Dead fish are seen in Zijiang River in Xinshao county, Central China's Hunan province, August 11, 2011. [Photo/Xinhua]

CHANGSHA - A large number of fish were found dead over the past week in a river in Central China's Hunan province, local environmental authorities said.

The dead fish, both wild and artificially bred, were found in a 55-km-long section of the Zijiang River in Xinshao county, said Luo Chunyi, head of the county's environmental protection bureau.

Over 130 families running fish farms in Xinshao have experienced a collective loss of more than 200 metric tons of fish, with direct economic losses reaching 20 million yuan ($3.1 million), said Xiao Songhai, the county's deputy head.

The exact number of dead wild fish cannot be ascertained at the present time, Xiao said.

Lei Xinhua, a fisherman from the county's village of Changtan, said he first noticed the dead fish last Friday after he saw a stream of "dark" water running down the river from the Yangxi Bridge, which is located upstream.

More dead fish were found later in the lower reaches of the river, primarily around Daxin Township and the village of Xiaoxi.

A test of the river's water conducted by the county's environmental authorities showed that the amount of dissolved oxygen in the river was significantly lower than what is required for fish to be able to survive.

The authorities said the test results ruled out the possibility of industrial pollution, but did not provide an explanation for the river's low oxygen levels.

The local water resources department is racing to clean up the dead fish, while the environmental protection bureau is now monitoring the river's water quality more closely.

The Zijiang River, one of the province's four major rivers, runs into Dongting Lake, the country's second-largest freshwater lake.