Tuna is a popular source of lean protein and heart-healthy omega-3 fatty acids, but it is also one of the most common sources of mercury in the modern diet. The level of mercury in a tuna, specifically methylmercury, is not uniform across all species. Instead, it is heavily influenced by the fish's size, age, and position in the food chain. Smaller, younger tuna accumulate less mercury during their shorter lives, making them the safest choices for consumption, while larger, older species have higher concentrations of the heavy metal.
The Core Principle: Bioaccumulation and Size
Mercury enters the marine environment from both natural and man-made sources, eventually converting into methylmercury, a potent neurotoxin. This compound is then absorbed by small marine organisms and travels up the food chain. This process, known as bioaccumulation, causes mercury concentrations to increase exponentially as you move from prey to predator. As a result, the largest and longest-living fish accumulate the highest levels of mercury. This principle is key to understanding the variation in mercury levels among different types of tuna.
Tuna Species and Their Mercury Profile
- Skipjack (Canned Light Tuna): Skipjack is a smaller species of tuna and is the primary variety used for canned "chunk light" tuna. Its shorter lifespan and smaller size mean it accumulates significantly less mercury than its larger relatives. The FDA and EPA classify skipjack as a "Best Choice" for consumption, allowing for more frequent servings per week. This makes it the lowest mercury option for most consumers.
- Albacore (Canned White Tuna): Albacore is a larger species than skipjack and is sold as "solid white" or "chunk white" tuna. It has nearly three times the mercury content of canned light tuna. While still a nutritious choice, consumption should be limited, especially for vulnerable populations like pregnant women and young children. The EPA classifies albacore as a "Good Choice," but with stricter guidelines on serving size and frequency.
- Yellowfin: Mercury levels in yellowfin tuna are comparable to or slightly higher than albacore. Some canned light tuna products may contain yellowfin, but fresh or frozen yellowfin steaks, which come from larger, older fish, can have elevated mercury levels.
- Bigeye and Bluefin: These are the largest tuna species and, as top predators, have the highest concentrations of mercury by a significant margin. Bigeye tuna, often used in sushi and sashimi, can contain mercury levels comparable to shark or swordfish. For this reason, these species should be consumed only very occasionally, if at all, and avoided entirely by pregnant people and young children.
Mercury Comparison Table
| Tuna Species | Average Mercury (PPM) | Typical Format | EPA/FDA Classification | Recommended Serving (per week)* |
|---|---|---|---|---|
| Skipjack | ~0.126 | Canned Light/Chunk Light | Best Choice | 2-3 servings |
| Albacore | ~0.350 | Canned White/Solid White | Good Choice | 1 serving |
| Yellowfin | ~0.354 | Fresh/Frozen Steaks | Good Choice | 1 serving |
| Bigeye | ~0.689 | Fresh/Frozen (Sashimi) | Should be avoided | Less than 1 serving |
| Bluefin | >0.500 | Fresh/Frozen Steaks | Should be avoided | Avoid or very limited |
*Note: Recommendations are based on adult consumption and assume no other high-mercury seafood is consumed. Serving sizes are typically 4 ounces, but guidelines can vary by brand and source.
Important Considerations for Safe Consumption
Brand Variation
It is important to remember that average mercury levels are not guaranteed for every can of tuna. Consumer Reports testing has found that mercury levels can spike unpredictably in individual cans, even within the same brand and type. Some brands, like Safe Catch, address this by individually testing each fish for mercury, guaranteeing a strict limit on their products. While other brands like Bumble Bee and StarKist also offer low-mercury options, this individual testing provides an extra layer of assurance.
Canned vs. Fresh Tuna
As a general rule, canned tuna products typically have lower mercury levels than fresh or frozen steaks. This is because the tuna used for canning is usually smaller and younger, particularly in canned light varieties. Fresh steaks, especially those labeled as ahi (a common market name for yellowfin and sometimes bigeye), are more likely to come from larger, older fish with higher mercury content.
Guidelines for Sensitive Populations
For those who are pregnant, breastfeeding, or are feeding young children, the guidance is more cautious. Due to the risk of unpredictable mercury spikes in individual cans, some organizations like Consumer Reports recommend that pregnant people avoid tuna altogether. Instead, they suggest substituting other low-mercury fish such as salmon, sardines, and trout. Always consult a healthcare provider for personalized dietary advice.
Recommended Consumption Frequency
- Canned Light (Skipjack): 2-3 servings (4oz each) per week for adults.
- Canned White (Albacore): 1 serving (4oz) per week for adults.
- Fresh/Frozen Steaks (Yellowfin/Bigeye): Limit consumption to infrequent servings.
For a full list of low-mercury seafood recommendations, consult the Environmental Defense Fund's Seafood Selector. The key to enjoying the benefits of tuna while minimizing risk is to diversify your seafood choices and stick to smaller, younger species when possible.
Conclusion
When choosing tuna, the species and size are the most significant factors determining mercury content. For the lowest mercury levels, opt for canned light tuna made from skipjack. This variety offers a balanced option for regular consumption compared to the higher mercury content found in canned albacore and fresh steaks from larger species like yellowfin and bigeye. By understanding these differences and following moderation, you can continue to enjoy tuna as part of a healthy diet while effectively managing your mercury exposure.
