Scientists looking for fish tainted by mercury found them in every fish and every river they sampled across the West, suggesting that industrial pollution generated around the world is likely responsible for at least some of it.
The survey of 2,707 fish randomly collected from 626 rivers in 12 states represents the biggest regional sampling yet of mercury in fish in the West, said Spencer Peterson, senior research ecologist EPA's National Health and Environmental Effects Research Laboratory in Corvallis.
The findings by scientists from the EPA and Oregon State University were reported in this month's issue of the journal Environmental Science & Technology and came out of an EPA survey of various environmental factors in rivers conducted between 2000 and 2004.
Though the survey found some fish with elevated mercury levels, suggesting a local source such as an old mercury mine, most levels were low, in line with canned tuna found in grocery stores, said Alan Herlihy, associate research professor in the OSU Department of Fisheries and Wildlife.
No attempt was made to specifically link the mercury in the fish to mercury in the atmosphere, but the low but widespread levels suggest the mercury came from deposition — mercury in the atmosphere falling to the earth in rain and snow, Herlihy added.
While generally below levels considered unsafe for people to eat from time to time, the mercury could pose a danger to fish and wildlife that depend on fish for their diet, said Robert Hughes, a fisheries and wildlife research associate professor at OSU who took part in the study.
Levels were generally higher in fish-eating fish, such as bass, walleye and pike, than in insect-eating fish, such as trout.
"What's important to note is that the levels are below what we consider a health concern in most fish," said EPA spokeswoman Suzanne Ackerman.
Elevated mercury levels have been linked to learning disabilities and developmental delays in children and to heart, nervous system and kidney damage in adults. Out of concern for the health effects, several states and the federal government have taken steps to cap mercury emissions.
It has long been understood that industrial emissions, such as coal-fired power plants, are responsible for some percentage of the mercury found in fish, said Steve Lindberg, a retired research fellow from the Oakridge National Laboratory, who did not take part in the study. The question is how much.
About two thirds of the mercury circulating in the atmosphere is generally considered to come from industrial or human sources, with the rest from things like volcanoes and other geologic sources, Lindberg said.
Lindberg said he is involved in research that has been tracking specific isotopes of mercury introduced into a lake watershed in Canada as they show up in fish, and will go on to see how mercury levels in the fish react when the mercury is cut off.
"It does suggest that concentrations in fish from mercury from the atmosphere are highly responsive to the amount of mercury being deposited from the atmosphere," he said. "We would argue in support of the notion of reducing industrial emissions to reduce the concentrations of mercury in fish."