HA LONG, Vietnam: Squinting in the bright light of a hot summer morning, Vu Thi Thinh perches on the edge of her small wooden boat and plucks a polystyrene block from the calm waters of Vietnam's Ha Long Bay.

It's not yet 9am, but a mound of styrofoam buoys, plastic bottles and beer cans sit behind her.

They are the most visible sign of the human impacts that have degraded the UNESCO World Heritage Site, famed for its brilliant turquoise waters dotted with towering rainforest-topped limestone islands.

"I feel very tired because I collect trash on the bay all day without much rest," said Thinh, 50, who has been working for close to a decade as a trash picker.

"I have to make five to seven trips on the boat every day to collect it all."

Since the beginning of March, 10,000 cubic metres of rubbish - enough to fill four Olympic swimming pools - have been collected from the water, according to the Ha Long Bay management board.

The trash problem has been particularly acute over the past two months, as a scheme to replace styrofoam buoys at fish farms with more sustainable alternatives backfired and fishermen chucked their redundant polystyrene into the sea.

Authorities ordered 20 barges, eight boats and a team of dozens of people to launch a clean-up, state media said.

Do Tien Thanh, a conservationist at the Ha Long Bay Management Department, said the buoys were a short-term issue but admitted: "Ha Long Bay ... is under pressure".