Rubbish threatens the economic and ecological health of the world's oceans by killing wildlife and poisoning marine habitats. Every year, an estimated 6.4 billion kilograms of trash are dumped in the sea, while millions of tonnes of plastic are floating around. There is so much plastic, in fact, that an enormous patch of discarded bottles, wrappers and other debris, covering an area 1,300 times the size of Hong Kong, is swirling around the Pacific Ocean.

More than 100,000 mammals and two million sea birds die each year from eating or becoming caught in marine debris. When fish or birds consume chemicals or plastic from ocean waste, the toxins are often passed on to humans at the dinner table. Fish have been found to contain unsafe levels of mercury, PCBs and dioxins, all of which build up over time in the human body, causing serious health complications.

This year 121 teams and more than 6,500 people participated in the Hong Kong cleanup. They cleared nearly 30,000 kilograms of waste from dozens of beaches. Most teams were corporate, with particularly strong showings from property developer Sino Group, whose team had 275 volunteers, and Nomura, a financial services company, which provided 215 volunteers. Outdoor education provider Outward Bound showed up with 250 participants, making it the largest non-corporate team. Smaller teams represented the Jewish Community Centre, Friends of Sai Kung and a plethora of other organisations.

'Gazetted beaches are cleaned 10 to 12 times a day,' Christensen said. 'The thing we have to understand is that if they weren't cleaned, they would be lost under piles and piles of rubbish.' That is the unfortunate fate of many of the city's beaches. Christensen was aghast when she saw the state of the shore near Sok Kwu Wan on Lamma Island. 'It was just trashed. We made a dent, but we were able to clear away maybe just one one-hundredth of what was there. We needed a thousand people to get the job done, so every weekend we took groups there and every single weekend it was devastating. There was always more trash to be picked up.'

In the end, 795 volunteers collected more than 3.5 tonnes of rubbish from the two-kilometre stretch of shore. 'Although the beach is looking a lot better, there's still tonnes more there, especially tangled up in the mangroves,' Christensen said.

Most debris consisted of food wrappers and containers, plastic bags, bottles and cigarette butts. Curiously, Bloomberg's team found a prosthetic leg on Lamma Island. Others found car doors, toilets and washing machines. Alarmingly, 160 syringes were recovered along with other dangerous waste such as batteries and bottles of bleach.

'Aside from cleaning the coastline, the Cleanup Challenge educates people about marine pollution and we collect data to track the sources of that pollution,' Christensen said. Most marine debris comes from the shore, which suggests that Hong Kong needs to reduce the amount of waste it produces. Ecovision is pressing the government to institute a deposit programme for bottles and cans, which has been shown elsewhere to significantly boost recycling rates.

Meanwhile, beach cleanups continue year-round. The Food and Environmental Hygiene Department employs about 200 people to clean up ungazetted beaches and stretches of shoreline, with up to 3,000 operations taking place every year, many involving the help of community groups, oceanside villages and non-governmental organisations.

'The actual Cleanup Challenge is really a celebration of what's being doing around the year,' Christensen said. 'We are pushing for behavioural change and policy change, encouraging companies to use smart packaging and fewer plastics. It's really a case of minimising our waste. The ocean is resilient - it can and will bounce back - but we all have to take responsibility for it.'