Which fish are safest to eat, and which have the most mercury?

Mercury varies dramatically by species. Mercury accumulates up the food chain through a process called biomagnification. Small, short-lived fish like sardines and anchovies contain very little mercury, while large predators like swordfish, shark, king mackerel, and tilefish can have mercury levels 10-100 times higher¹. The FDA and EPA advise pregnant women and young children to avoid these four highest-mercury species entirely and to limit albacore tuna to 6 ounces per week².

The SMASH fish breakdown:

• Sardines — Very high omega-3s (roughly 1,400 mg EPA+DHA per 3 oz serving), extremely low mercury, inexpensive canned. One of the most nutrient-dense foods available.
• Mackerel (Atlantic/Boston) — Excellent omega-3 source. Avoid king mackerel, which is a different species with high mercury levels.
• Anchovies — Tiny, low on the food chain, very low mercury. Often cheapest canned option.
• Wild Salmon (Alaskan) — Sockeye, pink, and coho are all good choices. Canned wild Alaskan salmon is typically much cheaper than fresh.
• Herring — High in omega-3s, low mercury, underappreciated in the U.S.

Reading labels matters. Key things to look for:

• "Wild-caught" vs. "farm-raised" — required by law on labels in the U.S.
• "Atlantic salmon" = almost always farmed. "Pacific salmon" or "Alaskan salmon" = wild-caught.
• Country of origin (COOL labeling) — helps you identify sourcing.
• Look for MSC (Marine Stewardship Council) certification for wild-caught or ASC certification for farmed.

Monterey Bay Aquarium Seafood Watch is the most widely trusted consumer guide for sustainable and safe seafood choices. Their recommendations use a green/yellow/red system: "Best Choice," "Good Alternative," and "Avoid." The guide considers both environmental sustainability and fishing/farming practices. Their app and website are free⁴.

Budget-friendly options. Eating healthy seafood doesn't require buying expensive fresh fillets. Canned wild Alaskan salmon, sardines, and anchovies are among the cheapest protein sources per gram of omega-3. Frozen wild-caught fish is also typically much cheaper than fresh and nutritionally equivalent. Some warehouse stores carry frozen wild sockeye salmon at a fraction of the fresh price.

Mercury in commercial fish. Karimi, Fitzgerald, and Fisher (2012) analyzed mercury concentrations across 51 commercially important fish and shellfish species using a large dataset of FDA monitoring data. They found that mercury concentrations spanned roughly three orders of magnitude across species. The lowest-mercury species included shrimp (0.009 ppm mean), sardines (0.013 ppm), and anchovies (0.017 ppm). Moderate-mercury species included canned light tuna (0.118 ppm) and halibut (0.241 ppm). The highest-mercury species included shark (0.979 ppm), swordfish (0.995 ppm), and king mackerel (0.730 ppm)¹. These data reinforce that species selection is the single most important factor in managing mercury exposure from seafood.

Selenium-mercury interaction. Kaneko and Ralston (2007) investigated the relationship between selenium and mercury in ocean fish, finding that most fish contain selenium in molar excess of mercury. Selenium binds mercury and may reduce its toxicity by preventing mercury from binding to selenoenzymes in the body. This selenium-to-mercury molar ratio (the "selenium health benefit value") is positive for most commonly consumed fish, meaning the selenium they contain may offset mercury risk. The exceptions are species where mercury approaches or exceeds selenium on a molar basis, primarily shark, pilot whale, and some swordfish. This research suggests that for typical seafood consumption, the selenium content of fish provides a degree of natural protection against mercury toxicity³.

FDA and EPA guidance. The joint FDA/EPA advisory on fish consumption (updated 2024) provides specific consumption guidance by life stage. For adults, the agencies recommend 2-3 servings (8-12 ounces) of low-mercury fish per week as part of a healthy diet. For pregnant and breastfeeding women and young children, the same 2-3 servings are recommended but from the "Best Choices" list (lowest-mercury species). The advisory explicitly names four species to avoid entirely for these populations: tilefish from the Gulf of Mexico, shark, swordfish, and king mackerel. Albacore tuna is classified as a "Good Choice" but limited to one serving per week due to higher mercury than light tuna².

Sustainability and sourcing. The Monterey Bay Aquarium Seafood Watch program evaluates fisheries and aquaculture operations using science-based criteria including stock health, ecosystem impacts, and management effectiveness. As of 2024, wild Alaskan salmon fisheries (all species) receive a "Best Choice" rating, reflecting healthy stocks, low bycatch, and strong management by the Alaska Department of Fish and Game. In contrast, most Atlantic farmed salmon operations receive "Good Alternative" or "Avoid" ratings depending on the production system, with land-based recirculating systems rated more favorably than open net pens⁴. The program's database covers over 2,500 seafood items and is freely accessible, making it the most comprehensive consumer-facing tool for navigating seafood choices.

For most people, the practical takeaway is straightforward: prioritize wild-caught SMASH fish, buy canned when fresh is too expensive, check labels for origin and catch method, and consult Seafood Watch when trying a less familiar species.