Mercury levels in Hawaiian yellowfin tuna are rising at a rate of at least 3.8% per year as a result of human activities leading to increased atmospheric levels of the toxin.
Researchers have long said rising mercury levels in open-ocean fish will be seen in response to changing atmospheric conditions, but evidence has so far remained elusive.
However, scientists at the University of Michigan have now found that mercury concentrations are increasing in yellowfin tuna, and if current rates continue, levels in the North Pacific will double by 2050.
The team compiled and re-analysed three previous reports on yellowfin tune caught near Hawaii.
Published in the journal Environmental Toxicology and Chemistry, the researchers looked at data from 1971, 1998 and 2008. In all studies, muscle tissue had been tested for total mercury.
Their analysis showed that mercury concentrations in the tuna did not change between 1971 and 1998, but that concentrations were far higher in 2008 – between 1998 and 2009, mercury levels rose by 3.8% per year.
Study author Paul Drevnick said: "The take-home message is that mercury in tuna appears to be increasing in lockstep with data and model predictions for mercury concentrations in water in the North Pacific. This study confirms that mercury levels in open ocean fish are responsive to mercury emissions."
Mercury is a potent toxin that can accumulate in high concentrations in fish. In the ocean, the primary source of mercury is atmospheric deposition from human activity, such as coal-fired power plants and gold mining.
The increasing levels pose a health risk to people who eat a lot of marine fish, such as swordfish and tuna. Mercury poisoning can cause toxic effects on the nervous, digestive and immune systems, and can affect the lungs, kidneys and eyes.
"Mercury levels are increasing globally in ocean water, and our study is the first to show a consequent increase in mercury in an open-water fish," Drevick said.
"More stringent policies are needed to reduce releases of mercury into the atmosphere. If current deposition rates are maintained, North Pacific waters will double in mercury by 2050."