This is not the first time dead animals have been found floating on the river, according to local residents.

The dead pigs in Shanghai were first reported on Thursday, when a netizen with the username Shaolinsidezhu1986 posted a picture online.

The picture showed at least seven dead pigs floating on the river. "This is the water we drink. In this water conservation area, dead animals' bodies can be easily seen. This is not the first time it happened," said the netizen.

"It's common to see dead pigs in the river, they were thrown into the water directly by farmers," said a villager in Songjiang district. The woman, quoted by Xinmin.cn, said the pigs may have drifted down from the upper streams.

Shanghai authorities are stepping up the coordination among the areas near the upstream of Huangpu River to track down the source of the dead pigs.

Local media reported that the pig carcasses may have come from farms on the upper tributaries in Zhejiang province.

The ear tags on the retrieved carcasses showed they are from Jiaxing and Pinghu in Zhejiang province, and some areas of Jiangsu province, according to Xinmin.cn.

An official from the agriculture department said Shanghai has a special system to treat dead pigs.

Under the system, the local community's agriculture department travels to farms to collect dead pigs and treat the carcasses for potential biohazards. Farmers receive compensation from the government. But this policy does not exist in neighboring provinces, the report said.

It added that sick or dead pigs are often thrown out in Jiaxing, Zhejiang province.

From last winter to this spring, a large number of pigs died in Zhulin village of Jiaxing, which is dependant on raising pigs, Jiaxing Daily reported on March 6.

The village has more than 1,400 households, and almost every family raises pigs. In January, 10,078 pigs died; in February, 8,326 pigs died. More than 300 pigs died each day, according to the report.

The pigs are living overcrowded conditions where bacteria breed easily. Consequently, many became sick and died. But there is not enough land to bury the pig carcasses, and many of dead animals were thrown away, it said.

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