Fish found dead in waterways in the Songjiang district of Shanghai may have been released by Buddhists to mark the birthday of a goddess, authorities said.

Some 250 kg of dead fish, mainly crucian carp, have been retrieved from a pond in Songjiang and from Dongjing port since April 3, raising fears over water quality.

The Songjiang district government said residents in Sijing town reported seeing people free fish in a river on March 30, the birthday of Guanyin, a Buddhist goddess, in the Chinese lunar calendar.

"Investigations at local markets found that many people spent hundreds or thousands of yuan on buying fish. Some even paid more than 10,000 yuan ($1,614)," according to a release from the government on Sina Weibo.

As so many fish were put into the river at the same time, many died because of the change in temperature and from other environmental factors, according to the release.

The district's water authority told China Daily on Sunday the water quality is stable and safe.