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

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Vietnam battles plastic pollution crisis at Unesco World Heritage site of Ha Long Bay

Vietnam battles plastic pollution crisis at Unesco World Heritage site of Ha Long Bay

“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 (350,000 cubic feet) of rubbish – enough to fill four Olympic swimming pools – have been collected from the water, according to the Ha Long Bay management board.

The rubbish 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.”

But the site’s popularity, and the subsequent rapid growth of Ha Long City – which is now home to a cable car, amusement park, luxury hotels and thousands of new homes – have severely damaged its ecosystem.

Conservationists estimate there were originally around 234 types of coral in the bay – now the number is around half.

There have been signs of recovery in the past decade, with coral coverage slowly increasing again and dolphins – pushed out of the bay a decade ago – coming back in small numbers, as a ban on fishing in the core parts of the heritage site expanded their food source.

But the waste, both plastic and human, is still a huge concern.

“There are so many big residential areas near Ha Long Bay,” says conservationist Thanh. “The domestic waste from these areas, if not dealt with properly, greatly impacts the ecological system, which includes the coral reefs.

“Ha Long City can now handle just over 40 per cent of its waste water.”

Single-use plastic is now banned on tourist boats, and the Ha Long Bay management board says general plastic use on board is down 90 per cent from its peak.

But rubbish generated onshore still lines parts of the beach, with a team of rubbish collectors not able to block the eyesore from tourists.

Pham Van Tu, a resident and freelance tour guide, says he has received a lot of complaints from visitors.

“They read in the media that Ha Long Bay is beautiful, but when they saw a lot of floating trash, they didn’t want to swim or go canoeing and they hesitated to tell their friends and family to visit,” he says.

The volume of leakage could more than double by 2030, the World Bank warns.

Larissa Helfer, 21, who travelled to Vietnam from her home in Germany, says Ha Long Bay was beautiful but the rubbish problem would be one of her strongest memories of the trip.

“Normally you [might say] ‘Look at the view! Look at the fishing villages!” she says. But here “you have to talk about the trash, [you say] ‘oh god … look at the plastic bottles and things in the sea’. And it makes you sad.”

Thinh, the rubbish collector, grew up in Ha Long and remembers a very different bay.

“It didn’t look so terrible,” she says. “Of course, a lot of work makes me tired and irritated. But we must do our work.”