Sewage discharged directly into Shenzhen city's Mangrove Nature Reserve has aroused concern among local residents who are worried the illegal waste could prove deadly to mangrove trees and migratory birds, Thursday's Daily Sunshine reported.

The sewage has been discharged for at least four years, the Daily Sunshine report said, quoting a security guard of a nearby scenic spot. But the source of the sewage remains unclear, it said.

The black and smelly water containing food packaging, wine bottles and toilet paper, flows into the nature reserve from outside the scenic spot's boundary fence, the report said. A dead egret has been found on the bank of a lake, its decomposed body attracting swarms of flies.

"A large amount of foul-smelling water is disposed of every day," said a man surnamed Zhang who lives nearby. "I have been worried the mangrove trees would be harmed. Every year, many migratory birds come here to spend the winter, I'm concerned the birds might be poisoned after eating the food from the filthy water."

Zhang said he had reported the illegal sewage discharge to relevant government agencies several times, but the problem has never been solved. "The outlet has been there for four years, but nobody knows where the sewage comes from."

Some mangrove trees near the lake have begun to wither, others are dying. The plants in the lake, polluted by the sewage, are no longer as exuberant as they were before. A warning sign by the lake says: "Dregs, sediment and construction waste are forbidden at the Mangrove Nature Reserve. Violators will be fined 2,000 yuan (US$250) for each cubic meter."

A woman surnamed Huang living in Bihaiyuntian housing estate said they had never opened the window facing the nature reserve since moving into the housing estate. "The smell is terrible," she said. "Clothes hanging outside become smelly and my child feels dizzy breathing in the air."

Huang said they can still see beautiful egrets flying in the nature reserve. But she was worried what would happen in a few years' time. "Who can guarantee we will still see those birds a few years from now?"

Editor: Yan