The marsh, at Three Fathoms Cove and near Sai Kung West country park, was once rated one of the top five freshwater sources and first for richness in fish species in Hong Kong.

The area is privately owned and unprotected by environmental laws. Parts belong to Sun Hung Kai Properties.

The developer has declined to comment and it is not clear who carried out the work that destroyed the wetland.

The fish, thought to be extinct in the wild for 60 years until its discovery in Hong Kong in the 1990s, live in four other sites in eastern Hong Kong.

The new programme aims to preserve the fish because the other locations, undisclosed for their protection, are also unprotected. About 80 fry at the Kadoorie Farm have survived since April.

'There has been some degree of cannibalism. I will feel better when the fry reach the juvenile stage,' the farm's conservation officer, Paul Crow, said.

The grey-brown fish with a distinct blue spot on their gill plates turn black during mating and measure about 10cm for adult males and slightly smaller for females.

The Agriculture and Fisheries Department has declined to put the fish on the protected species list, saying it may be a variant of the Chinese Paradise Fish, which is abundant in Hong Kong, and may also be found elsewhere.

But University of Hong Kong ecology and bio-diversity doctorate student Bosco Chan Pui-lok said he had failed to find any scientific literature or researchers who argued for the alternative classification except the department.

Mr Crow said: 'It is possible [the fish exists outside of Hong Kong] but until you have found it elsewhere, you have to treat it like it's the only one in the world.'