A type of fish so common that practically every American kid who ever dropped a fishing line and a bobber into a pond has probably caught one is being enlisted in the fight against terrorism. 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...