Study finds unsafe mercury levels in fish from Delta watershed
06/02/2013 12:00 AM
06/03/2013 8:28 AM
The first comprehensive study of rivers and streams in California has found that sport fish in the Sacramento-San Joaquin Delta watershed have higher concentrations of mercury and PCBs than anywhere else in the state.
The survey adds to the history of high mercury levels in sport fish in the Sacramento region and dovetails with recent research that found consumption of sport fish from certain Delta region streams remains high, despite knowledge of the high mercury levels.
The sport fish survey, conducted by the State Water Resources Control Board, surveyed 16 species from 63 locations in 2011.
"While past monitoring looked at fish contaminants in lakes, rivers and streams, it was not focused on providing a statewide picture," said Jay Davis, senior environmental scientist with the San Francisco Estuary Institute.
The survey piggybacks on similar surveys done on lakes and reservoirs as well as coastal areas – all of which found mercury to be the most common contaminant in fish. The survey is meant to provide information for future action and monitoring.
In the river survey, the highest contamination was found in sport fish high in the food chain – such as smallmouth and largemouth bass, striped bass and the Sacramento pikeminnow.
The river sites that yielded highly contaminated fish included the American River at Discovery Park and the south fork of the American River at Coloma. Fish tested from the San Joaquin River pier at Point Antioch and at Louis Park in Stockton also showed high mercury levels.
The tissue in these fish had a higher level of mercury, deemed as "not safe" for frequent consumption by the state's Office of Environmental Health Hazard Assessment. The most sensitive consumers in that assessment are children and women of child-bearing age.
Sport fish in rivers and streams outside the Sacramento-San Joaquin Delta had low or moderate contamination levels, as did fish lower in the food chain – such as rainbow trout, said Davis.
The survey also measured concentrations of other contaminants such as PCBs, dieldrin and DDT. These also were found in higher concentrations in the Delta watershed.
The issue of mercury, specifically the variant methylmercury, is a public health concern because sport fishing is practiced all year in and around the Delta watershed.
The high mercury rates are a legacy from the state's mining operations in the late 19th and early 20th centuries, when mercury ore was mined from 240 mines statewide and used to amalgamate gold and silver.
The mercury that eventually ran into streams and rivers was inorganic and not easily absorbed into the tissues of fish. However, once that mercury moves downstream, bacteria convert it into methylmercury, which can be absorbed.
As it moves up the food chain, it is found in highest concentration in the tissues of large and long-lived predatory fish at the top of the food chain – such as striped bass.
Methylmercury affects the brain and nervous system. In low doses, it can affect child development by delaying walking and talking, and can result in a shortening of attention span as well as in learning disabilities. In adults it can affect fertility and blood pressure regulation and can cause vision and memory loss.
Davis believes that the high methylmercury levels in fish should not be a surprise to many anglers.
"For the Delta region there are advisories in place for almost all of the areas that have problems in this survey," Davis said.
However, that has not deterred many from eating what they catch, said Fraser Shilling, researcher with the Department of Environmental Science and Policy at the University of California, Davis.
"The assumption is that once people know about it they change how they do things," Shilling said. "But we've found, in our surveys and others, that people do not necessarily change their behavior."
That much was gleaned by Shilling during his most recent research. In a 2008 survey, Shilling asked fishermen in streams and rivers in Sacramento, Stockton and Suisun Bay about their fish-eating behavior.
That survey established that approximately 75,000 people catch and eat fish from that area at a frequency that puts them at health risk. He identified 8,500 anglers as consuming the fish at 10 times the safe dosage.
"A lot of people favored striped bass and largemouth bass," he said.
Both species are known for having high methylmercury levels.
"This behavior may have a lot to do with a strong cultural preference for fish for economic reasons," Shilling said. "Once you buy your license, the fish becomes a free source of food. And in the lower elevations – closer to the urban areas – immigrants and ethnic minorities tend to be the main people catching fish."
Shilling is now doing a statewide study on the fishing and fish-eating habits of members of California Indian tribes.
So far, he has surveyed members of the Pomo Indian tribe from the Big Valley and Robinson Rancheria near Clear Lake, where methylmercury has been found in fish.
In one instance, he said, he encountered a tribal member who catches and eats fish every day. She has full knowledge that the fish are likely contaminated.
"For her, it's not an economic factor," Shilling said. "She says 'eating fish is my culture.' "
Shilling believes safeguarding public health is more a policy issue than a public awareness issue.
"We need to take the findings from these surveys – and then set policy," Shilling said. "That has not been happening – and the state has been falling down with that step."
Call The Bee's Edward Ortiz, (916) 321-1071. Follow him on Twitter @edwardortiz.
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