So says the Chinese officials. Pig farmers in the neighboring province have been dumping the dead pigs, supposedly died of cold weather.
The AP article below also mentions the "illicit trade of pork products harvested from diseased pigs", which the authorities are cracking down on.
From AP (3/12/2013):
China Pulls out 5,916 Pigs From Shanghai River
The number of dead pigs found floating in a river flowing into Shanghai has reached nearly 6,000.
The Shanghai municipal government said in an online announcement that 5,916 swine carcasses had been retrieved from Huangpu River by 3 p.m. Tuesday, but added that municipal water remains safe.
The surge in the dumping of dead pigs — believed to be from pig farms in the upstream Jiaxing area in the neighboring Zhejiang province — has followed police campaigns to curb the illicit trade of pork products harvested from diseased pigs.
Shanghai authorities said the city has taken proper measures to safely dispose of the pig carcasses and that the city's water plants are stepping up efforts to disinfect public water and testing for six common swine viruses.
The Shanghai government reported no major swine epidemic, widespread pig deaths or dumping of pigs within the city boundaries of Shanghai.
The state-run China News news agency said Monday that Zhejiang province had reported no swine epidemic but that a provincial agriculture official blamed cold weather for the deaths of the pigs.
The official, who was identified only by his family name Gu, told China News that the practice of dumping dead pigs into rivers lingers among some pig farmers in the city of Jiaxing. "We are still introducing the practice of collecting dead pigs," Gu was quoted as saying.
Shanghai authorities have been pulling out the swollen and rotting pigs, some with their internal organs visible, since Friday — and revolting images of the carcasses in news reports and online blogs have raised public ire against local officials.
Beijing-based writer Li Mingsheng expressed shock when he learned of the latest number of dead pigs in Shanghai.
"This is not only an environmental issue but also a public moral problem," Li wrote. "What's been polluted is not only Shanghai's river water but also the spirit of our country people."
Dumping something undesirable down the river seems to be a universal behavior, both physically and figuratively. So is selling something tainted with undesirable substances (disease virus, chemicals, radioactive cesium).