The tuna we’re eating is becoming more mercury-laden every year, and it’s most likely our own fault.

That’s according to new research from the University of Michigan, which found that mercury levels in yellowfin tuna caught in the Pacific Ocean have increased nearly 4 percent every year since 1998.

For seafood lovers, warnings of high mercury concentrations in fish are not new. But the study, published Monday in the journal Environmental Toxicology, shows that levels of the toxic metal aren’t declining—they’re getting worse.

“Our analysis suggests that mercury in fish is increasing at the same rate as the increase in mercury loading to the North Pacific Ocean,” said Paul Drevnick, an assistant research scientist at the University of Michigan and a coauthor of the study.

The researchers looked at yellowfin tuna caught in the waters off Hawaii in three years: 1971, 1998, and 2008. Drevnick said they made sure to compare fish based on their size, because they acquire more mercury from their environment as they age and grow.

The researchers found that the mercury levels in tuna were similar in 1971 and 1998 but increased between 1998 and 2008.

Should you change your seafood diet in response? Possibly, according to the United States Environmental Protection Agency’s food consumption guidelines, which advise avoiding fish with mercury concentrations above .3 parts per million.

Drevnick said that in 1971 and 1998, yellowfin tuna had mercury concentrations below the EPA guideline. But the 2008 sampling came in at .336 ppm—above the EPA limit.

Yellowfin, also called ahi, is popular as sushi and often sold as steaks. Small amounts of the fish are also mixed with skipjack in canned light tuna.

What’s caused the spike in mercury levels? Blaming our reliance on fossil fuels doesn’t add up, because we were spewing massive amounts of emissions long before the 1970s.

In a separate study published last summer, researchers at Woods Hole Oceanographic Institution found that since the industrial revolution, mercury levels in the upper levels of the ocean have risen threefold—a jump that isn’t reflected in yellowfin until very recently.

For a possible answer, Drevnick says to look east.

“Perhaps we are seeing the increase now because of the rapid industrialization of East Asia, where the emissions in the air are sent to the Pacific by prevailing winds,” he said.

But the links among mercury in the atmosphere, in the ocean, and in marine animals aren’t yet fully understood.

In the Woods Hole study, marine chemist Carl Lamborg said the mercury level increases expected over the next 50 years could add the same amount to the ocean that scientists observed over the past 150 years.

“The trouble is, we don’t know what it all means for fish and marine mammals,” Lamborg said in a statement. “It likely means some fish also contain at least three times more mercury than 150 years ago, but it could be more. The key is now we have some solid numbers on which to base continued work.”