The findings suggest that levels of the toxic metal in edible fish is likely to rise in the coming decades.
Even small amounts of mercury can damage nerves and be harmful to the heart and digestive and immune systems.
Mercury exposure in the womb can impair a baby's brain development, affecting thinking, memory and motor skills.
Scientists at the University of Michigan suspect mercury in the tissues of deep-living North Pacific fish caught near Hawaii originated in Asian industrial centres.
After being blown downwind for thousands of miles, the mercury was deposited into the ocean in rainfall.