Sina Weibo user "Happy as a Cat" Just a few of the fish

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: