Does Toro Have High Mercury? Separating Fact from Fishy Fiction

January 12, 2026 •

4 min read

According to a 2010 study published in Biology Letters, mercury levels are actually lower in bluefin toro, or fatty tuna, compared to its leaner counterpart, akami. This surprising finding challenges the common assumption that all bluefin tuna cuts are equally high in mercury and provides valuable context for consumers.

Quick Summary

This article explores the mercury content in toro (fatty tuna), distinguishing it from leaner bluefin cuts. It details how fat content and species influence mercury levels, offering guidance for mindful sushi consumption and informed decisions about seafood choices.

Key Points

Toro Has Lower Mercury than Lean Bluefin (Akami): Due to the high fat content, toro experiences a dilution effect, resulting in a lower mercury concentration per gram compared to the lean, protein-rich akami.
Fat Content is a Key Factor: Mercury has a higher affinity for muscle tissue proteins than fatty tissue, which is why leaner tuna cuts tend to have higher concentrations.
Bigeye Tuna Carries Higher Mercury Risk: Research indicates that bigeye tuna, a species often used for sushi, consistently has higher mercury levels than bluefin toro and yellowfin tuna.
Species and Size Matter Most: The main determinant of mercury risk is the species and size of the fish, as larger, longer-lived predatory fish accumulate more mercury over time.
Lower Mercury Tuna Alternatives Exist: Consumers can opt for lower-mercury fish for sushi, including salmon, crab, and canned light tuna, to reduce overall mercury intake.
Farmed Tuna May Contain Less Mercury: The controlled diet of farmed tuna can result in lower mercury levels compared to wild-caught specimens, offering a potentially safer alternative.
Moderation and Variety are Key: To minimize risk, enjoy high-mercury fish in moderation and incorporate a wide variety of seafood into your diet.

The Nuances of Mercury Accumulation in Tuna

For many sushi enthusiasts, the rich, melt-in-your-mouth texture of toro is the ultimate prize. However, the reputation of bluefin tuna as a high-mercury fish often causes concern. While bluefin tuna is a larger, predatory species known for accumulating higher mercury levels over its lifespan, the concentration is not uniform across all cuts. Contrary to what many might assume, mercury has a higher affinity for muscle tissue (akami) than fatty tissue (toro), leading to a lower concentration in the fatty cuts. A study using DNA barcoding confirmed that mercury levels were significantly higher in bluefin akami and bigeye tuna samples compared to bluefin toro.

The reason for this difference lies in the process of bioaccumulation. Methylmercury, the form of mercury that builds up in fish, binds to proteins in the muscle tissue. The fattier cuts, with a higher proportion of lipid content, essentially experience a dilution effect, resulting in lower mercury per gram of tissue compared to the leaner, protein-dense parts. This is a critical distinction that allows discerning diners to understand the true mercury profile of their sushi.

Comparing Mercury Levels: Toro vs. Other Tuna

Beyond just the different cuts of bluefin, it is helpful to compare mercury levels in toro to other species of tuna. Tuna species vary considerably in their mercury content, with larger, longer-lived species generally having higher levels. For instance, bigeye tuna is noted for its high mercury concentration, often exceeding levels in bluefin toro and yellowfin tuna. Yellowfin tuna, often harvested at a younger age, typically has lower mercury levels compared to the larger bigeye and bluefin varieties. Canned light tuna, usually made from skipjack, is among the lowest in mercury of all tuna varieties. This comparison highlights that not all tuna is created equal and reinforces the importance of knowing your source.

Factors Influencing Mercury Levels in Fish

Several key factors influence the mercury content of fish, and understanding these can help consumers make more informed choices:

Species and Trophic Level: The higher a fish is on the food chain, the more mercury it tends to accumulate through a process called biomagnification. Predatory fish like sharks, swordfish, and some tuna species are at the top of the food chain, leading to higher mercury levels.
Age and Size: Older and larger fish have had more time to accumulate mercury over their lifespan. A massive, older bluefin will have significantly higher levels than a smaller, younger specimen.
Diet: The diet of a fish is a primary source of its mercury intake. For example, farmed bluefin tuna that is fed a low-mercury diet can have significantly lower mercury levels than its wild counterpart, which feeds on other marine life in potentially contaminated areas.
Location: Mercury levels can also vary by the geographic location where a fish is caught, depending on local environmental factors and proximity to industrial pollutants.

The Mercury Comparison Table


Fish Type (Example)	Common Cut	Typical Trophic Level	Mercury Affinity	Key Takeaway
Bluefin Tuna	Toro (Fatty Belly)	High	Lower (dilution effect)	Lower mercury than lean bluefin muscle.
Bluefin Tuna	Akami (Lean Muscle)	High	High (protein binding)	Higher mercury than toro and most other tuna.
Bigeye Tuna	All Cuts	High	High	Consistently high mercury levels, often comparable to bluefin akami.
Yellowfin Tuna	All Cuts	Medium-High	Medium	Typically lower mercury than bluefin and bigeye due to smaller size and age.
Canned Light Tuna	All Cuts	Low	Low	Made from smaller skipjack, resulting in very low mercury levels.

How to Minimize Mercury Intake When Eating Sushi

For those who enjoy sushi but are concerned about mercury, mindful consumption is key. Here are several strategies to minimize your intake:

Choose Lower-Mercury Options: Opt for sushi made with fish known to have lower mercury levels. Good choices include salmon, crab, shrimp, and eel.
Vary Your Fish: Don't rely exclusively on high-mercury fish like tuna. Diversify your seafood choices to balance your nutrient intake and minimize heavy metal exposure.
Limit High-Mercury Tuna: When consuming bluefin or bigeye tuna, enjoy them in moderation. For bluefin, consider that the fatty toro has lower mercury than the lean akami.
Consider Sustainable and Farmed Options: When available, ask about the source of the fish. Farmed tuna, especially from operations that feed a controlled, low-mercury diet, can offer a safer alternative.
Be Aware of Portions: Pay attention to portion sizes. High-end restaurants sometimes serve larger, older tuna, which may contain more mercury.

The Final Word: Balancing Taste and Health

Ultimately, the question of whether toro has high mercury is more complex than a simple 'yes' or 'no.' While it comes from a species known for mercury accumulation, the specific cut of toro actually contains lower mercury levels than the leaner meat (akami) from the same fish due to the dilution effect of its high fat content. The highest mercury risk among common sushi tunas comes from bigeye tuna and bluefin akami. By understanding the nuances of how mercury accumulates in different fish and different cuts, consumers can confidently enjoy their sushi while making healthier, informed choices. A varied diet of fish, including lower-mercury options, remains the best approach to balancing the nutritional benefits of seafood against the potential risks of contaminants.

For additional guidance on safe fish consumption, you can consult advisories from authoritative sources like the U.S. Food and Drug Administration (FDA).

Conclusion

In summary, the assumption that all high-end bluefin tuna cuts are uniformly high in mercury is incorrect. The fat content of toro creates a dilution effect, making its mercury concentration significantly lower than the leaner akami cut from the same bluefin. However, the species itself is a key factor, with bluefin and bigeye tuna generally presenting higher mercury levels than smaller, lower-trophic fish. By understanding these distinctions and diversifying your seafood intake, you can continue to enjoy sushi safely while minimizing your exposure to mercury.

Frequently Asked Questions

No, toro is not mercury-free. All fish, including tuna, contains some level of mercury. However, due to its high fat content and the fact that mercury binds more to muscle protein, the mercury concentration in toro is significantly lower per gram compared to the leaner red meat (akami) from the same fish.

Akami is the lean, red muscle tissue of tuna, which has a higher concentration of mercury because the metal binds to its protein-rich muscle tissue. Toro, the fatty belly portion, has a lower mercury concentration due to a dilution effect from its higher fat content.

Among the tuna species commonly used for sushi, bigeye tuna is known to have some of the highest mercury levels. Lean bluefin tuna (akami) also has a higher concentration than its fattier toro cut.

Yes, you can still enjoy tuna sushi safely by consuming it in moderation and balancing your intake with lower-mercury options like salmon, crab, and shrimp. Being mindful of species and portion size can significantly minimize your risk.

Yes, toro offers several health benefits. It is a rich source of omega-3 fatty acids, which are beneficial for heart health and brain function. It also contains protein, vitamins, and minerals. The key is to balance these benefits against the potential risks of mercury intake.

According to some studies, farmed bluefin tuna can have lower mercury levels than wild bluefin. This is often attributed to a controlled diet with less mercury, especially when compared to wild tuna which feed on other marine life in potentially contaminated areas.

Regulatory bodies like the U.S. FDA provide guidelines on fish consumption, especially for sensitive populations like pregnant women and young children. They typically advise limiting consumption of certain higher-mercury fish and opting for lower-mercury varieties more often.