American mink - an invasive species

American mink, Neovison vison, are an invasive species in the UK, introduced by escapes and releases from fur farms in the 1950s and 60s. These carnivorous mustelids pose a threat to native wildlife, especially the water vole (Arvicola amphibius). 

Related to our native mustelid, the otter, the mink is similarly semi-aquatic but significantly smaller. Male mink are larger than females; males weigh 1-2kg and females 0.5-0.8kgs. Their fur is dark brown, except on the chin and throat where it's white. They are fiercely territorial - on average males' territories cover 5km of a watercourse. Females' territories are smaller, just 1-3km long, and are often inside or partially overlap males' territories.

Breeding occurs in spring, and females build dens in tree root cavities or rock piles at the water's edge, or in rabbit burrows. After a gestation period of 40-75 days, litters of 4-6 kits are born. Females have just one litter a year. Kits are weaned after around 6 weeks and become independent by late summer/autumn. 

Mink are opportunistic feeders and will eat fish, small mammals, birds and invertebrates. The primary species affected by mink predation is the water vole. Endangered in the UK and already threatened by habitat loss and pollution, the water vole is predicted to be driven to extinction in mainland Britain if no mink control occurs. Female mink are small enough to fit into water vole burrows and take young, and mink will also predate winter-surviving voles before they have had the chance to breed. Predation by mink can also have severe detrimental impacts on coastal ground nesting birds and waterfowl. This was seen in 1989 to 1995, when mink caused whole colony breeding failures of common gulls, black-headed gulls and common terns on islands off 1000km of Scottish coast. The most severe impact was on the black-headed gulls, whose local population decreased by 52% in this time.

With an estimated 122,000 mink across the UK, the problem of this invasive species is difficult to tackle. However, there are schemes aiming to eradicate mink from local and regional areas. 
The first stage of eradicating mink is using mink rafts to detect their presence. The rafts are placed in watercourses in potential mink territory with a clay pad inside a tunnel. If a mink walks through the tunnel it will leave footprints in the clay. Mink footprints can be identified as they are approximately 2.5cm wide and 2.5cm long, with 5 toe pads (though often only 4 are visible) in a steep arc over a relatively small heel pad. The surrounding area can also be checked for mink scats, which are 3-6cm long, twisted and have an extremely pungent odour. 


Mink footprints in the clay of a mink raft on a nature reserve, Hampshire

Once the presence of mink is established, the tunnel can be exchanged for a trap. Traps must be checked at least daily to reduce unnecessary stress on the animals and release any non-target species as soon as possible. It is illegal to release mink once caught, and generally they are killed as humanely as possible, by a shot to the head. Where funding is available, environmental DNA may be used to detect any remaining mink once the population density is too low for rafts to be effective. Cage-trapping of mink has proved to be effective in some locations - on North Uist in the Outer Hebrides mink have been successfully eradicated using this technique. 

Though the methods of removing mink are potentially controversial, eradication of mink, at least from some refuges, is essential to stop the decline and extinction of the water vole and other native species.



https://canalrivertrust.org.uk/enjoy-the-waterways/canal-and-river-wildlife/the-rogues-gallery-of-invasive-species/american-mink 
https://www.wildlifetrusts.org/wildlife-explorer/mammals/american-mink
https://thenorfolkminkproject.org.uk/
https://www.mammal.org.uk/species-hub/full-species-hub/discover-mammals/species-american-mink/
Long-term effects of North American Mink Mustela vison on seabirds in western Scotland


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