As pollution continues to impact the world’s oceans, methylmercury—the most toxic form of mercury—is increasingly accumulating in global fisheries. As a result, seafood consumption is the primary source of human exposure to methylmercury globally. 

Methylmercury is a poisonous compound associated with neurodevelopmental delays, impaired cardiovascular health and other serious health conditions. It forms from inorganic mercury that is primarily released from sources like fossil fuel combustion, industrial processes, and gold mining activities. Microorganisms in the water convert this inorganic mercury into methylmercury, which then accumulates in the aquatic food chain, including fish.

Given the critical role of the world’s fisheries in the global food supply and as an economic resource, keeping methylmercury out of seafood is of the utmost importance. 

Of all the seafood impacted by methylmercury, perhaps none is more prominent than one of the world’s most popular fish: tuna. In the United States alone, Americans consume around one billion pounds of canned and pouched tuna, making it the second most popular seafood product. Tuna provides many beneficial nutrients—from protein to B-vitamins and iron—but high levels of methylmercury in the fish could have far-reaching implications for the future public health of fish consumers.

A new University of Delaware study titled “Global Fishing Patterns Amplify Human Exposures to Methylmercury” was led by Mi-Ling Li, assistant professor of environmental chemistry and toxicology at UD, and published Sept. 23 in the Proceedings of the National Academy of Sciences. Co-authors include Colin P. Thackray and Elsie M. Sunderland from Harvard University, and Vicky W. Y. Lam and William W. L. Cheung from the University of British Columbia. 

Using high-resolution catch data for global fisheries and a spatial model for seafood methylmercury, the researchers examined the spatial origins and magnitudes of methylmercury extracted from the ocean. Results suggest that tropical and subtropical fisheries account for more than 70% of methylmercury fished from the ocean, largely because they are the major fishing grounds for commercially important fish like tuna and countries from all over the world want to harvest these fish. 

While these tropical and subtropical regions contain important fisheries, prior work has shown that these regions have been disproportionately impacted by anthropogenic mercury pollution—receiving half of the mercury deposited from the atmosphere. They also have a high amount of primary productivity, which enhances microbial activity in the seawater to make more bioavailable mercury for fish. 

Depending on the species, tuna can grow up to 15 feet long and weigh 1,500 pounds. As such, they serve as top predators in the food webs in which they are found. They typically have higher methylmercury concentrations and likely exceed regulatory limits for mercury concentrations established in the United States. Top predators like tuna caught in the tropical zones also have much lower levels of nutrients in terms of omega-3 fatty acids and selenium. 

Despite this, fisheries that target large species like tuna are expanding in these tropical and subtropical regions. While the total global seafood biomass harvested by wild-caught marine fisheries has declined since 1996, fresh and canned tuna have increased their commercial market presence. The global tuna catch has increased from 1950-2014 and, since 1980, many countries have developed important tuna fisheries. 

 “Our study shows that the current fishing pattern is amplifying human exposure to methylmercury,” said Li, “The fishing pattern is highly driven by the market. Humans choose to have more tuna.” 

While eating less seafood that is high in mercury might be an achievable goal for some, it is not a viable option for subsistence fishers who rely on self-caught seafood as a major protein source. The study estimates about 97% of global subsistence fishing communities likely exceed the U.S. EPA limit for human mercury exposure.

“Ultimately, we have to lower mercury emissions and that is a long-term remedy,” said Li. “The paper shows the need to reduce pollution on a global scale to keep mercury out of fisheries in the first place.”