The Science of Mercury Bioaccumulation
Methylmercury is a naturally occurring element that enters our waterways through both natural sources and industrial pollution. When in the water, it is absorbed by smaller fish and aquatic organisms. As larger predatory fish, like tuna, consume these smaller organisms, the mercury bioaccumulates in their tissue over their lifetime. This means that the larger and older the fish, the higher its potential mercury concentration. This fundamental principle is why different types of canned tuna, sourced from different species of varying sizes and lifespans, contain different levels of mercury.
Why Species and Size Matter
Understanding which species of tuna ends up in your can is the most crucial step to controlling your mercury exposure. The labels 'light' and 'white' are the most significant indicators, referring to the skipjack and albacore species, respectively.
- Canned Light Tuna: This product is most commonly made from skipjack tuna, a smaller and shorter-lived species. As a result, skipjack has a much lower mercury concentration, averaging 0.126 ppm according to FDA data. Canned light tuna is considered the best choice for individuals looking to minimize mercury intake, and the FDA recommends it for pregnant women and children.
- Canned White (Albacore) Tuna: Albacore are larger, longer-lived fish and thus accumulate more mercury. Canned albacore has an average mercury concentration of 0.350 ppm, nearly three times that of canned light tuna. While still a good source of omega-3s, its consumption should be more limited, especially for sensitive populations.
- Bigeye and Yellowfin Tuna: These species, often sold fresh or frozen for sushi (sometimes labeled 'ahi'), contain significantly higher levels of mercury than both canned light and canned albacore. Bigeye tuna, for example, can have up to 1.816 ppm and should be avoided by pregnant women and young children.
Comparison of Canned Tuna Types
| Tuna Type | Average Mercury (ppm) | Typical Species | FDA Ranking | Weekly Adult Serving Guideline |
|---|---|---|---|---|
| Canned Light Tuna | 0.126 | Skipjack | Best Choice | 2–3 servings (8–12 oz) |
| Canned White Tuna | 0.350 | Albacore | Good Choice | 1 serving (4 oz) |
| Bigeye Tuna | 0.689+ | Bigeye | Choice to Avoid | Limited or none |
The Role of Brand Transparency
While species is the main determinant, some brands go further to ensure lower mercury levels. Notably, the brand Safe Catch tests every single tuna before canning to ensure its mercury content is significantly lower than the FDA action limit. This process adds a layer of reassurance for consumers, especially those with increased health concerns. Other well-known brands like Bumble Bee, Chicken of the Sea, and StarKist offer lower mercury options in their chunk light varieties. It's worth noting that even with these brands, mercury levels can fluctuate from can to can, underscoring the benefit of brands that test each individual fish.
Other Considerations for Reducing Mercury Intake
- Diversify your seafood: Don't rely solely on tuna for your fish intake. Incorporate a variety of low-mercury fish like salmon, sardines, and anchovies to ensure a broader spectrum of nutrients.
- Packing liquid is irrelevant: Studies confirm there is no significant difference in mercury content whether tuna is packed in water, oil, or brine. The mercury is stored in the fish's tissue, not the packing fluid.
- Check the source: Some sources, like Oregon's Choice, claim locally-caught albacore from the Pacific Northwest has lower mercury due to environmental factors and catching younger fish. Supporting companies with such practices can be a beneficial choice.
Conclusion
For consumers seeking the lowest mercury canned tuna, the definitive answer is canned light tuna, which is predominantly skipjack. This smaller, younger species naturally accumulates less mercury, making it a safer and more versatile option for regular consumption. While canned albacore and other species remain nutritious, their higher mercury content dictates a more limited intake, particularly for sensitive groups like pregnant women and children. For maximum peace of mind, brands like Safe Catch, which test each fish, offer an additional layer of protection. By prioritizing canned light (skipjack) tuna and diversifying your seafood choices, you can confidently enjoy the nutritional benefits of fish while minimizing your mercury exposure. For more information on mercury levels in various seafood, consult the FDA's official data.
