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Last Edited: 5/12/2009

Lake Nacimiento

Mercury in Fish at Lake Nacimiento

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In March 2009, the Office of Environmental Health Hazard Assessment (part of the California Environmental Protection Agency) released recommendations for the safe consumption of fish from Lake Nacimiento. Click here for the complete report "2009 Update of California Sport Fish Advisories"


Mercury levels in Lake Nacimiento water are not a hazard. Mercury is found in fish because it enters the food chain through organisms in the sediment.

Safe Eating Guidelines for Fish from Lake Nacimiento

The Office of Environmental Health Hazard Assessment recommends that children and women who are pregnant or may become pregnant avoid eating carp, crappie, catfish, or any type of bass from Lake Nacimiento; however, they can eat one meal per week of sucker or sunfish such as bluegill from the lake. Men and women beyond childbearing age can safely eat two meals per week of sucker or bluegill such as sunfish, but only one meal per week of carp, catfish, or any type of bass from Lake Nacimiento. 

Why These Guidelines Are Recommended

The recommendations from the Lake Nacimiento fish advisory were developed on the basis of the results of an investigation conducted by the Environmental Health Investigations Branch of the California Department of Public Health (CDPH). In March 2006, CDPH worked with the California Department of Fish and Game to collect and analyze more than 150 fish from three locations in Lake Nacimiento. 

Fish all over Lake Nacimiento were found to have high levels of mercury. Predator fish, such as bass, have higher mercury levels. Mercury is a metal that can be harmful to the human nervous system when it is present in a form called “methylmercury.” Methylmercury can affect human development. (See the next section to read more about how mercury gets into fish).

Click here for the report on the collection and testing of fish from Lake Nacimiento [2/6/07]

The Environmental Health Investigations Branch analyzed fish at the lake as part of a broader investigation of possible exposure to mercury from the Klau and Buena Vista Mines. Learn more about the public health activities at the Klau and Buena Vista Mines.

How Mercury Gets Into Fish

When mercury is in the sediment of a lake, microbes transform it into methylmercury in their bodies. When invertebrates eat the microbes, they consume the sum of the mercury found in each of those organisms. Bottom feeding fish eat the invertebrates with the same effect up to the predator fish at the top of the food chain. Therefore, predator fish, such as white bass, have the highest levels of mercury.

If there is mercury in the fish, why is the water okay?

Because mercury accumulates in the sediment, its concentration in the water column remains well below levels known to have health effects on people who consume the water daily. Lake Nacimiento water has been monitored for mercury content since 1993, and all data show that the lake meets state and federal drinking water standards for mercury.

What about fish from other places?

Some fish come from contaminated areas, where chemicals have been found in the fish. When that happens, advisories are put in place to inform people how much fish is safe to eat. However, this does not mean all fish from all places should be avoided.

Fish should be part of a healthy diet, as it is an excellent source of nutrients. Women of childbearing age and children can safely eat up to two meals per week of most fish purchased in a store or restaurant. Some seafood from stores or restaurants that generally contain the lowest levels of mercury are shrimp, king crab, scallops, farmed catfish, wild ocean salmon, oysters, tilapia, flounder, and sole.

Shark and swordfish contain the highest amount of mercury and should not be eaten by children or women of childbearing age.



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