Whether man-made sources of mercury are contributing to the mercury levels in open-ocean fish has been the subject of hot debate for many years.

Data from over the past 50 years shows that mercury levels in Pacific yellowfin tuna, often marketed as ahi tuna, is increasing at 3.8% per year. The results were reported earlier this month in the journal Environmental Toxicology and Chemistry.

This finding, when considered with other recent studies, suggests that mercury levels in open-ocean fish are keeping pace with current increases in human-related, or anthropogenic, inputs of mercury to the ocean.

These levels of mercury – a neurotoxin – are now approaching what the EPA considers unsafe for human consumption, underscoring the importance of accurate data.

Ocean sensitivity

Motivated by the seminal environmental book Silent Spring, environmental chemists have long found widespread mercury pollution in wastewater from industrial activities.

Surprisingly, mercury also appeared far from point sources – in “pristine” lakes of Scandinavia and northeastern North America. It took many years and careers to understand why mercury wound up in these “pristine” lakes. Once emitted from natural or man-made sources, such as coal-burning power plants, mercury can travel as a gas many times around the globe before falling with rain, snow, or dust. Once out of the air and in the water, it can then be taken up by fish.

There has been a false perception, however, that the open ocean – far removed from point sources of pollution – is too voluminous to be polluted with mercury from atmospheric fallout.

The shorthand for saying oceans can’t be significant sinks for air-borne pollutants is “dilution is the solution to pollution.” The argument is that lakes are concentrated environments because they are in direct contact with their watersheds that collect rain and snow, but the deep open ocean is an extremely dilute environment.

Read more here http://theconversation.com/the-mercury-level-in-your-tuna-is-getting-higher-37147