China's coastal waters last year witnessed 68 red tides, or algae blooms, which feed off nutrients found in excess pollution and sap water of oxygen, killing off large amounts of sea life, Xinhua news agency said.

The State Oceanic Administration was cited as saying the algae blooms covered 13,700 square kilometres (5,500 square miles), an increase of more than 2,100 square kilometres over 2007, the report said.

While some experts said the red tides were a result of climate change and heavy rain, environmentalists believe they were largely due to sewage and agricultural pollutant run-off, it said.

In August last year, one algae bloom caused havoc for the sailing competition of the Olympic Games when it engulfed waters surrounding the sailing venue in eastern China's Qingdao city.

Up to 10,000 soldiers and volunteers were enlisted to clean up more than one million tonnes of the foul-smelling algae as they raced to clear the waters ahead of the competition, Xinhua said.

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