The encroachment of Canberra's urban edge on the Murrumbidgee River could cause the disappearance of a "unicorn", ecologists have warned.
The ACT government last week released plans to redevelop close to 90 hectares of river corridor and nature reserve between the river and Tuggeranong's town centre into a new suburb of Thompson.
But the river is one the last refuges of the rare Murray crayfish, Dr Chris Fulton from the Australian National University's Research School of Biology said.
He and Mae Noble, from the ANU Fenner School of Environment and Society, recently found the Snowy Mountain population had almost been wiped out after more than half of their preferred habitats in the Snowy Mountains disappeared.
The alpine streams of Goobarragandra were once a stronghold for the freshwater "spinies" but the clearing of riparian vegetation and an increase of sedimentation choking the deep, bouldered pools they live in has caused the population to decline by more than 90 per cent, Dr Fulton said.
"There was a relatively moderate breeding population of Murray crayfish, we're talking around 70 to 80 individuals in four or five kilometres of stream. Just six years later, we were lucky to find a dozen of these crayfish still clinging on," he said.
While a map released by the ACT government indicates about 87 hectares of nature reserve along the river will be preserved, he fears the ACT's population could see a similar decline if development continues along the Murrumbidgee's riparian corridor.
The species was listed as vulnerable in the territory and pockets of the Murrumbidgee were the only places where it was possible to see breeding individuals in the ACT, Dr Fulton said.
"The research we've just completed has found they're very sensitive to habitat loss because they're very choosy about where they want to live in a stream," Dr Fulton said.
"They like areas that have deep pools full of boulders and a relatively moderate flow but the key thing is they really like streambank vegetation which overhangs the stream and keeps it cool.
"That suggests if you have any clearing of that vegetation corridor on either side of the stream you could effectively be wiping out whole sections of streams habitable for these Murray crayfish."
Murray crays can live up to 25 years and only reach maturity between eight and 10 years of age.
They play a vital role in aquatic ecosystems, where they clean up dead and decaying matter and provide food for many other species.
Senior lecturer with the Department of Ecology, Environment and Evolution at La Trobe University's Wodonga campus Dr Susan Lawler describes them as the "unicorns of the river".
"The Murray crayfish are stunning creatures – if you take someone not familiar with them out in a boat and pull in a net with a few large animals in them, you will create a memorable moment," she said.
"They are difficult to catch and even harder to hold – their spines will make you bleed and only the initiated would even dare reach toward one if they were in defensive mode. When threatened, they raise their spiny claws and wave them about. Quite intimidating."
But despite their fierce nature, she said Murray crays were in decline and "nobody really knows what is driving that process".
"We know almost nothing about their young, where they live and what they eat, so we cannot study recruitment in this species," she said.
Dr Fulton said the loss of the Murray crayfish was particularly sad, given they were considered an icon of the Murray-Darling system and the Murrumbidgee.
"A lot of nature signs around the Cotter reserve have a drawing of a Murray crayfish on them. The irony is those Murray crayfish aren't in a large part of those areas anymore," he said.
He said the crayfish were not a "road block to be removed".
"They're on the edge. They've been in decline for 10 years and what concerns me is the government may do a survey and say they're very rare in this area and therefore it's not really going to be a threat," he said.
"Even just a few reproductive individuals are extremely valuable. They can really lead to the future of the population in a given area. We really need to consider this crayfish as one of the key reasons to keep riparian corridors intact along the Murrumbidgee."
If the suburb gets the green light, development would likely begin in 2018.
ACT planning minister Mick Gentleman said initial environmental investigations in the area had been completed but further planning would hinge on more study.
The ACT Conservation Council's executive director Clare Henderson previously told Fairfax Media more work was needed to identify endangered and threatened species in the area.