Dead fish found on the shorelines of a Shanghai river have sparked safety fears.

Small carp-type dead fish were first sighted last week in a river in Shanghai's Songjiang district, China Daily reported Monday, and larger dead carp over the weekend, hundreds of fish in all. So far about 551 pounds of fish have been retrieved.

The incident comes less than a month after thousands of rotting pig carcasses were found floating in another Shanghai river, the Huangpu, one of the city's primary sources of drinking water.

The dead pigs were thought to have been dumped from neighboring Zhejiang province.

An official from Songjiang Water Authority told the newspaper Sunday that the Songjiang river's water quality is "stable and safe."

Liu Fengqiang, vice-director of Shanghai's Songjiang district's environmental department said there was no link between the dead fish and the pig carcasses found in the Huangpu River.

"Local residents don't need to worry about water safety because the dead fish were found in a landscape river rather than a natural water source," said Liu, adding that there is also no connection between the fish incident and the bird flu strain which had so far claimed the lives of six people in China.

Based on accounts from local fisherman, the water authority official told China Daily that the fish died by electrocution and poisoning because of illegal fishing.

But another water authority official told the Shanghai Daily he thought the fish might have been drugged. "Small fish died earlier because they're more sensitive to toxins," he said.

Shi Hua, a resident of Songjiang, told China Daily that the water source for the area comes from the Huangpu River and he no longer drinks tap water, relying instead on bottled water. "I need to trust my sources of water. I'm still haunted by the dead pigs," he said.

Also last month, more than 1,000 dead ducks were found in the South River in Pengshan county in Southwestern China, stuffed into more than 50 woven plastic sacks, state broadcaster CCTV reported.

Groundwater contamination is also a problem in China.

The government of Hebei county, in a report released Sunday, said that levels of a cancer-causing element in its groundwater exceeded the national standard by more than 70 times, the Global Times reports.

Greenpeace East Asia has estimated that 320 million people in China don't have access to clean drinking water. And a study by China's ministry of environmental protection says that 64 of 118 Chinese cities had "seriously contaminated" groundwater supplies, The Guardian reports.