A tuna (also called tunny) is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family.

The fresh or frozen flesh of tuna is widely regarded as a delicacy in most areas where it is shipped, being prepared in a variety of ways for the sake of achieving specific flavors or textures. One way that fresh tuna is served is in sushi. When served as a steak, the meat of most species is known for its thickness and tough texture. In the U.K., supermarkets began flying in fresh tuna steaks in the late 1990s, which helped to increase the popularity of using fresh tuna in cooking; by 2009, celebrity chefs regularly featured fresh tuna in salads, wraps, and char-grilled dishes. Fresh bluefin tuna is considered a "cultural institution in Japan and there is still an enormous demand for this delicacy" at restaurants.

Seafood, which includes fish and shellfish, is a Protein Foods subgroup that provides beneficial fatty acids, notably the omega-3 fatty acids EPA and DHA). Eating about 8 ounces per week of a variety of seafood, the amount recommended for many adults, as part of a healthy diet, can support health. Some types of fish, such as salmon and trout are also natural sources of vitamin D, a nutrient that many people don't get enough of.

Seafood choices higher in EPA and DHA and lower in methylmercury are encouraged. Some example of seafood varieties commonly eaten in the United States that meet these goals include salmon, anchovies, sardines, Pacific oysters, and trout. Tilapia, shrimp, catfish, crab, and flounder are commonly consumed varieties that also are lower in methylmercury.

The amount of seafood recommended each week depends on your age, height, weight, and level of physical activity. People who are either pregnant or lactating should eat at least 8 and up to 12 ounces of seafood per week, from choices that are lower in methylmercury. Seafood intake during pregnancy is important because studies show positive associations with a child’s brain development. The U.S. Food and Drug Administration (FDA) and U.S. Environmental Protection Agency (EPA) provide joint advice for limiting methylmercury exposure for people who might become or who are pregnant or lactating and young children.

Fish and shellfish are an important part of a healthy diet. Fish and shellfish contain high-quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids. A well-balanced diet that includes a variety of fish and shellfish can contribute to heart health and children's proper growth and development. So, women and young children in particular should include fish or shellfish in their diets due to the many nutritional benefits.

However, nearly all fish and shellfish contain traces of mercury. For most people, the risk from mercury by eating fish and shellfish is not a health concern. Yet, some fish and shellfish contain higher levels of mercury that may harm an unborn baby or young child's developing nervous system. The risks from mercury in fish and shellfish depend on the amount of fish and shellfish eaten and the levels of mercury in the fish and shellfish. Therefore, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) are advising women who may become pregnant, pregnant women, nursing mothers, and young children to avoid some types of fish and eat fish and shellfish that are lower in mercury.

By following these 3 recommendations for selecting and eating fish or shellfish, women and young children will receive the benefits of eating fish and shellfish and be confident that they have reduced their exposure to the harmful effects of mercury.

1. Do not eat Shark, Swordfish, King Mackerel, or Tilefish because they contain high levels of mercury.

2. Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury.

• Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.
• Another commonly eaten fish, albacore ("white") tuna has more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week.

3. Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don't consume any other fish during that week.

Follow these same recommendations when feeding fish and shellfish to your young child, but serve smaller portions.

Frequently Asked Questions about Mercury in Fish and Shellfish:

1. What is mercury and methylmercury?
   Mercury occurs naturally in the environment and can also be released into the air through industrial pollution. Mercury falls from the air and can accumulate in streams and oceans and is turned into methylmercury in the water. It is this type of mercury that can be harmful to your unborn baby and young child. Fish absorb the methylmercury as they feed in these waters and so it builds up in them. It builds up more in some types of fish and shellfish than others, depending on what the fish eat, which is why the levels vary.

2. I'm a woman who could have children but I'm not pregnant - so why should I be concerned about methylmercury?
   If you regularly eat types of fish that are high in methylmercury, it can accumulate in your blood stream over time. Methylmercury is removed from the body naturally, but it may take over a year for the levels to drop significantly. Thus, it may be present in a woman even before she becomes pregnant. This is the reason why women who are trying to become pregnant should also avoid eating certain types of fish.

3. Is there methylmercury in all fish and shellfish?
   Nearly all fish and shellfish contain traces of methylmercury. However, larger fish that have lived longer have the highest levels of methylmercury because they've had more time to accumulate it. These large fish (swordfish, shark, king mackerel and tilefish) pose the greatest risk. Other types of fish and shellfish may be eaten in the amounts recommended by FDA and EPA.

4. What about fish sticks and fast food sandwiches?
   Fish sticks and "fast-food" sandwiches are commonly made from fish that are low in mercury.

5. The advice about canned tuna is in the advisory, but what's the advice about tuna steaks?
   Because tuna steak generally contains higher levels of mercury than canned light tuna, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of tuna steak per week.

6. What if I eat more than the recommended amount of fish and shellfish in a week?
   One week's consumption of fish does not change the level of methylmercury in the body much at all. If you eat a lot of fish one week, you can cut back for the next week or two. Just make sure you average the recommended amount per week.

Raw fish and shellfish should be kept in the refrigerator (40 °F/4.4 °C or less) only 1 or 2 days before cooking or freezing. After cooking, store seafood in the refrigerator 3 to 4 days. Any frozen fish or shellfish will be safe indefinitely; however, the flavor and texture will lessen after lengthy storage. For best quality, freeze (0 °F / -17.8 °C or less) cooked fish for up to 3 months. Frozen raw fish is best used within 3 to 8 months; shellfish, 3 to 12 months. Seafood should never be left out over two hours.

You can store commercially canned fish, such as tuna, for up to five years in the pantry; home canned fish, only one year.