Understanding Mercury in Seafood
Mercury is a naturally occurring element that, when released into the environment, can be converted into methylmercury by bacteria. This highly toxic compound then enters the aquatic food chain and bioaccumulates in the tissues of marine life, with levels increasing as it moves up the food chain. Therefore, larger, longer-lived predatory fish are more likely to contain higher concentrations of mercury. Both tuna and swordfish are predatory marine fish, but their positions in the food chain and their typical lifespan differ, leading to varying mercury concentrations.
The Science of Biomagnification
Biomagnification is the process by which concentrations of a substance, such as mercury, increase with each successive step in the food chain. It works like this:
- Plankton absorb methylmercury from the surrounding water.
- Smaller fish consume plankton, accumulating the mercury in their bodies.
- Larger fish, like tuna, eat many smaller fish, concentrating the mercury from all their prey.
- Apex predators, such as swordfish, consume large quantities of other fish, leading to the highest mercury levels in their tissue.
This is the fundamental reason why swordfish, being a top marine predator, carries a higher mercury load than most types of tuna. Tuna species vary in size and lifespan, which also influences their mercury content.
Comparison of Mercury Levels: Tuna vs. Swordfish
While the answer to which is higher in mercury is clear—swordfish—it's important to recognize that tuna comes in different varieties with different mercury levels. The U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) have issued guidelines to help consumers make safer seafood choices based on these differences.
| Fish Type | Average Mercury Concentration (ppm) | FDA/EPA Fish Advice Category |
|---|---|---|
| Swordfish | 1.00 ppm | Avoid |
| Bigeye Tuna | 0.69 ppm | Avoid |
| Albacore Tuna (Canned) | 0.35 ppm | Good Choice (1 serving/week) |
| Yellowfin Tuna | 0.35 ppm | Good Choice (1 serving/week) |
| Light Canned Tuna | 0.13 ppm | Best Choice (2-3 servings/week) |
Health Implications of Mercury Exposure
Exposure to high levels of methylmercury, most commonly from consuming contaminated fish, can cause adverse health effects, particularly on the nervous system. While most people have small amounts of methylmercury in their bodies without adverse effects, people at higher risk should be more cautious.
- Pregnant and Breastfeeding Women: Methylmercury can affect the developing brains and nervous systems of fetuses and young children, potentially causing impacts to cognitive thinking, memory, and motor skills. Women who are pregnant, breastfeeding, or planning to become pregnant are advised to avoid high-mercury fish entirely.
- Children: Young children are also more vulnerable to mercury's neurotoxic effects. Their consumption of fish, especially varieties with higher mercury levels, should be carefully monitored according to official guidelines.
For the general population, the key is to eat a variety of fish and to choose species lower in mercury more often. The benefits of eating fish, such as omega-3 fatty acids, can be enjoyed by selecting low-mercury options like salmon, shrimp, and light canned tuna. Cooking fish reduces the mercury content by a small amount, but the most impactful strategy is selecting lower-mercury species.
Making Smarter Seafood Choices
To enjoy the health benefits of seafood while minimizing mercury risk, consider these practical steps:
- Follow FDA/EPA guidelines: These agencies provide clear, updated advice on which fish to choose, which to limit, and which to avoid based on average mercury levels.
- Vary your fish intake: Instead of eating the same high-mercury fish frequently, include a variety of lower-mercury options in your diet.
- Choose canned light tuna over albacore: Canned light tuna, which typically uses smaller skipjack tuna, has significantly lower mercury than canned or fresh albacore tuna.
- Be aware of size: For any given species, smaller and younger fish will contain less mercury than larger, older ones due to less time for bioaccumulation.
Conclusion
In the debate over which is higher in mercury, tuna or swordfish, the evidence is conclusive: swordfish consistently contains higher levels of mercury due to its position as a larger, longer-lived apex predator. While most types of tuna also contain mercury, levels are generally lower and vary widely by species and whether it is fresh or canned. By understanding the principles of biomagnification and following official consumption advice from agencies like the FDA and EPA, consumers can confidently choose lower-mercury options to reap the nutritional benefits of fish while minimizing health risks.
