ABC7 News media partner the Mercury News reports fish tested in Almaden Lake in San Jose averaged nearly five times higher than the safe level to eat.
Mercury can affect the brain and nervous system.
The state water resources control board says four other bodies of water in Santa Clara County are in the top 20. Calero reservoir is 5th. Chesbro reservoir is 7th. Anderson Lake is 11th and Uvas Reservoir ranks 15th.