More than 3,000 workers from fishery departments of northeast China's Jilin and Liaoning provinces on Tuesday released 13 truckloads of carp fry into the Songhua River "in the hope of improving its ecology", Xinhua news agency said.

"Shortly after the release was completed, more than 1,000 residents in Jilin swarmed to the riverbank with nets and other fishing equipment," Xinhua said.

A dozen fishery workers patrolling the river confiscated poachers' nets but no arrests were made.

The release of the 70,000 yuan ($9,200) worth of young fish, coinciding with World Environment Day, took about two hours to complete.

Millions of residents of the city of Harbin had their taps turned off for weeks after a toxic spill in the Songhua River in 2005.

Many of China's lakes and rivers are threatened by run-off from fertilisers, dumped industrial waste and untreated sewage. Algae blooms can burst out in water rich in nutrients from farm and domestic run-off.