Steps taken to keep invasive mink out of Caithness and Sutherland
A Caithness-based countryside ranger has outlined issues faced by wildlife from the invasive American mink and steps taken to monitor the creature.
Paul Castle is the countryside ranger for Caithness and north Sutherland with High Life Highland and has revealed his methods of monitoring wild mink and how the animal has had a devastating effect on native species.
The American mink (Neovison vison or Mustela vison) is a small member of the Mustelid family of mammals, related to otters, stoats, weasels, and pine martin and an invasive species in the UK.
Paul said: “My journey with mink monitoring began in north Sutherland several years ago, where I took on the role of monitoring a mink raft. For years, I faithfully checked the raft for footprints, and while I never confirmed mink presence, I was often greeted by the tracks of otters and, occasionally, rats.
“These experiences gave me an insight into the lives of our native wildlife and the ever-present need for vigilance against invasive species.”
He says that though mink may look harmless given their size (smaller than an otter) their presence has “had devastating effects on native species”. Introduced to the UK in 1929 for fur farming, mink gradually escaped, either accidentally or through deliberate releases, over many years. They were eventually confirmed breeding in the wild in 1956.
In Scotland, the first mink farm opened in 1938, and by the 1980s, wild mink had spread north of the Great Glen, though they are yet to reach the far north of Scotland and some of the Scottish islands.
Mink are highly effective predators and once established they can have significant adverse impacts on local species. Prey includes birds and their eggs (native and domestic), small mammals, fish, amphibians, shellfish, and crustaceans, but their effect on water voles is particularly alarming.
Water voles (Arvicola amphibius), the UK’s fastest-declining mammal, have suffered a 95 per cent loss of range in the past century, partly due to habitat loss but increasingly because of mink.
Water voles of course have native predators, such as herons, owls, pike, and foxes but their defence against these natural predators was to retreat inside their bankside burrows. Unlike native predators, female mink can enter water vole burrows, wiping out entire colonies.
Mink are quick to colonise areas and easily cross waterbodies such as lochs, rivers and even sea channels to reach islands, this makes monitoring an essential part of conservation efforts.
Paul adds: “Currently, I’m responsible for the only mink raft in Caithness. It’s positioned along a small burn that flows from an important loch, home to a variety of wild bird species. The burn provides a lifeline for wildlife, but it’s also a potential corridor for mink, which makes the raft’s presence vital.”
Mink rafts are seen as a simple yet effective tool used to monitor mink activity. These anchored rafts float on the water’s surface and house a container of moist clay.
This surface layer is positioned in the centre of the tunnel. Mustelids are very inquisitive mammals and mink are no exception. They will naturally investigate any unusual objects found around water courses and tunnels seem irresistible to them. Passing through one of these raft tunnels, they leave their prints on the clay, which can then be identified by volunteers. Regular monitoring of these clay surfaces may indicate the presence of mink in an area.
“Over time, my Caithness raft took a battering from the elements. When I discovered it recently, it was in quite a sorry state and needed replacing.
“The tunnel was completely missing, it is anyone’s guess where that finally ended up after the wind carried it away. The plywood boards were also split and damaged yet amazingly the clay-lined container was still there, complete with faint otter prints.”
The West Sutherland Fisheries Trust, as part of the Scottish Invasive Species Initiative (SISI), stepped in to replace the damaged raft. “This initiative is another great example of cooperative conservation, bringing together 10 fishery trusts and over 350 volunteers to monitor invasive mink across 43 river catchments in northern Scotland.”
Paul says it has been an “extraordinary effort” and that there are around 650 rafts and traps regularly checked by dedicated volunteers.
“So, with a new raft in place, fresh clay and tunnel attached, back into the burn it went and I have resumed monitoring.
“The replacement Caithness raft has so far, only produced otter prints – and long may that continue. If mink tracks were to be identified, following careful consideration and approval, live trapping may be introduced. Live traps are also monitored by volunteers but any handling and dispatch of captured mink is only undertaken by fully trained and licensed individuals.
“I’m proud to be part of this collective effort. Several of my colleagues in the ranger service also monitor mink rafts in their areas. Every set of otter prints we find and every mink footprint we don’t find feels like a small victory.
“Volunteers are always needed to help with raft monitoring. Together, we can protect Scotland’s native wildlife. Conservation is a team effort, and together we aim to keep the Highlands a haven for native species.”
If you spot an American mink in Caithness and Sutherland please report it to the Scottish Invasive Species Initiative at: www.invasivespecies.scot/report-american-mink-sighting
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