spanish mackerels. Thunnini comprises fifteen species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max. length: 50 cm (1.6 ft), weight: 1.8 kg (4 lb)) up to the Atlantic bluefin tuna (max. length: 4.6 m (15 ft), weight: 684 kg (1,508 lb)). The bluefin averages 2 m (6.6 ft), and is believed to live for up to 50 years.

Their circulatory and respiratory systems are unique among fish, enabling them to maintain a body temperature higher than the surrounding water. An active and agile predator, the tuna has a sleek, streamlined body, and is among the fastest-swimming pelagic fish – the yellowfin tuna, for example, is capable of speeds of up to 75 km/h (47 mph). Found in warm seas, it is extensively fished commercially and is popular as a game fish. As a result of over-fishing, stocks of some tuna species, such as the Southern bluefin tuna, have been reduced dangerously close to the point of extinction.

The tuna is a sleek and streamlined fish, adapted for speed. It has two closely spaced dorsal fins on its back; The first being “depressible” – it can be laid down, flush, in a groove that runs along its back. Seven to 10 yellow finlets run from the dorsal fins to the tail, which is lunate – curved like a crescent moon – and tapered to pointy tips. The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side. The tuna’s dorsal side is generally a metallic dark blue, while the ventral side, or underside, is silvery or whitish, for camouflage.

Thunnus are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters between about 45 degrees north and south of the equator. All tunas are able to maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, bluefin can maintain a core body temperature of 25–33 °C (77–91 °F), in water as cold as 6 °C (43 °F). However, unlike typical endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.
Tuna achieve endothermy by conserving the heat generated through normal metabolism. The rete mirabile (“wonderful net”), the intertwining of veins and arteries in the body’s periphery, allows much of the heat from venous blood to be “re-claimed” and transferred to the arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling. This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain, which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish. In all tunas, however, the heart operates at ambient temperature, as it receives cooled blood, and coronary circulation is directly from the gills.
Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red myotomal muscles derive their color from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. The oxygen-rich blood further ables energy delivery to their muscles.
For powerful swimming animals like dolphins and tuna, cavitation may be detrimental, because it limits their maximum swimming speed. Even if they have the power to swim faster, dolphins may have to restrict their speed because collapsing cavitation bubbles on their tail are too painful. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.

Tuna is an important commercial fish. The International Seafood Sustainability Foundation (ISSF) compiled a detailed scientific report on the state of global tuna stocks in 2009, which includes regular updates. According to the ISSF, the most important species for commercial and recreational tuna fisheries are yellowfin (Thunnus albacares), bigeye (T. obesus), bluefin (T. thynnus, T. orientalis, and T. macoyii), albacore (T. alalunga), and skipjack (Katsuwonus pelamis).
The report further states:
Between 1940 and the mid-1960s, the annual world catch of the five principal market species of tunas rose from about 300 thousand tons to about 1 million tons, most of it taken by hook and line. With the development of purse-seine nets, now the predominant gear, catches have risen to more than 4 million tons annually during the last few years. Of these catches, about 68 percent are from the Pacific Ocean, 22 percent from the Indian Ocean, and the remaining 10 percent from the Atlantic Ocean and the Mediterranean Sea. Skipjack makes up about 60 percent of the catch, followed by yellowfin (24 percent), bigeye (10 percent), albacore (5 percent), and bluefin the remainder. Purse-seines take about 62 percent of the world production, longline about 14 percent, pole and line about 11 percent, and a variety of other gears the remainder 3.
The Australian government alleged in 2006 that Japan had illegally overfished southern bluefin by taking 12,000 to 20,000 tonnes per year instead of the their agreed 6,000 tonnes; the value of such overfishing would be as much as USD $2 billion. Such overfishing has severely damaged bluefin stocks. According to the WWF, “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 counters that Australian and New Zealand tuna fishing companies under-report their total catches of southern bluefin tuna and ignore internationally mandated total allowable catch totals.
In recent years, opening day fish auctions at Tokyo’s Tsukiji fish market have seen record-setting prices for bluefin tuna, reflecting market demand. In each of 2010, 2011, 2012 and 2013, new record prices have been set for a single fish – the current record is 155.4 million japanese yen (US $1.76 million) for a 221 kg (490 lb) bluefin, or a unit price of JP¥ 703,167/kg (US$ 3,603/lb). In November 2011, a different record was set when a fisherman in Massachusetts caught an 881-pound tuna. It was captured inadvertently using a dragnet. Due to the laws and restrictions on tuna fishing in the United States, federal authorities impounded the fish because it was not caught with a rod and reel. Because of the tuna’s deteriorated condition as a result of the trawl net, the fish sold for just under $5,000.

Increasing quantities of high-grade tuna are reared in net pens and fed bait fish. In Australia, former fishermen raise southern bluefin tuna,

Tuna cut in half for processing at the Tsukiji fish market in Tokyo

Thunnus maccoyii, and another bluefin species. Farming its close relative, the Atlantic bluefin tuna, Thunnus thynnus, is beginning in the Mediterranean, North America and Japan. Hawaiʻi approved permits for the first U.S. offshore farming of bigeye tuna in water 1,300 feet (400 m) deep in 2009.
Japan is the biggest tuna consuming nation and is also the leader in tuna farming research. Japan first successfully farm-hatched and raised bluefin tuna in 1979. In 2002, it succeeded in completing the reproduction cycle and in 2007, completed a third generation. The farm breed is known as Kindai tuna. Kindai is the contraction of Kinki University in Japanese (Kinki daigaku). In 2009, Clean Seas, an Australian company which has been receiving assistance from Kinki University managed to breed Southern Bluefin Tuna in captivity and was awarded the second place in World’s Best Invention of 2009 by Time magazine.

Canned tuna was first produced in 1903, quickly becoming popular. Tuna is canned in edible oils, in brine, in water, and in various sauces. In the United States, 52% of canned tuna is used for sandwiches; 22% for salads; and 15% for casseroles and dried, packaged meal mixes.
In the United States, only Albacore can legally be sold in canned form as “white meat tuna”; in other countries, yellowfin is also acceptable. While in the early 1980s canned tuna in Australia was most likely Southern bluefin, as of 2003 it was usually yellowfin, skipjack, or tongol (labelled “northern bluefin”).
As tunas are often caught far from where they are processed, poor interim conservation can lead to spoilage. Tuna is typically gutted by hand, and later pre-cooked for prescribed times of 45 minutes to three hours. The fish are then cleaned and filleted, canned, and sealed, with the dark lateral blood meat often separately canned for pet food. The sealed can itself is then heated under pressure (called retort cooking) for 2 to 4 hours. This process kills any bacteria, but retains the histamine that can produce rancid flavors. The international standard sets the maximum histamine level at 200 milligrams per kilogram. An Australian study of 53 varieties of unflavored canned tuna found none to exceed the safe histamine level, although some had “off” flavors.
Australian standards once required cans of tuna to contain at least 51% tuna, but these regulations were dropped in 2003. The remaining weight is usually oil or water. In the US, the FDA regulates canned tuna.

Tuna can be a good source of omega-3 fatty acids. It can contain 300 milligrams (0.011 oz) per serving. However, the level of omega-3 oils found in canned tuna is highly variable, since some common manufacturing methods destroy much of the omega-3 oils in the fish. Tuna is also a good source of protein.

Mercury content in tuna can vary widely. For instance, testing by Rutgers University reportedly found that a can of StarKist had 10 times more mercury than another can of similarly identified tuna. This has prompted a Rutgers University scientist whose staff conducted the mercury analysis to say, “That’s one of the reasons pregnant women have to be really careful … If you happen to get a couple or three cans in the high range at a critical period when you are pregnant, it would not be good.” Among those calling for improved warnings about mercury in tuna is the American Medical Association, which adopted a policy that physicians should help make their patients more aware of the potential risks.
A study published in 2008 found that mercury distribution in the meat of farmed tuna is inversely related to the lipid content, suggesting that higher lipid concentration within edible tissues of tuna raised in captivity might, other factors remaining equal, have a diluting effect on mercury content. These findings suggest that choosing to consume a type of tuna that has a relatively higher natural fat content might help reduce the amount of mercury intake, compared to consuming tuna with a low fat content.
The industry-sponsored group Center for Consumer Freedom, which does not release the names of its contributors, claims the health risks of methylmercury in tuna might be dampened by the selenium found in tuna, although the mechanism and effect of this is still largely unknown.
Due to their high position in the food chain and the subsequent accumulation of heavy metals from their diet, mercury levels can be high in larger species such as bluefin and albacore.

There are five main tuna fishery management bodies: the Western Central Pacific Ocean Fisheries Commission, the Inter-American Tropical

Tuna steak served in a French bistro

Tuna Commission, the Indian Ocean Tuna Commission, the International Commission for the Conservation of Atlantic Tunas and the Commission for the Conservation of Southern Bluefin Tuna. The five gathered for the first time in Kobe, Japan in January 2007. Environmental organizations made submissions on risks to fisheries and species. The meeting concluded with an action plan drafted by some 60 countries or areas. Concrete steps include issuing certificates of origin to prevent illegal fishing and greater transparency in the setting of regional fishing quotas. The delegates are scheduled to meet at another joint meeting in January or February 2009 in Europe.
In 2010, Greenpeace International has added the albacore, bigeye tuna, Pacific bluefin tuna, Atlantic bluefin tuna, southern bluefin tuna and the yellowfin tuna to its seafood red list. “The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries.”
It is widely accepted that bluefin tuna have been severely overfished, with some stocks at risk of collapse. According to the International Seafood Sustainability Foundation (a global, non-profit partnership between the tuna industry, scientists, and the World Wide Fund for Nature), Indian Ocean yellowfin tuna, Pacific Ocean (eastern & western) bigeye tuna, and North Atlantic albacore tuna are all overfished. In April 2009, no stock of skipjack tuna (which makes up roughly 60 percent of all tuna fished worldwide) was considered to be overfished. However, the BBC documentary, South Pacific, which first aired in May 2009 stated that, should fishing in the Pacific continue at its current rate, populations of all tuna species could collapse within 5 years. It highlighted huge Japanese and European tuna fishing vessels, sent to the South Pacific international waters after overfishing their own fish stocks to the point of collapse.
A 2010 tuna fishery assessment report, released in January 2012 by the Secretariat of the Pacific Community (SPC), supported this finding, recommending that all tuna fishing should be reduced or limited to current levels and that limits on skipjack fishing be considered.

Tuna, light, canned in oil, drained solids
Nutritional value per 100 g (3.5 oz)
Energy 830 kJ (200 kcal)
Carbohydrates 0 g
Fat 8 g
Protein 29 g
Water 60 g
Vitamin A equiv. 23 μg (3%)
Choline 29 mg (6%)
Vitamin D 269 IU (45%)
Calcium 13 mg (1%)
Iron 1.4 mg (11%)
Magnesium 31 mg (9%)
Phosphorus 311 mg (44%)
Potassium 207 mg (4%)
Zinc 0.9 mg (9%)
Percentages are roughly approximated
using US recommendations for adults.
Source: USDA Nutrient Database