Will Bruhns, spokesman for the Water Quality Control Board, said Stevens Creek fish also had among the highest levels of other contaminants found in fish.

Carp and channel creek catfish in Stevens Creek for example, were found to have high levels of PCBs—an industrial pollutant banned in 1977. Largemouth bass in Stevens Creek had levels of mercury five times the state's health standard.

Silicon Valley Water Control District spokesman Mike Di Marco said the pollutants come from a range of sources and had accumulated in the fish through decades of exposure.

Mercury is found naturally in the soil in the Stevens Creek watershed, which feeds the reservoir. It also can be found in air pollution drifting in from distant power plants—some from as far away as coal-burning plants in China—and in the wastes of old mines.

"There is evidence of mining 100 years ago in the Stevens Creek area," said Di Marco. Such mining—similar to that done in the Almaden mines to the south—occurred because of the mercury found naturally in the area's geological landscape.

The other pollutants are the residue of old industrial and farming practices. PCBs, for example, were commonly used in pesticides from the 1920s to the 1970s. These toxins were then washed into the reservoir by rainwater and then entered the food chain, including the reservoir's fish.

At the top of that food chain are the people who now catch and eat the fish.

The question now becomes what to do about it.

According to Bruhns, health risks occur through continuous, long-term exposure from eating such polluted fish.

For now, public health and environmental officials have issued a health advisory urging residents to be careful about what they eat. Women of childbearing age and children under the age of 18 are advised to not eat the fish from the reservoir or to sharply reduce their intake.

Bruhns said the health advisories are part of an ongoing investigation by the Water Quality Control Board in partnership with Santa Clara Valley Water District.

Since the pollutants come from disparate sources, he said there was not one place to point the blame. As a result, a "quick fix" cleanup is not possible.

However, some pollutants such as the PCBs will disappear over time, since they are no longer used.

In the meantime, the reservoir water is tested regularly and is still safe to drink. And lest residents forget, warning signs will be posted in English, Spanish and Vietnamese by the start of the fishing season in early 2005, reminding reservoir visitors to be careful about what they fish for—and what they catch.

For information about the health risks at Stevens Creek, and for information about the type and amount of fish to eat, visit http://www.oehha.ca.gov/fish/so_cal/bayareares.html.