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Fish caught at a popular fishing spot in the Laguna de Santa Rosa between Sebastopol and Santa Rosa had unacceptably high levels of mercury, well above the threshold where health officials normally recommend against eating them, according to a new state survey.

The State Water Resources Control Board surveyed fish caught in 63 well-known river fishing holes statewide during 2011 in an effort to get a better sense of how safe California's game fish species are for human consumption. The findings, released last week, largely confirmed what officials suspected: that except for a handful of spots, mercury levels are moderate to low, suggesting most fish are safe to eat.

The one unexpected finding, however, came in the 22-mile Laguna de Santa Rosa, the remnant of the sprawling wetlands that used to cover much of the west county. Bass caught at the fishing spot near the bridge along Occidental Road showed an average mercury level of .53 parts per million, well above the .44 ppm threshold at which the state recommends avoiding any consumption. Carp averaged .35 ppm, a level at which the state usually recommends limiting consumption to one serving per week.

"Everybody is caught a little by surprise," said David Bannister, executive director of the Laguna de Santa Rosa Foundation, a group dedicated to studying, preserving and promoting the waterway.

State and local officials have known for years that there is mercury in the water, he said, but it appeared to be from naturally occurring sources and was not high on the priority list for restoring the slow-moving, meandering waterway that drains into the Russian River. Mercury contamination certainly fell well behind other Laguna issues such as agricultural and urban runoff and pesticides, which are currently under study and discussion among regulators and area landowners.

But, Bannister said, "it appears that with these findings, there should be some signs warning people about consumption."

Regional water quality officials say the new report will boost the priority of dealing with the mercury, but there has been very little study of the element in the waterway so far.

"This is frankly so new to us that we don't know what it means in this case," said David Leland, acting assistant executive officer of the North Coast Regional Water Quality Control Board.

State health officials, meanwhile, say the survey is too preliminary to issue any kind of warning. Researchers tested only a handful of fish from each fishing hole, an average of about nine per site. While the findings at the Occidental Road site are noteworthy, they would need to be verified by more specific and detailed science, said Sam Delson, spokesman for the state Office of Environmental Health Hazard Assessment.

The survey, released May 23, was not intended to make specific recommendations about fish consumption, water control board spokesman Tim Moran said, but rather to begin to establish a base of data to measure changes of contamination over time. There are some historical data on mercury and other contaminants in fish, but the reliability and survey methods vary widely, making it hard to draw conclusions.

The water control board said the Laguna was the only spot in the new survey that showed such high mercury levels in fish but does not have an official advisory against eating the catch.

A previous survey of reservoirs and lakes showed that largemouth bass in both Lake Mendocino and Lake Sonoma have unacceptable levels of mercury, and the state has issued advisories against eating bass caught there.

There are posted warning signs at the reservoirs, and fliers are handed out explaining the safe guidelines for bass and other species of fish that live there, said J.D. Hardesty, spokesman for the Army Corps of Engineers, which manages both fisheries.

There are similar state do-not-eat advisories for bass in Napa County's Lake Berryessa and Marin County's Soulejoule reservoir.

Mercury is a toxic element that has been associated with nervous system damage, according to the U.S. Centers for Disease Control. It is particularly toxic for developing fetuses and young children, so state advisories urge extra caution by children and women of child-bearing age.

Mercury tends to accumulate in the flesh of creatures that consume it. Large predator fish, such as bass and carp, are particularly notorious for accumulations of mercury since they eat the smaller creatures in a contaminated food chain.

Mercury occurs naturally in Northern California soils, and there were once working mercury mines throughout the area. The element also is washed down from the Sierra foothills as a byproduct of mining in the Gold Rush era, eventually reaching San Francisco Bay, and may be precipitating out of clouds of pollution from power plants in Asia, according to state officials.

More than half of the sites surveyed by the water control board showed low levels of mercury. Only about 13 percent showed concentrations above the do-not-eat threshold. Laguna de Santa Rosa was the only waterway in the survey outside the Sacramento-San Joaquin Delta system that showed such high contamination, according to the report.

The state's advisories on safe fish consumption, including do-not-eat recommendations, are not binding, Delson said. The choice of whether to post public warning signs is up to local health officials.

Dr. Karen Holbrook, deputy health officer for the Sonoma County Department of Health Services, said her office is concerned about the report but wants to have more detailed information before taking any specific action. She recommended that people who are worried simply limit themselves to no more than one of these types of fish per week until authorities learn more.

"We don't want to scare people," she said. "We want people to appreciate and continue to eat fish . . . but we do know that there are toxic chemicals in the water."

Currently, there are no warning signs of any sort at the Occidental Road fishing spot -- just an open gate and footpath to the Laguna.

Visits by The Press Democrat this week turned up no fishermen near the bridge, but there was plenty of evidence of human activity: fire pits, beer bottles and assorted trash, including bait cups and scraps of fishing line along the bank.

"I see cars down there every day all summer long, and they're fishing," said Bannister, whose office is within sight of the fishing-spot turnoff.

Michael Kyes, mayor of nearby Sebastopol, said the findings are alarming enough that the county should post warnings immediately, even without more definitive guidance from the state.

Kyes said that the green, sluggish water in the Laguna is the recipient of a soup of runoff from cattle and horse farms, vineyards, private yards, and on occasion the city of Santa Rosa's regional sewage treatment plant.

"I wouldn't eat the fish in there even without the mercury," he said.

You can reach Staff Writer Sean Scully at 521-5313 or sean.scully@pressdemocrat.com.