XINING, Oct. 3 -- Over 7 million rare fish fry have been released into the upper reaches of the Yellow River, China's second largest, since 2009 to help to repair its ecosystem, said local fishery authorities.

Qinghai Province in northwest China, where the Yellow River and Yangtze River originate, plans to release about 900,000 rare, captive-bred, native fish this year to replenish stocks, said Wang Guojie, deputy chief of the province's fishery environment supervision office.

The Ministry of Agriculture, in charge of fishery affairs, announced in 2007 that one third of the 150 fish species in the Yellow River were believed to be extinct due to human encroachment and low rainfall.

Over fishing, dumping and hydropower projects degraded the environment and led to a shrinking fish population. In Qinghai, 22 fish species are native and most of them are only found on the Qinghai-Tibet plateau.

"Fish grow much more slowly on the plateau than at low altitude, due to cold weather. Plateau fish, living at around 4,000 meters above the sea level, are extremely resistant to cold, but only add an average of half kilogram in weight every ten years," said Wang.

Once the stocks are damaged, it's hard to restore quickly, he said.

"Our survey shows a rise in fish stocks in some key areas over the past five years, meaning the released fry have adapted well," said the official.

Qinghai has plans to build more fish farms and expand the project's release area.