The San Francisco Public Utilities Commission says the city has begun using common bluegill fish to guard against terror attacks on the water system.

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Commission General Manager Susan Leal said San Francisco and New York have become the first cities to use the process, which uses non-contact sensors placed in the aquarium, which is hooked into the water system, to monitor the fish's behavior and watch for indicators of toxic conditions, KPIX-TV San Francisco reported Monday.

If toxins are suspected, the system triggers an alarm, takes water samples for analysis, and notifies staff 72 hours before the water reaches the public.

"It gives us a real edge on early detection on any changes in our water quality," said Leal.

San Francisco purchased three of the $110,000 systems and plans to install them at monitoring sites within the water supply system.