Understanding Mercury and Bioaccumulation
Mercury is a naturally occurring element that enters the ocean through both natural sources, like volcanic activity, and human activities, such as coal burning. In marine environments, microorganisms convert elemental mercury into a toxic organic form called methylmercury. This methylmercury is then consumed by small aquatic organisms, which are eaten by progressively larger predators in the food chain. This process is known as bioaccumulation, where mercury concentrations increase with each successive trophic level. Since tuna are large, long-lived predators, they are particularly susceptible to accumulating high levels of methylmercury in their muscle tissue.
Tuna Species with the Highest Mercury Levels
Certain species of tuna are known to contain significantly higher levels of mercury than others, primarily due to their size, age, and position in the marine food chain. The largest, oldest, and deepest-diving tuna species tend to accumulate the most.
Bigeye Tuna (Thunnus obesus)
Among the most common tuna species, bigeye tuna is consistently found to have the highest mercury concentration. Often sold as fresh or frozen steaks and commonly used for sushi (known as ahi), bigeye is a large, predatory fish. Research shows that it tends to accumulate more mercury than other species partly because it dives deeper into zones of higher methylmercury production and consumes larger prey.
Albacore Tuna (Thunnus alalunga)
Often referred to as "white" tuna, albacore has significantly higher mercury levels than canned light tuna. While generally lower than bigeye, studies confirm that albacore, especially larger and older specimens, can have mercury concentrations that are concerning for frequent consumption. It is crucial for consumers to distinguish between canned albacore and canned light tuna due to this difference.
Tuna Species with Lower Mercury Levels
For those looking to reduce their mercury intake while still enjoying tuna, focusing on smaller, younger species is the best strategy.
Skipjack Tuna (Katsuwonus pelamis)
This smaller, shorter-lived tuna is the species most commonly used for canned "light" tuna. Its rapid growth and lower position on the food chain mean it has the lowest mercury levels among popular tuna types. The FDA considers canned light tuna a "low in mercury" choice, suitable for more frequent consumption for most adults.
Yellowfin Tuna (Thunnus albacares)
Found in both fresh and canned forms, yellowfin tuna generally falls between albacore and skipjack in terms of mercury content. While some fresh yellowfin can approach albacore levels, especially in larger individuals, it is still a lower-mercury option than bigeye. Canned yellowfin is often included in the broader category of "light" tuna.
Comparison of Mercury Levels in Tuna
To make an informed decision, here is a quick overview of typical mercury levels in different tuna products based on FDA data.
| Tuna Species | Common Form(s) | Average Mercury Concentration (ppm) |
|---|---|---|
| Bigeye | Fresh/Frozen (for sushi) | 0.689 |
| Albacore | Fresh/Frozen, Canned (White) | ~0.350 - 0.358 |
| Yellowfin | Fresh/Frozen, Canned (Light) | ~0.175 - 0.354 |
| Skipjack | Canned (Light) | 0.126 - 0.144 |
Factors That Influence Mercury Content
Several biological and environmental factors influence the amount of mercury found in a specific tuna fish.
- Trophic Level: Predatory fish like tuna that feed on other fish high up the food chain will have higher mercury levels. Larger species like bigeye are at a higher trophic level than smaller skipjack.
- Age and Size: Because mercury accumulates over a fish's lifetime, older, larger tuna have had more time to build up methylmercury in their tissue. This is why canned light tuna, typically from younger skipjack, is a safer bet.
- Catch Location: The concentration of mercury can vary by region. Some ocean areas have higher levels of environmental mercury, which can affect the fish caught there.
Safer Seafood Alternatives
While low-mercury tuna is a safe option, diversifying your seafood intake can further reduce mercury exposure. Here is a list of excellent, low-mercury seafood alternatives:
- Salmon: Rich in Omega-3s and low in mercury.
- Sardines: A small, oily fish with very low mercury levels.
- Shrimp: A popular and very low-mercury shellfish.
- Tilapia: A versatile fish with minimal mercury.
- Catfish: Another freshwater option with low mercury content.
Official Guidance on Fish Consumption
Organizations like the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) provide advice on consuming fish, particularly for vulnerable populations such as pregnant or breastfeeding women and young children. Their guidance categorizes fish into "best choices," "good choices," and "choices to avoid" based on mercury content. Canned light tuna falls under "best choices," while albacore is typically a "good choice" with a recommended consumption limit. Bigeye tuna is often listed as a "choice to avoid" or consume very rarely. You can find detailed consumption advice on the FDA website.
Conclusion
The question of which tuna is highest in mercury can be clearly answered by distinguishing between the species. Bigeye tuna has the highest mercury levels, followed by Albacore, while Skipjack tuna contains the least. Consumers should be aware that "canned light" tuna is predominantly the low-mercury skipjack, and "canned white" or albacore tuna has a higher concentration. By understanding the differences between species and adhering to consumption advisories, especially for sensitive groups, individuals can safely enjoy the nutritional benefits of tuna while minimizing mercury exposure.
