Mercury levels are rising in Hawaiian yellowfin tuna, often marketed as ahi, at a rate of nearly four percent a year as the oceans absorb the pollutant from the air, researchers said Monday.

Coal-fired power plants and artisanal gold mining operations produce mercury, a potent toxin that makes its way into the world's water and poses a health risk to people who eat certain fish.

"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," University of Michigan researcher Paul Drevnick said.

Scientists have long expected to see rising mercury in fish, as a consequence of increasing air pollution due to industrialization, but evidence has been hard to find.

For this study, published in the Environmental Toxicology and Chemistry journal, Drevick and colleagues took a second look at data from three studies that sampled the same yellowfin tuna population near Hawaii in 1971, 1998 and 2008.

The studies tallied mercury levels in the muscle of captured yellowfin tuna.

The team's re-analysis included yellowfins between 48 and 167 pounds (22-76 kilograms) and used a computer model that controls for the effect of fish body size.

In all, 229 fish were analyzed: including 111 from 1971 and 104 from 1998. In those years, no significant rise in mercury could be seen.

However, when researchers compared 1998's sample to 14 yellowfin tuna -- a far smaller size sample -- from 2008, they found mercury had risen at a rate of about 3.8 percent per year.

Yellowfin tuna, often marketed as ahi, is popular in sushi and is already considered a "high mercury" species by the Natural Resources Defense Council.

Drevnick said the study suggests that at the current rate, North Pacific waters will double in mercury by 2050.

"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," said Drevnick, an assistant research scientist at the University of Michigan School of Natural Resources and Environment.

"This study confirms that mercury levels in open ocean fish are responsive to mercury emissions."