Life is beginning to change for the American mink on the riverbanks of the Upper Thames and its tributaries in Oxfordshire. For decades the mink has been dominant — if officially an intrusive — mammal in the wildlife of Britain.

It is known to be one of the reasons why the native water vole has been in decline as the mink hunt it down for food.

Loss of wetlands and waterside vegetation to intensifying agriculture have been factors in the reduction of water voles.

Yet there are signs that the mink is not going to have everything its way in the future, with the recovery of otters and polecats in the Thames region, according to researchers with WildCRU, the wildlife conservation reserach unit of the Department of Zoology of Oxford University.

So the gradual reappearance of the otter and polecat means that the mink now have some competition on the riverbanks and are beginning to adjust their patterns of behaviour.

However, no one expects the invasive mink to decline in the way that the native otters, polecats and water voles have in the past.

Prof David Macdonald, founding director of WildCRU and Oxford’s first professor of wildlife conservation, said: “American mink are an intrusive mammal into Britain and they had an open door into the countryside as they were introduced here exactly when otters and polecasts had been driven into steep decline.”

Historically, American mink began to be imported into this country from the USA in the 1920s when there was a worldwide fashion for long furs, of which they were a fashionable example.

“We must distinguish between the American mink and the European mink, the latter being native to mainland Europe. The reason American mink became attractive to the fashion world was because it had longer and more luxuriant fur than the European mink,” said Prof Macdonald.

So it became economic to set up American mink farms in this country — the first was in 1926 — and similar farms were established in France, Germany, Holland and Denmark.

“Inevitably, there were escapes by mink from the farms and in more recent years many mink were released by the animal rights movement, although a lot of those mink probably died,” he said.

But sufficient mink were hardy enough to survive and are now found in the wild all over Britain — with the exception of the northern half of Scotland.

Just why mink have not spread all over Scotland is a subject for research by WildCRU.

Many regard the American mink as a menace. Prof Macdonald said: “They are beautiful and fascinating mammals and it is not their fault they are here and are so problematic. They were brought into this country by people.”

Although the mink is seen as a problem in the upper Thames region and everywhere else, there is a parallel situation in Russia and the former Soviet republics.

“The Russians thought the American mink would be an advantage for their fur trappers. So they were imported and the result has been that the native European mink has severely declined in those countries too, due to the competition from the American invader,” added Prof Macdonald.

To assess the impact of American mink on our riverbanks, WildCRU carried out a four-year project on the Upper Thames region — that is the stretch north of Oxford.

Tom Moorhouse and Lauren Harrington, both post doctoral researchers with WildCRU, were among the team involved in the work on the impact of mink on the water vole population. Mink were fitted with lightweight radio receivers and were tracked over their territory and some water voles, bred by specialists, were reintroduced into various stretches of the river.

Mr Moorhouse said: “We found that water voles had a better survival rate when the margin of vegetation at the water edge was 6m rather than 1m, although the voles usually lived in the first 1m from the water’s edge so they could easily escape into the water if danger appeared.”

One way to help water voles survive and check the presence of mink was to anchor mink rafts in the water. The rafts were designed by the Game Conservancy and Wildlife Conservancy Trust.

Mink would leave their footprints in clay that lined the raft.

If it was decided by landowners and water bailiffs that the mink should be humanely killed, a trap would be put at one end of the raft. However, to keep mink out of a stretch of river, there would have to be an ongoing programme of eradication using the rafts and traps, otherwise the mink would normally recolonise.

Mr Moorhouse added: “It is a job for the landowners and gamekeepers or water bailiffs to install the rafts and remove the mink. But then there is the question of the time and cost that would be involved.”

Ms Harrington added: “We found that with the return of the otter and polecat the mink have changed from their normal noctural habits to be active in the day-time, presumably to avoid being bullied by the otters, which are much larger than the mink. This is a rare example of a mammal changing its pattern of activity to avoid competition.”

Polecats, mink, weasels and stoats are all members of the Mustelidac family, which Prof Macdonald irreverently called “the smellies”.

Members of the weasel family all have strong odours as part of their scent-marking communcations.

The mink versus water vole question is just one faced by conservationists when considering how far people should go to try to control invasive mammals.

Prof Macdonald added: “There are difficult conservation problems with financial and philosophical implications in how far we can go to control the invasive species such as the American mink, the grey squirrel and the muntjac deer. All these introduced species pose problems for native wildlife and sometimes it is justifiable to try to put the clock back by removing them.

“But in other cases it may be better to try to save them — after all rabbits and fallow deer, for example, were non-native originally and they are now accepted as part of Britain’s wildlife. Conservation is all about difficult judgements,” said Prof Macdonald.