San Francisco and New York City both use a highly efficient system to detect the presence of toxins in their city water supply, toxins that may be a possible sign of a terrorist attack. The two cities have found that the best tool for monitoring such threats are bluegills, those little fish many people catch on a lazy summer afternoon. According to an article by the Associated Press, these two cities keep a small number of bluegills in a tank at the bottom of their water treatment plants because bluegills are highly attuned to chemical imbalances in their environment. When a disturbance is present in the water, the bluegills react against it. If the computerized system of the treatment plant detects even the slightest change in a bluegill’s vital signs, it sends out an e-mail alert. Bill Lawler, the co-founder of the corporation that makes and sells these bluegill monitoring systems, said, “Nature’s given us pretty much the most powerful and reliable early warning center out there.