Sanitation officials said yesterday they expect this summer to remove almost 800 tons of a floating plant that threatens fish stocks in Suzhou Creek.

The plant, known as duckweed or water lentil, grows on the surface of the creek and can grow at an amazing rate - doubling in size every 48 hours.

It feeds off nitrogen and phosphorous contained in sewage and river runoff. As the plant dies and rots, it absorbs oxygen, killing fish and threatening the livelihood of thousands of fishermen.

Workers from Shanghai Public Sanitation Bureau is removing duckweed from the creek around the clock at a rate of 20 to 30 tons a day, and the bureau will draft in extra help during July and August, when the plant's growth is at its highest.

Last summer, workers removed over 600 tons of duckweed and this year they expect to remove 800 tons.

Liu Weiguang, an official from the bureau, said the worst-affected part was the upper reaches of the river.

"After we set several devices to stop the plant from floating along the river, the situation is much better in downtown area," Liu said.

To control the spread of duckweed, water quality must be improved, officials said. Duckweed is a common plant usually found in ponds, marshes, lakes and streams.