The number of dead pigs collected from waterways near Shanghai has begun to decline following days of continuous salvage efforts, the city government said Wednesday while reassuring the public that official tests show drinking water remains "unaffected."

Analysts urged local governments to reflect on their inadequate supervision and loose management of environmental issues while painstakingly pursuing economic growth.

By 3 pm Wednesday, 685 dead pigs had been fished from the Huangpu River, 43.8 percent fewer than the day before, and raising the total number of swine carcasses to 6,601, said a statement released by the Shanghai municipal government.

Officials continued their testing for several viruses including porcine circovirus and salmonella. They said no contamination has been found in the city's drinking water produced by the major water treatment plants, according to the statement released on the government's official Weibo.

The Huangpu River is a major source of drinking water for Shanghai, home to 23 million people.

"This is not the first time we have found dead pigs. Farmers in the neighboring Zhejiang Province habitually throw them into rivers, putting Shanghai at risk of water pollution," Xu  Yonghua, a deputy director with the Jinshan district water authority, told the Global Times.

Xu said never before has the city had to remove about 1,000 dead pigs a day from the waterway.

Ear tags that identify a pig's place of birth are showing that many were born in Jiaxing, Zhejiang. Recovered tags led authorities to a pig farmer in Jiaxing who was detained Wednesday for investigation after admitting he had thrown pigs in the river.