The last refuge for many of China's rarest and most economically important wild fish has mere days to secure public support before it is trimmed, dammed and ruinously diminished, conservationists warned today.

The alarm was raised after the authorities in Chongqing quietly moved to redraw the boundaries of a crucial freshwater reserve on the Yangtze, which was supposed to have been the bottom line for nature conservation in one of the world's most important centres of biodiversity.

The Upper Yangtze Rare and Endemic Fish Nature Reserve was created in the 1990s as a haven for species that were threatened by the Three Gorges dam, the world's biggest hydroelectric plant.

Among the hundreds of species it protects are four types of wild carp that experts say are essential to China's food security because they provide the diverse genetic stock on which fish farms depend for healthy breeding.

In recent years, the importance of this 400km-long ecological hold-out has increased as China's hunger for energy has driven power companies to build two more mega-dams – Xiangjiaba and Xiluodu – that have swamped the shoals and stilled the rapids along thousands of kilometres of Asia's biggest river.

Downstream, the combination of dams, pollution, overfishing and river traffic have decimated fish stocks, wiped out at least one species – the Baiji or Yangtze river dolphin – and left others – like the giant Yangtze sturgeon (Acipenser dabryanus), the Chinese paddlefish or the finless porpoise – critically endangered.

Upriver, the state has promised to safeguard the last untamed stretch. A coalition of scientists and conservationists has opposed development in the reserve. Premier Wen Jiabao has expressed unease about the impact of excess dam-building on environmentally important areas.

But this goal has run up against the interests of the Three Gorges Project Development Corporation and local officials, who want to build yet another hydroelectric plant at Xiaonanhai that would choke the river to power the development of the poor local economy.

The developers appear to have gained the upper hand last week when the Ministry of Environmental Protection announced plans to redraw the boundary of the reserve so that it would no longer encompass the area of the proposed dam. This leaves less than 10 days for public discussion, according to conservationists who are dismayed there has been almost no domestic coverage, partly because many of the 29 endangered fish species – such as Chinese paddle, Yangtze sturgeon and Chinese sucker – are unknown outside of expert circles.

"This is the last hold-out for much of China's freshwater biodiversity. It is a rare situation when one project can do so much damage," said Ma Jun of the Institute for Public and Environmental Affairs, one of the country's leading green campaign groups. "Part of the problem is that unlike pandas, snub-nosed monkeys or Tibetan antelopes, most people have not heard of or seen the fish affected."

Local government insists no decision has been made on the dam, but past precedent suggests that construction will begin before the formal environmental impact assessment is made, by which time developers will argue that it would be a huge waste of money to cancel.

Less often calculated is the economic loss of biodiversity. With fewer wild carp to bolster farm stocks, environmental experts say China is taking a risk with a primary source of protein. Since the Three Gorges was built, the downstream carp population has crashed by 90%, according to Guo Qiaoyu, Yangtze River project manager at The Nature Conservancy.

"This is economically important. We eat a lot of these fish. We need to help people realise its important to protect fish reserves and not just tap the power of the river," said Guo. "If we lose this reserve, the wild population will almost be wiped out.

It is rare for the Nature Conservancy to oppose dam construction, which they accept as important to China's development. But the US-based NGO has sent a letter to the government, urging full protection of the Yangtze reserve, which looks set to be a test case of the authorities' willingness to conserve.

"I feel very frustrated. This reserve was first set up as a compensation for the Three Gorges Dam. And then for other dams on the Jingsha cascade," said Guo. "If you change this reserve again to make way for another dam, it shows that we don't have a baseline for conservation, that anything can be overridden in the interests of economic development. It sets a terrible precedent."