Japan is to use tiny fish in the battle against terrorism.

Ricefish, measuring less than two inches and commonly kept as pets, react rapidly to contaminated water.

The fish, which is pale orange, pushes its face close to the surface when it experiences breathing difficulties or simply dies in the manner of canaries in gas-filled mines.

Whereas sophisticated and expensive filters may take up to 15 hours to detect a problem, ricefish typically react to contamination within a couple of hours. So far, they have been used on only an experimental basis at two water plants but they will now be deployed at the central water supply system for Shizuoka prefecture, south of Tokyo.

Japan has a history of studying animal behaviour for possible indications of disaster.

Catfish are watched carefully because they are believed to thrash about when an earthquake is looming.

It is suggested that they are unusually sensitive to electric fields, which are disturbed when a fault line is about to slip.