Alien invaders: American mink removed from Scotland

Before we can even see the animal, its pungent, musky scent has filled the air.

We walk down to inspect the floating trap that is tethered to the riverbank, and sure enough, a mink is inside.

It is quickly checked over: this one is a large male with, unusually, grey fur instead of a brown coat.

Then it is taken away to a quiet spot along this catchment of the River Don, and killed: a single shot to its head.

It is not easy to witness, and the conservationists I am with say it is a difficult part of their job.

"It's not something I get any satisfaction out of, but I am trained to do this, and we dispatch them as quickly and humanely as possible to cause minimal distress to the animal," says Jamie Urquhart, a river biologist from the River Don Trust.

One by one, American mink like this are being removed from rivers in Scotland, in what has been described as the biggest conservation scheme of its kind in a paper in the journal Biological Conservation.

These animals first arrived in the UK in the 1950s, imported from America to be farmed for their fur.

But after some escaped - or were released - they have spread around the country, and scientists estimate there are now tens of thousands throughout the UK.

And the predators have had a devastating impact on local river wildlife, affecting birds such as moorhens, coots, widgeon and teal, fish and most markedly water voles, which have declined by more than 95% over the last 50 years.

Conservationists say they are left with a stark choice: either leave the mink alone and allow the UK's native wildlife to continue to decline, or begin the colossal effort to remove them, however difficult this may be.

Water voles have been badly affected by the mink invasion

Jamie Urquhart says: "It can be a contentious issue for some, but given it is an invasive species that is not supposed to be here - it is our fault it is here in the first place - I feel I would like to rectify that where possible by doing my part and helping in this project."

Even animal welfare organisations like the Royal Society for the Prevention of Cruelty to Animals make exceptions to their strong stance against killing wild animals when it comes to invasive species.

A spokesman said: "The RSPCA is as a general principle against the culling of wild animals, but accepts that in the case of some mink eradication projects there may be strong scientific grounds to support one.

"But it is of utmost importance that any cull be carried out by trained operators using humane, established methods."

Mass removal

So far, the University of Aberdeen led project, which was funded by the Tubney Charitable Trust, Scottish Natural Heritage (SNH), Cairngorms National Park Authority, the Natural Environment Research Council , and local fisheries trusts, has removed mink from more than 10,000 sq km (4,000 sq miles) of north-east Scotland.

Volunteers are given a raft to look after, which allows them to monitor mink

It is a process that took three years and involved 186 volunteers from local communities in the area.

Project leader Xavier Lambin, professor of ecology from the University of Aberdeen, says: "We have been gradually spreading, starting from the headwaters, and moving down each river in turn."

The idea is simple: each volunteer is assigned one or several mink rafts, which are small floating platforms.

Each raft has a "footprint plate" made from soft clay, and these show whether any animals have decided to take a look inside the raft's tunnel.

Volunteers check their rafts every fortnight, and if any mink footprints are spotted, then a trap is set, a small metal cage, which captures and contains any mink that crawl inside.

The raft is then checked daily, which is a legal requirement, and if a mink is caught, it is then killed by a trained member of the team (the Wildlife & Countryside Act 1981 makes it an offence to release mink or allow them to escape into the wild, and the Mink Keeping (Scotland) Order 2003 prohibits the keeping of the species in Scotland except under licence).

A clay plate records the footprints from curious mink

Professor Lambin says that clearing mink from the rivers has already had an impact on local wildlife.

He explains: "We very much find evidence that water voles are making a comeback - some areas that have not seen water voles for 10, 20 years are now being reoccupied.

"And we expect species like moorhens will make a recovery. They used to be abundant, but we don't see many now.

"There is scope, but it will be a slow gradual process."

Global impact

Invasive species - plants, animals or microorganisms that are not native to an area, but have been introduced, either accidentally or deliberately, by humans - are a growing problem around the world.

Scientists say they pose a huge threat to the world's biodiversity and cost the global economy hundreds of billions of pounds.

But removing them once they have established and spread has proved difficult - especially from the mainland.

Professor Lambin says: "If you work on an island, you have a finite task - once you have removed them, it is unlikely they will come back.

"In an area like mainland Scotland, we have to guard against recolonisation. This means putting into place a continuous monitoring scheme behind our control front so any mink that reinvade the area will be picked up and removed."

However, he says that the initial success of this project over such a large scale offers hope that the tide of established invasive species can be turned.

He says: "This is currently the largest project of this kind in the world. It is a rare success that will provide optimism: If you work with communities, you will have something that works."

The team now plans to double the area of mink removal to 20,000 sq km (8,000 sq miles), working with the rivers and fisheries trusts of Scotland (Rafts), Scottish Wildlife Trust, the University of Aberdeen and the SNH, and they say that eventually, the approach could be rolled out across the whole of the UK.

Professor Lambin tells the BBC: "It is important to be strategic about how we spread so we minimise reinvasion of the area, but anything is possible when you work with volunteers, people are willing to invest in their neighbourhood, and are grateful to be given the chance to contribute to biodiversity."

However, even then, he warns that it could be decades before the UK can be declared a mink-free zone.