While mercury concentrations in freshwater fish are much higher than in saltwater fish, Duke University researchers have found that saltwater fish—including tuna, mackerel and shark—are a greater health threat to humans.  

In freshwater, harmful methylmercury latches onto decayed plants and animal matter, which sunlight can more easily break down. But in seawater, methylmercury latches onto chloride (salt), which doesn’t degrade as easily, and marine life ingests it. 

Fish and shellfish have a natural tendency to store methylmercury in their organs, which makes them the leading source of mercury ingestion for humans. A potent neurotoxin, methylmercury can cause kidney problems, neurological disorders and even death, says Heileen Hsu-Kim, PhD, an assistant professor of civil and environmental engineering at Duke’s Pratt School of Engineering. Fetuses exposed to it can develop the same conditions, as well as learning disabilities.  

Ingested mercury accumulates in the human body, and Dr. Hsu-Kim says Americans have a high rate of exposure. In fact, 8% of U.S. women exhibit levels that exceed national guidelines. 

Dr. Hsu-Kim believes scientists and policymakers should focus on the effects of mercury in oceans instead of freshwater. (Currently, the Environmental Protection Agency and Food and Drug Administration make no distinction.)  

As you make your grocery list, check mercury levels in specific fish and seafood by visiting the Environmental Defense Fund’s Seafood Selector.