Police helicopters and boats have started patrolling a reservoir to protect Shanghai's drinking water supply.

The patrols, which began on Thursday, will support 24-hour surveillance to prevent anyone breaking into the Qingcaosha Reservoir at the estuary of the Yangtze River.

According to the city's regulations on the protection of drinking water sources, implemented in March 2010, people are barred from entering the reservoir region, unless authorized for water supply purposes.

But despite the 50-kilometer-long reservoir being surrounded by a barbed-wire fence, many still manage to sneak in, some with boats, mainly for fishing, said Qin Yongbo, head of the border defense team under the public security department of Chongming county.

Altogether, 147 people and 55 boats had been caught and hundreds of fishing nets seized.

"Diesel oil leaked from some of the fishing boats in those cases. That would affect the water quality if it happened too much, although a small amount of oil is negligible in a reservoir that's the size of 9,000 football pitches," Qin said.

The surveillance from land, water and air will ensure the safety of the reservoir that supplies 70 percent of tap water in Shanghai, said Hu Yunwang, deputy head of the border defense division for the city.

The helicopter will spot any suspicious boats, and inform the police speedboats and cars, which will deal with the situation on the ground, Hu said.

"We will also work with government agencies to give fishermen other job opportunities, such as planting or doing factory work," he said. "Because many of them fished here for a long time before it was fenced off to protect water supply."

zhouwenting@chinadaily.com.cn