The Risk of Methylmercury
Mercury is a naturally occurring element, but industrial activity releases it into the environment, where bacteria convert it into methylmercury. This organic compound then enters aquatic ecosystems, where it is absorbed by fish. The danger arises from a process called bioaccumulation, where methylmercury levels increase as it moves up the food chain. Larger, longer-living predatory fish consume many smaller fish, accumulating significantly higher concentrations of this neurotoxin in their tissue. For humans, prolonged exposure to high levels of methylmercury can cause adverse effects on the brain and nervous system, and poses a particular risk to the developing fetus and young children.
High-Mercury Fish: Choices to Limit or Avoid
For most people, limiting exposure is a matter of moderation. However, for some of the highest-mercury species, the FDA advises certain vulnerable populations, such as pregnant or nursing women and young children, to avoid them completely. For the general population, these are the fish you should not eat more than once a week, and ideally, much less frequently or not at all.
List of Fish to Strictly Limit or Avoid
- Shark: As a top predator, shark meat contains some of the highest mercury levels. It is on the FDA's 'Choices to Avoid' list for a reason.
- Swordfish: Another large, long-lived predator, swordfish accumulates significant amounts of mercury. It is also on the 'Choices to Avoid' list.
- King Mackerel: The larger, older king mackerel are especially high in mercury and should be avoided.
- Gulf of Mexico Tilefish: Mercury levels in tilefish from the Gulf of Mexico are consistently high and warrant being on the 'Choices to Avoid' list. Note that Atlantic tilefish have lower levels.
- Bigeye Tuna (Ahi): This species of tuna has much higher mercury content than canned light tuna and is listed as a 'Choice to Avoid' by the FDA.
- Orange Roughy: This long-living deep-sea fish is another species with very high mercury concentrations, and should be avoided.
- Marlin: Much like swordfish, marlin is a large predatory fish with high mercury levels, placing it on the list of fish to avoid.
Moderate-Mercury Fish: The Once-a-Week Candidates
For those who enjoy certain types of fish, the FDA has a 'Good Choices' category, which suggests consuming one serving (about 4 ounces) per week for most adults. These include species that contain more mercury than the 'Best Choices' but less than the high-mercury list.
Examples of Fish to Limit to One Serving Per Week
- Albacore Tuna (canned white): This tuna species contains more mercury than canned light (skipjack) tuna. Health experts recommend limiting intake to about one serving per week.
- Grouper: A popular but larger fish, grouper contains moderate mercury levels.
- Halibut: Both Atlantic and Pacific halibut are considered 'Good Choices' for once-a-week consumption.
- Mahi Mahi: This fast-growing fish can still contain moderate mercury, so it's best to limit intake.
High vs. Low Mercury Fish Comparison
| Fish Category | Examples | Average Mercury Level (PPM) | Recommended Frequency | Risk Level |
|---|---|---|---|---|
| Highest Mercury | Shark, Swordfish, King Mackerel, Gulf of Mexico Tilefish, Bigeye Tuna | > 0.46 | Avoid | High |
| Moderate Mercury | Albacore Tuna, Grouper, Halibut, Mahi Mahi | > 0.15 to 0.46 | 1 serving per week | Moderate |
| Low Mercury | Salmon, Sardines, Tilapia, Shrimp, Canned Light Tuna | ≤ 0.15 | 2-3 servings per week | Low |
Safer Alternatives: Low-Mercury Fish
To reap the nutritional benefits of fish, such as omega-3 fatty acids, without the mercury risk, focus on varieties on the FDA's 'Best Choices' list. These are typically smaller, shorter-lived fish.
Best Choices for More Frequent Consumption
- Salmon: A fatty fish rich in omega-3s and low in mercury. Wild-caught Alaskan salmon is a particularly good choice.
- Sardines: Small, oily, and with very low mercury levels, sardines are a nutrient-dense option.
- Tilapia: A mild-flavored, widely available fish with very low mercury.
- Shrimp: One of the most popular seafood options, shrimp has very low mercury content.
- Canned Light Tuna: Skipjack tuna, used for canned light tuna, has significantly less mercury than albacore and bigeye tuna, making it a better option for occasional consumption.
Best Practices for Safer Seafood Consumption
Beyond choosing the right fish, how you consume seafood can further reduce your exposure to contaminants. For fish caught from local waters, always check for local advisories, as pollution can vary by location. If no advisory exists, limit consumption to one serving per week and avoid other fish that week. Cooking method does not remove mercury, as it is bound to the fish's tissue, so smart choices are your best defense. However, for other contaminants like PCBs, trimming fat and properly preparing the fish can be helpful.
Conclusion While the nutritional benefits of eating fish are well-documented, making informed decisions about which fish to eat is crucial for minimizing mercury exposure. The rule of thumb is to limit your consumption of larger, longer-lived predatory fish like shark, swordfish, and bigeye tuna, and prioritize smaller, low-mercury options such as salmon, sardines, and shrimp. By diversifying your seafood choices and paying attention to consumption guidelines from organizations like the FDA, you can enjoy the health benefits of fish safely and responsibly. More information can be found on the FDA's official advice page.
