They have been campaigning for the reserve's protection for the past decade. The rest of the river is blocked by dams, polluted by industrial discharge or disturbed by busy transport.

A meeting of experts decided last month to rezone the Upper Yangtze National Nature Reserve for Rare and Endangered Fish, covering a stretch of about 330 kilometres, to make way for unspecified development.

The area to be conceded by the reserve, a 20 to 30 kilometre stretch of the Yangtze and its banks, is the site of the proposed Xiaonanhai hydro- electric station, which environmentalists have been campaigning against since the 1990s.

According to Chinese law, no construction is allowed in a national- level nature reserve. If officials want to embark on a project, they must first apply to rezone the nature reserve to move the would-be construction site outside the reserve.

The municipality and the Ministry of Environmental Protection have already approved the application, but it still needs the approval of the State Council, China's cabinet.

Building a dam would be environmentalists' worst nightmare, said Dr Guo Qiaoyu , a China executive of The Nature Conservancy.

For many Yangtze fish species, hatching requires up to 400 kilometres of undisturbed river flow, which would be impossible if a dam is built.

Ma Jun , director of the Institute of Public and Environmental Affairs, said environmentalists were also saddened that the experts at the meeting, who had previously voiced their concern over the dying out of the Yangtze's bio-resources, were made to change their attitude and have remained silent since.

Analysts noted that the main source of political pressure may have been the officials of Chongqing municipality, who are pursuing their ambitious economic development programme. Chongqing environmental authorities refused to comment. Nonetheless, seven environmental groups have signed a joint petition to the central government seeking the release of all details of the rezoning decision and, when possible, holding open hearings about the rezoning.

They said they hoped the State Council will overturn the approval.

They are also drafting a petition letter to Vice Premier Li Keqiang, widely speculated as China's next premier in 2013, and chairman of the China Committee of the International Year of Biodiversity.

Grounds for the rezoning decision have not been published yet, although official media said the meeting did take place.

Ma said the particular section of river could be used for building a modern waterfront or a shipping centre.

But environmentalists fear the rare fish reserve will make way for the long-shelved Xiaonanhai dam and hydroelectric station - a project which, though estimated to cost 23.9 billion yuan (HK$27.9 billion) and not among the largest in the nation, will surely be a boost to the city's low-carbon profile and be a new guarantee to its cheap energy supply.

Any rezoning attempt would threaten the species in the fish reserve, because many of them need long natural flows of water to hatch their young, explained Sun Shan, a scientist and member of Shan Shui Conservation Centre. 'If a dam is erected, the last section of natural flow will disappear on the Yangtze and fish-hatching will become more difficult, no matter how much area of the original reserve is still kept,' she said.

Moreover, Chongqing's rezoning would set an example and cause a domino effect. 'The nearby cities may ask for further rezoning of the fish reserve to make way for their own development projects in due time, and eventually nibble away the entire area,' Sun said.

As reported in June of last year, environmentalists said that the Xiaonanhai Dam - proposed to be built near Luohuang town, around 30 kilometres upstream from downtown Chongqing and 700 kilometres upstream from the Three Gorges Dam - is a pet project of Bo Xilai , the municipal Communist Party chief.

In contrast with the lack of dissent at the recent experts' meeting, previously concerns were expressed by both natural scientists and environmental officials about the proposed Xiaonanhai project.

Last year, Wan Bentai, chief engineer with the Ministry of Environmental Protection, said Xiaonanhai would pose a threat to rare fish in the Yangtze. He advised the Chongqing officials to adhere to laws and heed environmental concerns while pursuing their economic goals.

The Yangtze used to be the home of China's largest number of fresh water fish.

Zhao Yimin , an official with Yangtze Fishery Administrative Committee, claimed that its biodiversity, consisting of 370 species, was once the largest of all rivers on earth except the Amazon.

'Now all this is becoming history,' he was once quoted by mainland media as saying.

Excessive development in the lower reaches has already led some species to die out quickly. In Jiangsu , 50 species became extinct in less than three decades, experts said.

Professor Cao Wenxuan , scientist with the Chinese Academy of Sciences' Institute of Hydrobiology, called for a 10-year ban on fishing in Yangtze in July 2008, but the government refused to act.

Sun said: 'If the fish reserve on the upper Yangtze is rendered useless, we will not only lose all the fish - some as old as dinosaurs and as precious as the giant pandas.

'We will also lose the entire fishing industry and a lifestyle that has run down for thousands of years. We will lose part of China's culture.'