Why Does Tuna Have Mercury in It? Unpacking the Marine Food Chain

December 10, 2025 •

4 min read

According to the World Health Organization, mercury is one of the top ten chemicals of major public health concern. This highly toxic element finds its way into the ocean and eventually contaminates fish, leading to the well-known question: why does tuna have mercury in it? The answer lies in a complex environmental process involving pollution, bacterial transformation, and the marine food web.

Quick Summary

Mercury enters the ocean through natural and human activities, where bacteria convert it to toxic methylmercury. This compound then accumulates in marine organisms, becoming more concentrated in tuna due to its position as a large predatory fish through a process called biomagnification.

Key Points

Sources of Mercury: Mercury enters oceans from natural sources and, significantly, from human activities like burning coal.
Bacterial Transformation: Aquatic bacteria convert elemental mercury into the more toxic methylmercury.
Bioaccumulation and Biomagnification: Methylmercury accumulates in organisms (bioaccumulation) and concentrates up the food chain (biomagnification).
Tuna's Predatory Status: Tuna, as large, long-lived predators, accumulate higher mercury levels.
Varying Mercury Levels: Larger and older tuna species like bigeye and albacore contain more mercury than smaller skipjack tuna.
Health Concerns: Methylmercury is a neurotoxin, posing risks, especially to developing fetuses and young children.
Informed Choices: Minimize exposure by choosing low-mercury tuna (e.g., canned light skipjack) and diversifying seafood, following health guidelines.

The Journey of Mercury from Earth to Ocean

Mercury, a naturally occurring element, has seen its presence in the environment significantly increased by human activities. The entry of mercury into the atmosphere and water is a result of both natural and anthropogenic factors.

Where does the mercury come from?

Anthropogenic Sources: Human activities, primarily coal-fired power plants, artisanal gold mining, and industrial processes, are major contributors to mercury release. Waste incineration and cement production also play a role.
Natural Sources: Volcanic eruptions and geothermal vents naturally introduce elemental mercury into the environment.
Legacy Mercury: Historical industrial emissions have left mercury in deep ocean sediments, which can be remobilized.

From Elemental Mercury to Methylmercury

In aquatic environments, microorganisms convert inorganic mercury into the more toxic methylmercury (MeHg). This is the form that accumulates in marine organisms and is harmful to human health.

The Role of Bioaccumulation and Biomagnification

This is central to understanding why does tuna have mercury in it. It's a cumulative process within organisms and across the food chain.

Bioaccumulation

Bioaccumulation is the buildup of methylmercury in an organism's tissues over its life. Microscopic organisms absorb mercury from the water, and its concentration increases in their bodies because they cannot easily excrete it.

Biomagnification

Biomagnification is the increase in toxin concentration at successively higher levels of the food chain. As larger fish eat many smaller, contaminated organisms, the mercury becomes more concentrated in the predators. Tuna, being high-level predators with long lives, accumulate significant levels of mercury through this process. The mercury concentration increases with the age and size of the tuna.

Comparing Mercury Levels in Different Types of Tuna

Mercury levels vary among tuna species depending on their size, age, and diet. This is important for consumers.


Tuna Type	Typical Characteristics	Average Mercury Level (Relative)	Consumer Guideline (FDA/EPA)
Skipjack (Canned Light)	Smallest, shortest lifespan.	Lowest	Up to 3 servings per week for non-pregnant adults.
Yellowfin (Fresh)	Medium-sized, often younger.	Moderate	Moderation is key; less frequent consumption than canned light.
Albacore (Canned White)	Larger species; longer lifespan.	Higher	Limit to one serving per week due to higher average mercury.
Bigeye (Fresh)	Large, top-level predator, long-lived.	Highest	Should be avoided or eaten very rarely, especially by sensitive populations.

Minimizing Mercury Exposure from Tuna

Balancing the nutritional benefits of fish with mercury risks is recommended.

Choose lower-mercury options: Opt for canned light tuna (skipjack) over canned white (albacore) and minimize fresh bigeye.
Diversify your seafood: Include fish with lower mercury, like salmon and sardines.
Follow official guidelines: Vulnerable groups like pregnant women and children have specific recommended limits.
Be aware of serving sizes: A typical serving is 4 ounces cooked.

Health Effects of Methylmercury

While moderate tuna consumption is generally safe for most adults, high or prolonged methylmercury exposure can be harmful, particularly to the developing nervous system in fetuses and young children. Severe poisoning can lead to neurological issues like vision loss and impaired motor function. It is a persistent neurotoxin.

Conclusion: Responsible Consumption in a Polluted World

The presence of mercury in tuna is a result of environmental mercury, both natural and from pollution, being converted to methylmercury and accumulating up the marine food chain, especially in long-lived predators like tuna. Understanding this helps consumers make informed choices to enjoy fish's benefits while minimizing exposure. Choosing lower-mercury options and diversifying seafood intake are key strategies.

For more information on balancing fish consumption with safety, consult the U.S. Environmental Protection Agency's {Link: consumer guidelines https://www.epa.gov/mercury/guidelines-eating-fish-contain-mercury}.

The Scientific Reason Tuna Has High Mercury

High mercury in tuna is due to widespread pollution, bacterial conversion to methylmercury, bioaccumulation, and biomagnification as tuna are top predators.

Why Methylmercury is Especially Toxic

Methylmercury's toxicity allows it to cross the blood-brain barrier and placenta, harming the central nervous system, particularly during development.

How Your Location Affects Tuna's Mercury

Mercury levels can vary based on catch location, influenced by regional pollution, natural inputs, and oceanic conditions.

Why Older Tuna Has More Mercury

Older, larger tuna accumulate more mercury over their lifespan and from eating more contaminated prey.

Mercury in Canned Tuna vs. Fresh Tuna

Levels differ by species: canned light (skipjack) from younger fish generally has less mercury than fresh yellowfin or canned white (albacore) from larger species.

How Mercury Levels are Monitored

Food safety agencies monitor seafood mercury and provide consumer guidelines.

The Main Source of Human Exposure to Mercury

The primary exposure for humans is through eating contaminated fish and shellfish.

What are the symptoms of methylmercury poisoning?

High-level poisoning can cause neurological symptoms like vision loss and coordination issues.

Can mercury be removed from tuna after it's caught?

No, mercury is bound in muscle tissue and cannot be removed by cooking or processing.

Why mercury is particularly dangerous for pregnant women

Mercury can cross the placenta and harm fetal brain development.

How industrial pollution impacts tuna worldwide

Pollution increases global ocean mercury, affecting tuna populations.

Frequently Asked Questions

Bioaccumulation is the buildup of a substance within a single organism. Biomagnification is the increase in concentration of that substance as it moves up the food chain.

Canned light tuna, usually skipjack, typically has the lowest mercury levels compared to larger species like fresh bigeye and canned white albacore.

Yes, mercury can harm a fetus's developing brain. Pregnant and nursing women are advised to limit certain fish intake and follow specific guidelines.

No, mercury is bound in the fish's muscle tissue and cannot be removed by preparation methods.

High exposure to methylmercury can cause neurological damage, particularly in vulnerable groups, affecting vision, coordination, and speech.

Safety depends on the type. Canned light (skipjack) is generally safer for regular consumption within guidelines, while canned white (albacore) should be eaten less often due to higher mercury.

Lower-mercury options include salmon, sardines, shrimp, tilapia, and sole.

Human activities, especially burning coal, mining, and industrial waste, are major sources of mercury emissions that contribute significantly to ocean pollution.

Microorganisms in water and sediment convert inorganic mercury into methylmercury, the toxic organic form that accumulates in fish.

Tuna's position as a large, long-lived predator high in the food chain means it accumulates significantly more methylmercury from the fish it consumes through biomagnification.

Mercury levels in individual tuna increase with age and size due to bioaccumulation. Global mercury levels in oceans can also change over time influenced by pollution levels.