Unlike the semi-mythical carnivores of Bodmin and Dartmoor, there is proof of this creature's existence. The bodies of two of the otters have been found, killed by cars as they tried to cross roads.
The otter, which is half the size of its British counterpart and has a noticeable "tidemark" running from eye to ear, is the latest in a long line of creatures to have gone native after being introduced by man and deliberately or accidentally released. The list includes the rabbit, brought to Britain by the Normans, the grey squirrel, introduced as a pet in the 19th century, and several species of deer, including muntjac, sika, Pere Davids and Chinese water deer.
More recent escapees include wallabies in the Peak District, mink, which went native in Devon in the Fifties after being farmed for their fur and now number 100,000, and coypu, which arrived in Britain in theThirties and were exterminated because of the damage they caused to drainage systems in East Anglia.
Despite stronger controls on the keeping of wild animals, exotic pets are more popular than ever before and wildlife parks have proliferated, leading many naturalists to fear that more alien species are moving into the countryside. One theory is that the short-clawed otter may have escaped from a park owned by the Virgin Airlines boss, Richard Branson.
The short-clawed otter's successful establishment in the wild has taken conservationists by surprise. Don Jefferies, Britain's foremost otter expert, said: "It is a tropical species so we thought they probably would not breed. However, what happens if we have another really cold winter, like 1962-63, is another matter."
The first hint of an escape came in the early Eighties, when a local resident told Dr Jefferies that otters were breeding again in the Upper Thames Valley and sent him a photograph of one on a log to prove it. The native otter had become locally extinct in the Seventies because of hunting and the use of organochlorine pesticides. When Dr Jefferies measured the log, and compared it with the otter in the picture, he decided it was the much smaller Asian species.
Reports since then, including two of otters killed on the roads in 1986 and 1991, suggest that three or four generations of otters may have bred in the wild.
According to Dr Jefferies, the escape of the short-clawed otter and its breeding success have provided an "unplanned experiment" in water cleanliness, proving that the waters of the Thames were "sufficiently unpolluted to allow inhabitation by otters of whatever species and that [the native otter] was absent solely because there was none left to occupy the area".
However, a report to be published later this month by the Vincent Wildlife Trust shows that the native otter is now returning to many areas from which it has been absent for several decades, and threatening to push the Asian otter from its new-found ecological niche.
In Asia the two species of otter co-exist by eating different foods - the bigger species eats fish, the smaller one insects - or occupying different parts of a river system. However, Dr Jefferies believes the British otter, as it returns, may either kill its smaller rival or force it into less hospitable waterways, making it much harder for it to survive.
Several other exotic species of pet, including porcupines, gerbils and hamsters, established a foothold in the wild in Britain but died out or were exterminated later. Mongolian gerbils were released by a film company on the Isle of Wight in the early Seventies and colonies of golden hamsters have appeared in Bootle, Bury St Edmunds and Barnet. American red-eared terrapins and bullfrogs are among the latest intruders.
Dr Jefferies added: "Every time an alien species escapes there are people who talk about how useful it might be. They said the coypu might keep vegetation under control, for instance. Instead it chomped its way through long rows of sugar beet and made colossal holes in the banks. There is not one alien introduction I can think of that doesn't cause a problem. If the Asian short-clawed otter did disappear, I would not regret it."Reuse content