Tuna are marine fish of the genus Thunnus, in the family Scombridae, though some other fish belonging to the same family are also commonly called tuna. Of the true tuna, belonging to the genus Thunnus, there are nine different species. Tuna live in oceans all over the world, and all kinds are edible.

Tuna meat differs from that of many other fish because it is pink or red rather than white, and tuna is one of the most widely consumed types of fish. However, some varieties of tuna are endangered and should not be eaten by the environmentally cautious. In addition, the high levels of mercury in some kinds of tuna is a health concern, particularly for children and pregnant or nursing women.

The different species of tuna are:

• Albacore
• Bigeye tuna
• Blackfin tuna
• Karasick tuna
• Longtail tuna
• Northern Bluefin tuna
• Pacific Bluefin tuna
• Southern Bluefin tuna
• Yellowtail tuna

In addition, Skipjack tuna, or Katsuwonus pelamis, is commonly marketed as tuna, and most canned light tuna is of this variety.

Tuna has been a popular fish for human consumption for centuries and has significantly suffered from overfishing as a result. Tuna farming has recently become more popular as a way to harvest tuna meat more safely. The different varieties of Bluefin tuna are most widely used in these farming operations.

However, Monterrey Bay Aquarium's Seafood Watch advises against eating any kind of Bluefin tuna at this time because the species are severely endangered. Bluefin tuna is the most popular type of tuna used in sushi and considered a delicacy, and it has suffered extensive overfishing as a result. In addition, the great majority of Bluefin tuna is still wild-caught using methods that endanger other marine life, such as dolphins and sea turtles.

Other types of tuna are environmentally sound seafood choices, but make sure they are caught through trolling, handline, or pole fishing. Longline fishing endangers other fauna and should not be supported as an industry. However, if longline fished tuna is your only option, be aware that longline fishing in the United States is heavily regulated and produces much less bycatch.

Because tuna are predatory fish, high up on the food chain, they accumulate large amounts of mercury from the smaller fish they eat. Therefore, people should limit their intake of tuna in order to avoid health problems associated with the consumption of mercury. Skipjack tuna is a safer choice in this respect.

Five tuna fishing management commissions from around the world met in Kobe, Japan in January 2007 to develop guidelines for safely farming Bluefin tuna with a view towards conservation. Their stricter safeguards against illegal farming and overfishing were adopted by approximately 60 countries. A follow-up meeting is planned for early 2009.