Researchers in Singapore have developed a system that detects water pollution by monitoring the behaviour of fish.

The technology automatically counts the number of live and dead fish in tanks connected to the public water supply and raises the alarm if the number of dead fish rises suddenly.

The system currently detects pollution only when it has grown severe enough to kill fish.

But enhancements under development will equip it to detect unusual movements or other distressed behaviour, giving a much earlier warning that something toxic is in the water supply.

The system analyses images from video cameras pointed at fish tanks to detect the motion vectors and positions of the fish.

Many public water treatment plants already incorporate human-monitored fish tanks among various methods of checking water quality.

The technology was developed jointly by researchers and engineers at Singapore's state water supply body, the Public Utilities Board, and the A*STAR Institute for Infocomm Research. Work on the project began in 2006.

Chong Hou Chun, a researcher at the Public Utilities Board, said: "With such a system, the operators will be more effective in their round-the-clock monitoring of the water quality in the water supply system."

Other applications include CCTV monitoring in trains to detect sudden collapse of passengers from exposure to colourless and odourless gas, and in swimming pools as a drowning alert warning systems, according to a statement.

As a densely-populated island nation with limited land area, Singapore is forced to rely on the neighbouring country of Malaysia for up to half of its water supply.

However, an agreement which provides one third of the water flowing from Malaysia expires in 2011, and negotiations over future water prices have been contentious.

As a result, Singapore's government has strongly encouraged the development of alternative water sources such as energy-efficient desalination of seawater and recycling of public waste water into drinkable water.