Dead animals keep turning up in China under mysterious circumstances

Chinese authorities have hauled thousands of dead fish—around 550 pounds, to be exact—from a Shanghai river over the past four days. The fish were not related to the H7N9 avian flu virus afflicting humans, officials say, and no chemical substances (video) have been found in water samples. Authorities have also cleared nearby factories as a cause.

But no one really knows why 550 pounds of fish suddenly went belly-up. Here’s a roundup of other mystery mass floating animal carcass incidents that have, er, surfaced in China this spring:

• 16,000 dead pigs. Drifting for weeks down Shanghai’s Huangpu River, a major source of drinking water for the city, the pigs were infected with porcine circovirus, which is not harmful to humans. The reasons for farmers dumping these pigs in the first place remains unclear.
• 1,000 dead ducks. In late March, 50 to 60 woven plastic bags packed with a thousand duck corpses washed up on the shores of Nanhe River in Sichuan. Though cleared of any threat to humans, the cause of the duck massacre was unclear due to the carcasses’ extreme state of decomposition.
• Five black swans. These unusual birds, part of a flock on the campus of Anhui University, died in late March, as Danwei reports. Possible causes of death include a large dog, toxic garbage, or water pollution.