Grebes at Lake Berryessa show state’s highest levels of mercury contamination

Lake Berryessa - the remote reservoir in the hills of eastern Napa County, would seem to be an ideal home for grebes, a long-necked water bird known for its distinctive call and courtship dance in which pairs run side-by-side across the water.

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The birds live and breed at Berryessa by the thousands, diving below the surface to feast on an abundance of small fish in the lake that also draws fishermen from far and wide intent on landing lunker-sized bass.

But Lake Berryessa is contaminating the grebes, a species that tops the food chain in a mercury-tainted waterway where humans are warned to eat game fish sparingly.

Nearly nine out of 10 grebes from Lake Berryessa had a level of mercury in their blood high enough to put them at high risk of reproductive impairment, a newly published study has found.

None of the other 24 lakes and reservoirs surveyed in California as part of the government study had grebes with a rate of mercury poisoning anywhere close to that level.

The large percentage of Lake Berryessa grebes at high risk for harm is “cause for concern for that population,” said Josh Ackerman, a Dixon-based U.S. Geological Survey wildlife biologist and ecotoxicologist who is the lead author of the report, released last month.

The USGS study, based on field work done in 2012-13, is the first to confirm that mercury contamination in California waterways and fish - documented by previous studies - has spread to other wildlife, Ackerman said. The survey did not assess mercury risks to humans.

Grebes were chosen as the “indicator species” because they are prevalent statewide and are at the top of the aquatic food chain, which is infused with highly toxic mercury from the bottom up. Algae absorb mercury, which accumulates in the tissues of increasingly larger organisms, and, in a process called biomagnification, maxes out in top predators.

Grebes feast almost exclusively on prey fish, 2 to 4 inches long, slowly amassing in their svelte, feathered bodies a concentration of mercury that has biologists concerned.

Overall, only 6 percent of the 354 grebes sampled at 25 waterways around the state had a mercury blood level at or above 3 parts per million, the high-risk threshold. At Lake Berryessa, a popular recreation spot for humans, 88 percent of the grebes were at that level of contamination, and all the rest were at 1 part per million or more, the threshold for moderate risk of reproductive impairment, such as reduced hatching success.

Five other lakes had between 6 percent and 13 percent of grebes with 3 parts per million of mercury in their blood.

Half of the grebes at Lake Mendocino near Ukiah were at moderate risk, with none at high risk. Clear Lake in Lake County, a major grebe habitat, had 32 percent of grebes at moderate risk and 8 percent at high risk.

At Lake Hennessey, a small reservoir east of St. Helena in Napa County, 78 percent of grebes were at moderate risk and none at high risk.

Peter Kilkus, president of the Lake Berryessa Chamber of Commerce, said he's been boating on the lake for 17 years and enjoyed passing hundreds of the black and white birds with red eyes and swan-like necks.

“It's like you're in a grebe heaven,” he said. “Everybody loves them.”

On jet skis, a person can get within 15 to 20 feet of grebes, which eventually dive down and pop up 50 feet away, said Kilkus, a retiree and former mayor of San Anselmo who moved to Lake Berryessa in 2005.

Grebes are the most plentiful bird species on the 15-mile- long reservoir, which holds four times more water than Lake Sonoma and is home to bald eagles, osprey and seagulls.

Sidney Silverberg, a veteran Berryessa fishing guide, said the Western and Clark's grebes are as reliable as modern technology in locating bass and other prized game fish. Just head for flocks of hundreds of grebes on the water, signaling the presence of minnows, which they share with bigger fish.

“You don't need a fish finder,” he said.

The State Water Resources Control Board lists 74 California lakes and reservoirs as mercury-impaired waterways, including Lakes Sonoma, Mendocino, Pillsbury and Berryessa. A water board study of nearly 5,000 sport fish from 272 lakes and reservoirs in 2007-08 found that one in five waterways had at least one species - primarily bass or carp - with a mercury level high enough to merit a state recommendation that women of child-bearing age and children up to age 17 should not eat them. The four North Bay area reservoirs also made that list.

The safe eating guidelines for fish from Lake Berryessa, issued by the state Office of Environmental Health Hazard Assessment, advise women 18 to 45 and children not to eat bass, catfish or Chinook salmon and only one serving a week of trout. Men over 17 and women over 45 can eat one serving a week of bass, catfish and salmon or three servings of trout.

Unborn babies and children are more susceptible to brain damage from ingested mercury, prompting the more restrictive fish consumption standards, the state agency said. There are specific fish consumption guidelines for 71 California creeks, rivers and lakes, as well as a standard recommendation for all other waterways.

Mercury contamination of California waters is a legacy of mercury mining in the Coast Range, with a heavy concentration in Sonoma, Napa and Lake counties, and the use of mercury in Sierra foothills gold mining. But mercury is a global pollutant, and its impact on California lakes is “comparable to the average condition” found across the U.S., according to the water board's sport fish survey.

The new USGS survey is an indicator that any species eating food from Lake Berryessa, such as swallows, kingfishers and osprey, may be affected. Mink and bald eagles that eat adult grebes, as well as seagulls that consume grebe eggs and chicks, are also absorbing mercury, Ackerman said.

There were measurable amounts of mercury in all the fish tested in the survey, which also sampled prey fish, game fish and grebe eggs.

“There's mercury in most fish all over the world,” said Silverberg, a Berryessa fishing guide for 16 years. He doesn't eat any fish from the lake, and serious anglers typically release their catch to help sustain the game fish population, he said.

But he also knows “people of all ages that eat fish all the time,” Silverberg said.

He's seen as many as 1,000 grebes on the lake, and only four dead birds in the water, and he snorkels in the shallows to check on juvenile fish hiding from predators in the grass. Silverberg thinks Lake Berryessa and the grebes are in good shape.

“I would hate to see them go,” he said.