Groth, E. 2010. Ranking the contributions of commercial fish and shellfish varieties to mercury exposure in the United States: Implications for risk communication. Environmental Research http://dx.doi.org/10.1016/j.envres.2009.12.006. Synopsis by Jennifer F. Nyland

Experts send a mixed message to consumers when it comes to eating fish: it’s good for your heart health but beware of the methylmercury. A new way of organizing and ranking the pollutant’s levels in fish and shellfish may help consumers navigate this apparent contradiction, according to the study’s author.

The reanalysis of the government-collected data could provide consumers in the United States with an easier method to evaluate dietary exposure to mercury through the fish they eat, making it more likely they will choose the types and amounts of seafood that are safest to eat. In particular, the calculation of a “mercury input factor” weights both the amount of a particular fish sold and its mercury content.

When sorted by market share, the news was good. Of the top five selling fish and shellfish in the United States – shrimp, tuna, salmon, pollock, and catfish – only tuna was ranked at the above-average or higher mercury level.

Most human exposures to mercury are in the form of methylmercury and from eating contaminated fish. Yet, consumer information on which fish have high levels of mercury is sometimes difficult to find and decipher – it can be confusing and contradictory. For instance, the fish advisory on mercury issued by the Food and Drug Administration and the US Environmental Protection Agency in 2004 is primarily available through the Internet, which many people still cannot access.

Methylmercury can interfere with brain development and is known to affect learning and behavior at levels below those designated as acceptable by federal agencies. Although everyone can be affected, women of childbearing age and children are especially susceptible. These groups are advised to choose seafood with low mercury levels and limit amounts to other species to reduce their exposure to the metal. Posing the biggest hazard are the older, predatory fish such as tuna, swordfish and the angler-caught fish from local contaminated waters.

Based on data gathered from the FDA and the National Marine Fisheries Service, the 51 varieties of fish and shellfish that makeup the seafood market in the United States were classified by their mercury content and compared to pounds sold per year. They were then sorted into categories based on mercury and market: mercury levels, market share, consumption, mercury input factor and mercury content.

The highest mercury levels were found in tilefish from the Gulf of Mexico, followed by shark, swordfish, king mackerel and orange roughy. The lowest levels were found in salmon, shrimp, sardines and tilapia.

Consumers who eat fish with a low- or below-average mercury levels are unlikely to be exposed at levels of concern set forth by federal agencies, according the study’s author. On the other hand, it may be prudent to limit consumption of fish with above-average or higher mercury levels.