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 toxicity [3].
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 tuna [2].
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 pens [4]. 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.