San Francisco, New York, Washington and other big cities are using bluegills — also known as sunfish or bream — as a sort of canary in a coal mine to safeguard their drinking water.

Small numbers of the fish are kept in tanks constantly replenished with water from the municipal supply, and sensors in each tank work around the clock to register changes in the breathing, heartbeat and swimming patterns of the bluegills that occur in the presence of toxins.

"Nature's given us pretty much the most powerful and reliable early warning center out there," said Bill Lawler, co-founder of Intelligent Automation Corporation, a Southern California company that makes and sells the bluegill monitoring system. "There's no known manmade sensor that can do the same job as the bluegill."