By Alex Kirby
BBC News Online environment correspondent
A British mammal redolent of a bygone countryside idyll, the otter, is steadily returning to parts of England it had abandoned.
Recovering: But a long way to go (Image: Andy Graham, Worcestershire Wildlife Trust)
A study of English otters has found the area they inhabit grew fivefold in 25 years.
Half a century ago, otters were found only in parts of eastern and south-west England.
Better water quality has helped their recovery, though road traffic is now a growing menace.
The study, the National Otter Survey of England, is the work of the Environment Agency (EA) and the Wildlife Trusts, with help from English Nature and the privatised water companies.
The survey, the fourth since the late 1970s, found the areas otters now live in has increased by 527% since the first survey.
Of 3,327 riverbank and wetland sites surveyed, 1,137 (almost 35%) showed evidence of otters, either paw prints or spraints (droppings). The 1977 figure was 5.8%.
Against the odds
Otters declined steeply across Europe in the 1960s, with conservationists blaming the widespread use of certain pesticides. Apart from remnant populations clinging on in remote parts of England, their British strongholds were reduced to Wales and Scotland.
But improved water quality and fish stocks, coupled with changes in riverbank management, have reversed the English otters' fortunes.
Otters are related to badgers, weasels and mink
Their spraints smell of jasmine tea mixed with fish paste
They are not natural swimmers, but learn fast
They are an important indicator species, and their return holds out hope for other creatures and for the entire aquatic environment.
Andrew Crawford of the EA, the study's author, says the otters are "on the road to recovery".
He said: "Overall the survey suggests a real and continued increase in otter range, which in turn reflects a considerable increase in population."
But the animals' spread is patchy, with some areas seeing only very slow increases.
Alastair Driver, the agency's national conservation manager, said: "We can't become complacent - otters are not increasing as fast as we would like in some areas.
"We will need to concentrate on ways to protect the otter from the motor car, which continues to be one of the biggest threats."
Allison Crofts, habitats and species manager for the Wildlife Trusts, told BBC News Online: "Potentially, road traffic is one of the most serious limiting factors for the future.
"This is especially true of south-east England, with its dense transport network. We're going to struggle to see the otters make a full recovery there, and they may never return to their historic levels.
The otter recovery is slow, not spectacular (Image: Wildlife Trusts)
"The danger's worst when the rivers are in flood in winter and early spring. We're pushing for mitigation measures on all new road schemes - underpasses or ledges to keep the otters off the roads, or else fencing."
Researchers predicted in 1996 that there would be signs of otters at 43% of all sites surveyed by 1999, and at 56% by 2006. The latest survey acknowledges the results fall far short, but says: "The trend is still upwards."
Otters used to be hunted, but in 1978 they were given legal protection, which has helped their comeback.
The survey says: "Illegal killing of otters undoubtedly still occurs, and there are serious concerns in a limited number of areas.
"But persecution is no longer considered a general threat for otter populations."
The Trent river network in the English Midlands has seen the largest growth of evidence of otters, from 5% of areas surveyed in 1991-94 to 24.4% now.
But parts of north-west and south-east England have seen only small increases.
Apart from improving water quality, the companies have helped to build otter dens (holts), restore riverbanks and construct road avoidance schemes.