The water vole, once a familiar sight along UK rivers, is finding it harder than ever to maintain its population.
The water vole likes earth banks
The State of Britain's Mammals Report 2005, published on Wednesday, says the rodent is "a contender for the UK's most rapidly declining mammal".
Monitoring along 436km of watercourses last year showed that in three-quarters of the areas surveyed, evidence for water vole presence had become scarcer.
But conservationists believe they can turn the species around by 2010.
"I'm actually quite optimistic," said water vole expert Rob Strachan, from Oxford University's Wildlife Conservation Research Unit.
"The water vole is a good colonist. Where we've done habitat enhancement next to existing populations, we have shown that water voles will quickly spread into the new areas and start breeding," he told the BBC News website.
"In a pilot study we did in Sussex, we tripled numbers in three years - and that's a dramatic increase for any mammal."
Water voles (Arvicola terrestris) prefer places with ample waterside plant growth to provide food, and steep earth banks for making their burrows.
But the intensification of agriculture in the second half of the 20th Century led to huge changes in wetland habitats.
Once strong water vole populations were pushed back by land drainage and by the canalisation of rivers with iron and concrete banks.
The new mink traps have proved very efficient (Image: Jonathan Reynolds/The Game Conservancy Trust)
Another significant factor in the downturn has been the introduction of the American mink, many of which escaped from fur farms.
The alien invader has driven out the water vole in many areas.
Mink are better swimmers than voles, and a female mink with young to feed can kill all the voles in her territory in less than a year. Other predators include stoats, owls, herons, large fish, foxes and domestic cats.
There are now, though, promising initiatives in place to constrain the mink population.
These involve the use of a raft-cum-trap device that conservationists can set by the waterside.
Developed by the Game Conservancy Trust, it incorporates a "tracking cartridge" containing a moist sand/clay mixture that records the footprints of animals visiting the raft.
Once mink are detected, and if they are to be removed from the site, managers can then install traps on the rafts to catch the animals.
"It is very efficient and by spacing the rafts along a watercourse and working at a large scale, you can take out the mink very quickly," said Mr Strachan.
"Two thousand and three saw the last mink farm close, so there are no new sources; and mink themselves are now a mammal in decline in some areas."
Conservation efforts that have concentrated on trying to restore the water voles' habitat should be helped by new agri-payment schemes that switch farming subsidies away from food production to environmental enhancement of the countryside.
The UK Biodiversity Action Plan (Bap), which aims to help the most threatened species, has set specific goals for vole recovery.
PINE MARTEN - MARTES MARTES
Strongest populations are found in north-west Scotland
Belongs to the mustelid family - along with stoats and weasels
Suffered years of persecution by gamekeepers, and from hunters who valued its fur
One target sought to return water vole numbers to 1970 levels by 2010. This is now recognised to be unachievable.
Instead, a Smart (Specific, Measurable, Achievable, Relevant, and Time bound) target is now being developed which in five years' time would see the decline halted and the species begin a concerted bounce back in large areas of the UK (there are no water voles in Northern Ireland).
The State of Britain's Mammals 2005 report is the fourth in a series of annual updates produced by the Mammals Trust UK.
As well as highlighting the water vole, this year's report looks in some detail at the pine marten, Britain's second rarest carnivore (the wildcat is the rarest).
The mustelid's numbers are strongest in Scotland; its population size in the rest of the UK is tiny, although accurate figures are hard to come by.
"The pine marten seems to be more uncertain in terms of how many there are," explained Jill Nelson, chief executive of Mammals Trust UK.
"They're known reasonably well in Scotland, less so in England and Wales. This is a case where there is more to be done in terms of monitoring so that we can be sure of what a future conservation strategy would be."
The trust believes the pine marten (Martes martes) does not currently have the appropriate conservation status.
At the moment it is listed simply as a Species of Conservation Concern and the charity wants the animal elevated to a Priority Species.