What You Should Know About Tuna

Tuna Trivia
Scientific name: Thunnini

Fresh Tuna Steak

Known for its distinct flavor, global availability and versatility in the kitchen, tuna has risen to be one of the most consumed fish on the market today. However, there are alarming pitfalls to eating tuna, and if you're not careful, you can fall victim to its damaging effects. Read on to learn important facts about this seafood, including why it is not advisable for human consumption, the fraud behinds its commercialization and the dangers it poses to your health.

What Is Tuna?

Tuna is a marine fish that belongs to the mackerel (Scombridae) family, similar to albacore and bonito,1 but specifically falls under the subgroup Thunnini.2 There are several species of tuna, all of which are nomadic, meaning they do not remain in one place their entire life, and instead change their location frequently.3 Most tuna species today are found in the Atlantic Ocean, Mediterranean Sea and Black Sea. They thrive in subtropical and temperate waters.4

Tuna fish can grow to very large sizes. The Atlantic blue fin tuna, for example, can grow to a whopping 2 meters (6.5 feet) long, weighing 250 kilograms (550 pounds).5 However, there are larger recorded specimens. In fact, the largest ever recorded tuna was an Atlantic Bluefin. It was caught off Nova Scotia and weighed 1,496 pounds.6

Despite their large size, tunas are very agile and can swim very fast, covering long distances at high speeds. They can swim at speeds up to 43 miles per hour.7 Tunas are carnivores, and usually feed on squids, crustaceans and eels.8

Different Types of Tuna

There are at least 15 different species of tuna fish,9 each with their distinct characteristics. However, there are only five species most consumers are familiar with:10

  • Skipjack tuna (Katsuwonas pelamis) — Of all the commercial tuna species, this is the smallest and most abundant. Skipjacks are known for their streamlined bodies that do not have scales, with their backs having a dark purple-blue coloring. Their lower sides and bellies are both silver, and if you look closer, you'll notice four to six dark bands.

    Skipjack tuna mainly reside in the Indian, Atlantic and Pacific Oceans, with the majority residing near the equator. They can live for eight to 10 years. Skipjack are also the main species used for canned tuna.11

  • Yellowfin tuna (Thunnus albacares) — You can distinguish yellowfin by their yellow sides, silver belly and torpedo-like shape. Their backs have a dark metallic blue color, while their finlets are bright yellow. They also have very long anal and dorsal fins. This species can live up to six or seven years, and they migrate all the time. Yellowfins reside throughout the Atlantic, Pacific and Indian Oceans.12
  • Bluefin tuna (Thunnus maccoyii, Thunnus orientalis and Thunnus thynnus) — Not only are bluefins the largest species, but they're also the most long-lived, with some living up to 40 years. Their build can be likened to torpedoes, giving them the advantage of speed. Their eyes are set flush to their body and their fins are retractable.

    There are three species of bluefins: Southern (Thunnus maccoyii), Atlantic (Thunnus thynnus) and Pacific (Thunnus orientalis). Of these three, the Atlantic is the most endangered. In addition, Bluefin tunas are impressive predators, owing to their large size, and they start young. They often beginning hunting the moment they hatch, preying on smaller fish like mackerel and herrings, and even eels.13

  • Bigeye tuna (Thunnus obesus) — This species is also long-lived, reaching up to 15 years old. Bigeyes reside in the tropical and subtropical regions of the Indian, Pacific and Atlantic Oceans (but not the Mediterranean Sea). Their upper sides and back have a dark metallic blue color, but their lower sides and belly are white.14
  • Albacore tuna (Thunnus alalunga) — Albacore can be identified by their very long pectoral fins and bullet-shaped body. They're smaller than other species. They have a dark blue back and lighter blue-gray sides and belly.

    Albacores tend to keep within their species, and unlike other tunas do not blend in with other species. They can be found migrating through all ocean waters, even the Mediterranean.

Because tuna is one of the most widely harvested fish species today — with France, Japan, the U.S., Spain and Taiwan as the top harvesters — there's growing concerns about the longevity of this food.15

A major concern surrounding tuna is its mercury contamination.16 Like many large fish species, tuna accumulates mercury, a heavy metal that is toxic to neurons in the brain.17

The Dangers of High Mercury Levels in Tuna

Tuna actually contains a wide array of vitamins, minerals and nutrients, including proteins, selenium, potassium and iron, to name a few.18 But unfortunately, its high mercury levels far outweigh its potential health benefits, and is the primary reason why tuna is not recommended for human consumption.

According to the EPA, high levels of mercury exposure "can harm the brain, heart, kidneys, lungs, and immune system of people of all ages." However, it's particularly damaging to unborn babies and young children, as it may interfere with their developing nervous system, causing them to have poor cognitive function.19 For this reason, the FDA advises pregnant women to moderate their consumption of certain types of seafood, including tuna, to avoid passing on mercury's damaging effects on their unborn child.20

According to a research published in 2010, which looked at the contributions to total mercury in U.S. seafood from 51 different shellfish and fish varieties, tuna was found to be responsible for over one-third of Americans' total exposure to methylmercury.21

There's no telling how much tuna is actually safe to eat — remember that mercury is extremely toxic, so ideally you should not be exposed to any. What's more, methylmercury harms people's nervous systems to differing degrees, depending on how much mercury they have accumulated.

If you think that you cannot get mercury poisoning from tuna served in fancy restaurants, think again. Studies show that the contamination is worse in these establishments. Toxicology testing found that levels of mercury in tuna served in restaurants are actually higher compared to store-bought varieties.

This is because restaurants tend to choose certain tuna species, such as bigeye and bluefin akami, which contain higher mercury levels than other species like yellowfin and bluefin toro. Bigeye and bluefin akami are leaner, but because mercury accumulates to a greater degree in muscle than fat, this means that these highly prized varieties contain more mercury.22

Another alarming fact about tuna is that it is often mislabeled. According to the conservation group Oceana, almost 60% of the fish labeled "tuna" in the U.S. is not tuna but is instead cheaper common fish varieties. Sushi restaurants are particularly large offenders, as 84% of the "white tuna" they sell is actually escolar, a fish that's been linked to acute and serious digestive problems, even if consumed in small amounts.23

Tuna Nutrition Facts

In a perfect world, people would be able to get all the important nutrients below from tuna and other seafood. Unfortunately, the risk of mercury exposure is much too great to recommend this food for consumption. This is quite saddening, because tuna, if clean and untainted with heavy metals, can provide you with a wide array of nutrients.24

Tuna Nutrition Facts

Serving Size: 100 grams, Tuna, fresh, raw
  Amt. Per
Calories 109  
Total Fat 0.49 g  
Saturated Fat 0.116 g  
Trans Fat    
Cholesterol 39 mg  
Sodium 45 mg  
Total Carbohydrates 0 g  
Dietary Fiber 0 g  
Sugar 0 g  
Protein 24.4 g  
Vitamin A 18 µg Vitamin C 0 mg
Calcium 4 mg Iron 0.77 mg

Instead of tuna, opt for safer seafood varieties like wild-caught Alaskan salmon, which come from waters that are not tainted with mercury. Smaller fish like sardines and anchovies also pose a low mercury risk because their small bodies do not accumulate mercury.